# Overview

Ruby is an object-oriented scripting language developed by Yukihiro Matsumoto ("Matz"). The main web site for Ruby is ruby-lang.org. Development began in February 1993 and the first alpha version of Ruby was released in December 1994. It was developed to be an alternative to scripting languages such as Perl and Python.[1] Ruby borrows heavily from Perl and the class library is essentially an object-oriented reorganization of Perl's functionality. Ruby also borrows from Lisp and Smalltalk. While Ruby does not borrow many features from Python, reading the code for Python helped Matz develop Ruby.[1]

Mac OS X comes with Ruby already installed. Most Linux distributions either come with Ruby preinstalled or allow you to easily install Ruby from the distribution's repository of free software. You can also download and install Ruby on Windows. The more technically adept can download the Ruby source code[2] and compile it for most operating systems, including Unix, DOS, BeOS, OS/2, Windows, and Linux.[3]

## Features

Ruby combines features from Perl, Smalltalk, Eiffel, Ada, Lisp, and Python.[3]

### Object Oriented

Ruby goes to great lengths to be a purely object oriented language. Every value in Ruby is an object, even the most primitive things: strings, numbers and even true and false. Every object has a class and every class has one superclass. At the root of the class hierarchy is the class Object, from which all other classes inherit.

Every class has a set of methods which can be called on objects of that class. Methods are always called on an object — there are no “class methods”, as there are in many other languages (though Ruby does a great job of faking them).

Every object has a set of instance variables which hold the state of the object. Instance variables are created and accessed from within methods called on the object. Instance variables are completely private to an object. No other object can see them, not even other objects of the same class, or the class itself. All communication between Ruby objects happens through methods.

### Mixins

In addition to classes, Ruby has modules. A module has methods, just like a class, but it has no instances. Instead, a module can be included, or “mixed in,” to a class, which adds the methods of that module to the class. This is very much like inheritance but far more flexible because a class can include many different modules. By building individual features into separate modules, functionality can be combined in elaborate ways and code easily reused. Mix-ins help keep Ruby code free of complicated and restrictive class hierarchies.

### Dynamic

Ruby is a very dynamic programming language. Ruby programs aren’t compiled, in the way that C or Java programs are. All of the class, module and method definitions in a program are built by the code when it is run. A program can also modify its own definitions while it’s running. Even the most primitive classes of the language like String and Integer can be opened up and extended. Rubyists call this monkey patching and it’s the kind of thing you can’t get away with in most other languages.

Variables in Ruby are dynamically typed, which means that any variable can hold any type of object. When you call a method on an object, Ruby looks up the method by name alone — it doesn't care about the type of the object. This is called duck typing and it lets you make classes that can pretend to be other classes, just by implementing the same methods.

### Singleton Classes

When I said that every Ruby object has a class, I lied. The truth is, every object has two classes: a “regular” class and a singleton class. An object’s singleton class is a nameless class whose only instance is that object. Every object has its very own singleton class, created automatically along with the object. Singleton classes inherit from their object’s regular class and are initially empty, but you can open them up and add methods to them, which can then be called on the lone object belonging to them. This is Ruby’s secret trick to avoid “class methods” and keep its type system simple and elegant.

### Metaprogramming

Ruby is so object oriented that even classes, modules and methods are themselves objects! Every class is an instance of the class Class and every module is an instance of the class Module. You can call their methods to learn about them or even modify them, while your program is running. That means that you can use Ruby code to generate classes and modules, a technique known as metaprogramming. Used wisely, metaprogramming allows you to capture highly abstract design patterns in code and implement them as easily as calling a method.

### Flexibility

In Ruby, everything is malleable. Methods can be added to existing classes without subclassing, operators can be overloaded, and even the behavior of the standard library can be redefined at runtime.

### Variables and scope

You do not need to declare variables or variable scope in Ruby. The name of the variable automatically determines its scope.

• x is local variable (or something besides a variable).
• $x is a global variable. • @x is an instance variable. • @@x is a class variable. ### Blocks Blocks are one of Ruby’s most unique and most loved features. A block is a piece of code that can appear after a call to a method, like this: laundry_list.sort do |a,b| a.color <=> b.color end  The block is everything between the do and the end. The code in the block is not evaluated right away, rather it is packaged into an object and passed to the sort method as an argument. That object can be called at any time, just like calling a method. The sort method calls the block whenever it needs to compare two values in the list. The block gives you a lot of control over how sort behaves. A block object, like any other object, can be stored in a variable, passed along to other methods, or even copied. Many programming languages support code objects like this. They’re called closures and they are a very powerful feature in any language, but they are typically underused because the code to create them tends to look ugly and unnatural. A Ruby block is simply a special, clean syntax for the common case of creating a closure and passing it to a method. This simple feature has inspired Rubyists to use closures extensively, in all sorts of creative new ways. ### Advanced features Ruby contains many advanced features. • Exceptions for error-handling. • A mark-and-sweep garbage collector instead of reference counting. • OS-independent threading, which allows you to write multi-threaded applications even on operating systems such as DOS. (this feature will disappear in 1.9, which will use native threads) You can also write extensions to Ruby in C or embed Ruby in other software. ## References 1. a b Bruce Stewart (November 29, 2001). "An Interview with the Creator of Ruby". O'Reilly. Retrieved 2006-09-11. 2. "Download Ruby". Retrieved 2006-09-11. 3. a b "About Ruby". Retrieved 2006-09-11. # Installing Ruby Ruby comes preinstalled on Mac OS X and many Linux distributions. In addition, it is available for most other operating systems, including Microsoft Windows. To find the easiest way to install Ruby for your system, follow the directions below. You can also install Ruby by compiling the source code, which can be downloaded from the Ruby web site. ## Operating systems ### Mac OS X Ruby comes preinstalled on Mac OS X. To check what version is on your system: 1. Launch the Terminal application, which is located in the "Utilities" folder, under "Applications". 2. At the command-line, enter: ruby -v If you want to install a more recent version of Ruby, you can: • Buy the latest version of Mac OS X, which may have a more recent version of Ruby. • Install Ruby using RVM. (This is the most popular way because you can manage ruby versions and install many other ruby packages) • Install Ruby using Fink. • Install Ruby using MacPorts. • Install Ruby using Homebrew. ### Linux Ruby comes preinstalled on many Linux systems. To check if Ruby is installed on your system, from the shell run: ruby -v If ruby is not installed, or if you want to upgrade to the latest version, you can usually install Ruby from your distribution's software repository. Directions for some distributions are described below. #### Debian / Ubuntu On Debian and Ubuntu, install Ruby using either the graphical tool Synaptic (on Debian, only if it is installed; it is included with Ubuntu) or the command-line tool apt. #### Fedora Core If you have Fedora Core 5 or later, you can install Ruby using the graphical tool Pirut.[1] Otherwise, you can install Ruby using the command-line tool yum. #### Arch Linux If you have Arch Linux you can install Ruby using the command-line tool pacman. #### Mandriva Linux On Mandriva Linux, install Ruby using the command-line tool urpmi. #### PCLinuxOS On PCLinuxOS, install Ruby using either the graphical tool Synaptic or the command-line tool apt. #### Red Hat Linux On Red Hat Linux, install Ruby using the command-line tool RPM. ### Windows Ruby does not come preinstalled with any version of Microsoft Windows. However, there are several ways to install Ruby on Windows. • Download and install one of the compiled Ruby binaries from the Ruby web site. • Download and run the one click RubyInstaller. • Install Cygwin, a collection of free software tools available for Windows. During the install, make sure that you select the "ruby" package, located in the "Devel, Interpreters" category. #### Windows is slow Currently Ruby on windows is a bit slow. Ruby isn't optimized for windows, because most core developers use Linux. Though 1.9.2 passes almost all core tests on windows. Most of today's slowdown is because when ruby does a require 'xxx'  it searches over its entire load path, looking for a file named xxx, or named xxx.rb, or xxx.so or what not. In windows, doing file stat's like that are expensive, so requires take a longer time in windows than linux. 1.9 further complicates the slowdown problem by introducing gem_prelude, which avoids loading full rubygems (a nice speedup actually), but makes the load path larger, so doing require's on windows now takes forever. To avoid this in 1.9.2, you can do a require 'rubygems'  which reverts to typical load behavior. If you want to speed it up (including rails) you can use http://github.com/rdp/faster_require Which have some work arounds to make loading faster by caching file locations. Also the "rubyinstaller" (mingw) builds are faster than the old "one click" installers If yours comes from rubyinstaller.org, chances are you are good there. NB that Jruby tends to run faster but start slower, on windows, than its MRI cousins. Rubinius is currently not yet windows compatible. ## Building from Source If your distro doesn't come with a ruby package or you want to build a specific version of ruby from scratch, please install it by following the directions here. Download from here. ## Compile options ### Building with debug symbols If you want to install it with debug symbols built in (and are using gcc--so either Linux, cygwin, or mingw).  ./configure --enable-shared optflags="-O0" debugflags="-g3 -ggdb"  ### Optimizations Note that with 1.9 you can pass it --disable-install-doc to have it build faster. To set the GC to not run as frequently (which tends to provide a faster experience for larger programs, like rdoc and rails), precede your build with $ export CCFLAGS=-DGC_MALLOC_LIMIT=80000000


though you might be able to alternately put those in as opt or debug flags, as well.

## Testing Installation

The installation can be tested easily.

$ruby -v  This should return something like the following: ruby 1.8.7 (2009-06-12 patchlevel 174) [i486-linux]  If this shows up, then you have successfully installed Ruby. However, if you get something like the following: -bash: ruby: command not found  Then you did not successfully install Ruby. ## References 1. "yum". Fedora Wiki. Retrieved 2006-09-13. # Ruby editors Although you can write Ruby programs with any plain text editor, some text editors have additional features to aid the Ruby programmer. The most common is syntax highlighting. Here is a spreadsheet of the various options available. # Notation conventions ## Command-line examples In this tutorial, examples that involve running programs on the command-line will use the dollar sign to denote the shell prompt. The part of the example that you type will appear bold. Since the dollar sign denotes your shell prompt, you should not type it in. For example, to check what version of Ruby is on your system, run: $ ruby -v


Again, do not type the dollar sign – you should only enter "ruby -v" (without the quotes). Windows users are probably more familiar seeing "C:\>" to denote the shell prompt (called the command prompt on Windows).

An example might also show the output of the program.

$ruby -v ruby 1.8.5 (2006-08-25) [i386-freebsd4.10]  In the above example, "ruby 1.8.5 (2006-08-25) [i386-freebsd4.10]" is printed out after you run "ruby -v". Your actual output when you run "ruby -v" will vary depending on the version of Ruby installed and what operating system you are using. ### Running Ruby scripts For simplicity, the following convention is used to show a Ruby script being run from the shell prompt. $ hello-world.rb
Hello world


However, the actual syntax that you will use to run your Ruby scripts will vary depending on your operating system and how it is setup. Please read through the Executable Ruby scripts section of the Hello world page to determine the best way to run Ruby scripts on your system.

### Running irb

Ruby typically installs with "interactive ruby" (irb) installed along with it. This is a REPL that allows you to experiment with Ruby, for example:

$irb >> 3 + 4 => 7 >> 'abc' => "abc"  # Interactive Ruby When learning Ruby, you will often want to experiment with new features by writing short snippets of code. Instead of writing a lot of small text files, you can use irb, which is Ruby's interactive mode. ## Running irb Run irb from your shell prompt. $ irb --simple-prompt
>>


The >> prompt indicates that irb is waiting for input. If you do not specify --simple-prompt, the irb prompt will be longer and include the line number. For example:

$irb irb(main):001:0>  A simple irb session might look like this. $ irb --simple-prompt
>> 2+2
=> 4
>> 5*5*5
=> 125
>> exit


These examples show the user's input in bold. irb uses => to show you the return value of each line of code that you type in.

### Cygwin users

If you use Cygwin's Bash shell on Microsoft Windows, but are running the native Windows version of Ruby instead of Cygwin's version of Ruby, read this section.

To run the native version of irb inside of Cygwin's Bash shell, run irb.bat.

By default, Cygwin's Bash shell runs inside of the Windows console, and the native Windows version of irb.bat should work fine. However, if you run a Cygwin shell inside of Cygwin's rxvt terminal emulator, then irb.bat will not run properly. You must either run your shell (and irb.bat) inside of the Windows console or install and run Cygwin's version of Ruby.

## Understanding irb output

irb prints out the return value of each line that you enter. In contrast, an actual Ruby program only prints output when you call an output method such as puts.

For example:

$irb --simple-prompt >> x=3 => 3 >> y=x*2 => 6 >> z=y/6 => 1 >> x => 3 >> exit  Helpfully, x=3 not only does an assignment, but also returns the value assigned to x, which irb then prints out. However, this equivalent Ruby program prints nothing out. The variables get set, but the values are never printed out. x=3 y=x*2 z=y/6 x  If you want to print out the value of a variable in a Ruby program, use the puts method. x=3 puts x  # Mailing List FAQ ### Etiquette There is a list of Best Practices[1]. ### What is the Best editor? Several editors exist for Ruby. Commercial: The editor of preference on OS X is Textmate. RubyMine has also received good reviewed[2] Free: NetBeans offers a version with Ruby support. RadRails is a port of eclipse to support Ruby syntax. Eclipse has a plugin DLTK that offers ruby support[3]. On windows for rails projects there is RoRed. See some more questions answered. # Basic Ruby - Hello world The classic "hello world" program is a good way to get started with Ruby. ## Hello world Create a text file called hello-world.rb containing the following code: puts 'Hello world'  Now run it at the shell prompt. $ ruby hello-world.rb
Hello world


You can also run the short "hello world" program without creating a text file at all. This is called a one-liner.

$ruby -e "puts 'Hello world'" Hello world  You can run this code with irb, but the output will look slightly different. puts will print out "Hello world", but irb will also print out the return value of puts – which is nil. $ irb
>> puts "Hello world"
Hello world
=> nil


Like Perl, Bash, and C Shell, Ruby uses the hash symbol (also called Pound Sign, number sign, Square, or octothorpe) for comments. Everything from the hash to the end of the line is ignored when the program is run by Ruby. For example, here's our hello-world.rb program with comments.

# My first Ruby program
# On my way to Ruby fame & fortune!

puts 'Hello world'


You can append a comment to the end of a line of code, as well. Everything before the hash is treated as normal Ruby code.

puts 'Hello world'                # Print out "Hello world"


You can also comment several lines at a time:

=begin
This program will
print "Hello world".
=end

puts 'Hello world'


execute by clicking the link "run" online Although block comments can start on the same line as =begin, the =end must have its own line. You cannot insert block comments in the middle of a line of code as you can in C, C++, and Java, although you can have non-comment code on the same line as the =end.

=begin This program will print "Hello world"
=end puts 'Hello world'


execute online

## Executable Ruby scripts

Typing the word ruby each time you run a Ruby script is tedious. To avoid doing this, follow the instructions below.

### Unix-like operating systems

In Unix-like operating systems – such as Linux, Mac OS X, and Solaris – you will want to mark your Ruby scripts as executable using the chmod command. This also works with the Cygwin version of Ruby.

$chmod +x hello-world.rb  You need to do this each time you create a new Ruby script. If you rename a Ruby script, or edit an existing script, you do not need to run "chmod +x" again. Next, add a shebang line as the very first line of your Ruby script. The shebang line is read by the shell to determine what program to use to run the script. This line cannot be preceded by any blank lines or any leading spaces. The new hello-world.rb program – with the shebang line – looks like this: #!/usr/bin/ruby puts 'Hello world'  If your ruby executable is not in the /usr/bin directory, change the shebang line to point to the correct path. The other common place to find the ruby executable is /usr/local/bin/ruby. The shebang line is ignored by Ruby – since the line begins with a hash, Ruby treats the line as a comment. Hence, you can still run the Ruby script on operating systems such as Windows whose shell does not support shebang lines. Now, you can run your Ruby script without typing in the word ruby. However, for security reasons, Unix-like operating systems do not search the current directory for executables unless it happens to be listed in your PATH environment variable. So you need to do one of the following: 1. Create your Ruby scripts in a directory that is already in your PATH. 2. Add the current directory to your PATH (not recommended). 3. Specify the directory of your script each time you run it. Most people start with #3. Running an executable Ruby script that is located in the current directory looks like this: $ ./hello-world.rb


Once you have completed a script, it's common to create a ~/bin directory, add this to your PATH, and move your completed script here for running on a day-to-day basis. Then, you can run your script like this:

$hello-world.rb  #### Using env If you do not want to hard-code the path to the ruby executable, you can use the env command in the shebang line to search for the ruby executable in your PATH and execute it. This way, you will not need to change the shebang line on all of your Ruby scripts if you move them to a computer with Ruby installed in a different directory. #!/usr/bin/env ruby puts 'Hello world'  ### Windows If you install the native Windows version of Ruby using the Ruby One-Click Installer, then the installer has setup Windows to automatically recognize your Ruby scripts as executables. Just type the name of the script to run it. $ hello-world.rb
Hello world


If this does not work, or if you installed Ruby in some other way, follow these steps.

2. Run the standard Windows "Command Prompt", cmd.
3. At the command prompt (i.e. shell prompt), run the following Windows commands. When you run ftype, change the command-line arguments to correctly point to where you installed the ruby.exe executable on your computer.
$assoc .rb=RubyScript .rb=RubyScript$ ftype RubyScript="c:\ruby\bin\ruby.exe" "%1" %*
RubyScript="c:\ruby\bin\ruby.exe" "%1" %*


For more help with these commands, run "help assoc" and "help ftype".

# Basic Ruby - Strings

Like Python, Java, and the .NET Framework, Ruby has a built-in String class.

## String literals

One way to create a String is to use single or double quotes inside a Ruby program to create what is called a string literal. We've already done this with our "hello world" program. A quick update to our code shows the use of both single and double quotes.

puts 'Hello world'
puts "Hello world"


Being able to use either single or double quotes is similar to Perl, but different from languages such as C and Java, which use double quotes for string literals and single quotes for single characters.

So what difference is there between single quotes and double quotes in Ruby? In the above code, there's no difference. However, consider the following code:

puts "Betty's pie shop"
puts 'Betty\'s pie shop'


Because "Betty's" contains an apostrophe, which is the same character as the single quote, in the second line we need to use a backslash to escape the apostrophe so that Ruby understands that the apostrophe is in the string literal instead of marking the end of the string literal. The backslash followed by the single quote is called an escape sequence.

## Single quotes

Single quotes only support two escape sequences.

• \' – single quote
• \\ – single backslash

Except for these two escape sequences, everything else between single quotes is treated literally.

## Double quotes

Double quotes allow for many more escape sequences than single quotes. They also allow you to embed variables or Ruby code inside of a string literal – this is commonly referred to as interpolation.

puts "Enter name"
name = gets.chomp


### Escape sequences

Below are some of the more common escape sequences that can appear inside of double quotes.

Try out this example code to better understand escape sequences.

puts "Hello\t\tworld"

puts "Hello\b\b\b\b\bGoodbye world"

puts "Hello\rStart over world"

puts "1. Hello\n2. World"


The result:

Say
hello
to
the
world


## print

In contrast, Ruby's print function only prints out a newline if you specify one. For example, try out the following code. We include a newline at the end of print's argument list so that the shell prompt appears on a new line, after the text.

 print "Say", "hello", "to", "the", "world", "\n"


The result:

$hello-world.rb Sayhellototheworld  The following code produces the same output, with all the words run together. print "Say" print "hello" print "to" print "the" print "world" print "\n"  ## See also String literals # Basic Ruby - Alternate quotes In Ruby, there's more than one way to quote a string literal. Much of this will look familiar to Perl programmers. ## Alternate single quotes Let's say we are using single quotes to print out the following path. puts 'c:\bus schedules\napolean\the portland bus schedule.txt'  The single quotes keep the \b, \n, and \t from being treated as escape sequences (the same cannot be said for wikibooks' syntax highlighting). But consider the following string literal. puts 'c:\napolean\'s bus schedules\tomorrow\'s bus schedule.txt'  Escaping the apostrophes makes the code less readable and makes it less obvious what will print out. Luckily, in Ruby, there's a better way. You can use the %q operator to apply single-quoting rules, but choose your own delimiter to mark the beginning and end of the string literal. puts %q!c:\napolean's documents\tomorrow's bus schedule.txt! puts %q/c:\napolean's documents\tomorrow's bus schedule.txt/ puts %q^c:\napolean's documents\tomorrow's bus schedule.txt^ puts %q(c:\napolean's documents\tomorrow's bus schedule.txt) puts %q{c:\napolean's documents\tomorrow's bus schedule.txt} puts %q<c:\napolean's documents\tomorrow's bus schedule.txt>  Each line will print out the same text – "c:\napolean's documents\tomorrow's bus schedule.txt". You can use any punctuation you want as a delimiter, not just the ones listed in the example. Of course, if your chosen delimiter appears inside of the string literal, then you need to escape it. puts %q#c:\napolean's documents\tomorrow's \#9 bus schedule.txt#  If you use matching braces to delimit the text, however, you can nest braces, without escaping them. puts %q(c:\napolean's documents\the (bus) schedule.txt) puts %q{c:\napolean's documents\the {bus} schedule.txt} puts %q<c:\napolean's documents\the <bus> schedule.txt>  ## Alternate double quotes The %Q operator (notice the case of Q in %Q) allows you to create a string literal using double-quoting rules, but without using the double quote as a delimiter. It works much the same as the %q operator. print %Q^Say:\tHello world\n\tHello world\n^ print %Q(Say:\tHello world\n\tHello world\n)  Just like double quotes, you can interpolate Ruby code inside of these string literals. name = 'Charlie Brown' puts %Q!Say "Hello," #{name}.! puts %Q/What is "4 plus 5"? Answer: #{4+5}/  # Basic Ruby - Here documents For creating multiple-line strings, Ruby supports here documents (heredocs), a feature that originated in the Bourne shell and is also available in Perl and PHP. ## Here documents To construct a here document, the << operator is followed by an identifier that marks the end of the here document. The end mark is called the terminator. The lines of text prior to the terminator are joined together, including the newlines and any other whitespace. puts <<GROCERY_LIST Grocery list ------------ 1. Salad mix. 2. Strawberries.* 3. Cereal. 4. Milk.* * Organic GROCERY_LIST  The result: $ grocery-list.rb
Grocery list
------------
2. Strawberries.*
3. Cereal.
4. Milk.*

* Organic


If we pass the puts function multiple arguments, the string literal created from the here document is inserted into the argument list wherever the << operator appears. In the code below, the here-document (containing the four grocery items and a blank line) is passed in as the third argument. We get the same output as above.

puts 'Grocery list', '------------', <<GROCERY_LIST, '* Organic'
2. Strawberries.*
3. Cereal.
4. Milk.*

GROCERY_LIST


You can also have multiple here documents in an argument list. We added a blank line at the end of each here document to make the output more readable.

puts 'Produce', '-------', <<PRODUCE, 'Dairy', '-----', <<DAIRY, '* Organic'
1. Strawberries*
2. Blueberries

PRODUCE
1. Yogurt
2. Milk*
3. Cottage Cheese

DAIRY


The result:

$grocery-list.rb Produce ------- 1. Strawberries* 2. Blueberries Dairy ----- 1. Yogurt 2. Milk* 3. Cottage Cheese * Organic  We have been using the puts function in our examples, but you can pass here documents to any function that accepts Strings. ## Indenting If you indent the lines inside the here document, the leading whitespace is preserved. However, there must not be any leading whitespace before the terminator. puts 'Grocery list', '------------', <<GROCERY_LIST 1. Salad mix. 2. Strawberries. 3. Cereal. 4. Milk. GROCERY_LIST  The result: $ grocery-list.rb
Grocery list
------------
2. Strawberries.
3. Cereal.
4. Milk.


If, for readability, you want to also indent the terminator, use the <<- operator.

puts 'Grocery list', '------------', <<-GROCERY_LIST
2. Strawberries.
3. Cereal.
4. Milk.
GROCERY_LIST


Note, however, that the whitespace before each line of text within the here document is still preserved.

$grocery-list.rb Grocery list ------------ 1. Salad mix. 2. Strawberries. 3. Cereal. 4. Milk.  ## Quoting rules You may wonder whether here documents follow single-quoting or double-quoting rules. If there are no quotes around the identifier, like in our examples so far, then the body of the here document follows double-quoting rules. name = 'Charlie Brown' puts <<QUIZ Student: #{name} 1.\tQuestion: What is 4+5? \tAnswer: The sum of 4 and 5 is #{4+5} QUIZ  The result: $ quiz.rb
Student: Charlie Brown

1.	Question: What is 4+5?
Answer: The sum of 4 and 5 is 9


Double-quoting rules are also followed if you put double quotes around the identifier. However, do not put double quotes around the terminator.

puts <<"QUIZ"
Student: #{name}

1.\tQuestion: What is 4+5?
\tAnswer: The sum of 4 and 5 is #{4+5}
QUIZ


To create a here document that follows single-quoting rules, place single quotes around the identifier.

puts <<'BUS_SCHEDULES'
c:\napolean's documents\tomorrow's bus schedule.txt
c:\bus schedules\the #9 bus schedule.txt
BUS_SCHEDULES


The result:

$bus-schedules.rb c:\napolean's documents\tomorrow's bus schedule.txt c:\new documents\sam spade's bus schedule.txt c:\bus schedules\the #9 bus schedule.txt  # Basic Ruby - Introduction to objects Like Smalltalk, Ruby is a pure object-oriented language — everything is an object. In contrast, languages such as C++ and Java are hybrid languages that divide the world between objects and primitive types. The hybrid approach results in better performance for some applications, but the pure object-oriented approach is more consistent and simpler to use. ## What is an object? Using Smalltalk terminology, an object can do exactly three things. 1. Hold state, including references to other objects. 2. Receive a message, from both itself and other objects. 3. In the course of processing a message, send messages, both to itself and to other objects. If you don't come from Smalltalk background, it might make more sense to rephrase these rules as follows: 1. An object can contain data, including references to other objects. 2. An object can contain methods, which are functions that have special access to the object's data. 3. An object's methods can call/run other methods/functions. ## Variables and objects Let's fire up irb to get a better understanding of objects. $ irb --simple-prompt
>> comedian = "Stephen Colbert"
=> "Stephen Colbert"


In the first line, we created a String object containing the text "Stephen Colbert". We also told Ruby to use the variable comedian to refer to this object.

Next, we tell Ruby to also use the variable favorite_comedian to refer to the same String object.

>> favorite_comedian = comedian
=> "Stephen Colbert"


Now, we have two variables that we can use to refer to the same String object — comedian and favorite_comedian. Since they both refer to the same object, if the object changes (as we'll see below), the change will show up when using either variable.

## Methods

In Ruby, methods that end with an exclamation mark (also called a "bang") modify the object. For example, the method upcase! changes the letters of a String to uppercase.

>> comedian.upcase!
=> "STEPHEN COLBERT"


Since both of the variables comedian and favorite_comedian point to the same String object, we can see the new, uppercase text using either variable.

>> comedian
=> "STEPHEN COLBERT"
>> favorite_comedian
=> "STEPHEN COLBERT"


Methods that do not end in an exclamation point return data, but do not modify the object. For example, downcase! modifies a String object by making all of the letters lowercase. However, downcase returns a lowercase copy of the String, but the original string remains the same.

>> comedian.downcase
=> "stephen colbert"
>> comedian
=> "STEPHEN COLBERT"


Since the original object still contains the text "STEPHEN COLBERT", you might wonder where the new String object, with the lowercase text, went to. Well, after irb printed out its contents, it can no longer be accessed since we did not assign a variable to keep track of it. It's essentially gone, and Ruby will dispose of it.

## Reassigning a variable

But what if your favorite comedian is not Stephen Colbert? Let's point favorite_comedian to a new object.

>> favorite_comedian = "Jon Stewart"
=> "Jon Stewart"


Now, each variable points to a different object.

Let's say that we change our mind again. Now, our favorite comedian is Ellen DeGeneres.

>> favorite_comedian = "Ellen DeGeneres"
=> "Ellen DeGeneres"


Now, no variable points to the "Jon Stewart" String object any longer. Hence, Ruby will dispose of it.

# Basic Ruby - Ruby basics

As with the rest of this tutorial, we assume some basic familiarity with programming language concepts (i.e. if statement, while loops) and also some basic understanding of object-oriented programming.

We'll deal with variables in much more depth when we talk about classes and objects. For now, let's just say your basic local variable names should start with either a lower case letter or an underscore, and should contain upper or lower case letters, numbers, and underscore characters. Global variables start with a $. # Program flow Ruby includes a pretty standard set of looping and branching constructs: if, while and case For example, here's if in action: a = 10 * rand if a < 5 puts "#{a} less than 5" elsif a > 7 puts "#{a} greater than 7" else puts "Cheese sandwich!" end  [As in other languages, the rand function generates a random number between 0 and 1] There will be plenty more time to discuss conditional statements in later chapters. The above example should be pretty clear. Ruby also includes a negated form of if called unless which goes something like unless a > 5 puts "a is less than or equal to 5" else puts "a is greater than 5" end  Generally speaking, Ruby keeps an if statement straight as long as the conditional (if ...) and the associated code block are on separate lines. If you have to smash everything together on one line, you'll need to place the then keyword after the conditional if a < 5 then puts "#{a} less than 5" end if a < 5 then puts "#{a} less than 5" else puts "#{a} greater than 5" end  Note that the if statement is also an expression; its value is the last line of the block executed. Therefore, the line above could also have been written as puts(if a < 5 then "#{a} less than 5" else "#{a} greater than 5" end)  Ruby has also adopted the syntax from Perl where if and unless statements can be used as conditional modifiers after a statement. For example puts "#{a} less than 5" if a < 5 puts "Cheese sandwich" unless a == 4  while behaves as it does in other languages -- the code block that follows is run zero or more times, as long as the conditional is true while a > 5 a = 10*rand end  And like if, there is also a negated version of while called until which runs the code block until the condition is true. Finally there is the case statement which we'll just include here with a brief example. case is actually a very powerful super version of the if ... elsif... system a = (10*rand).round #a = rand(11) would do the same case a when 0..5 puts "#{a}: Low" when 6 puts "#{a}: Six" else puts "#{a}: Cheese toast!" end  There are some other interesting things going on in this example, but here the case statement is the center of attention. # Writing functions In keeping with Ruby's all-object-oriented-all-the-time design, functions are typically referred to as methods. No difference. We'll cover methods in much more detail when we get to objects and classes. For now, basic method writing looks something like this (save the following code in a file called func1.rb): # Demonstrate a method with func1.rb def my_function( a ) puts "Hello, #{a}" return a.length end len = my_function( "Giraffe" ) puts "My secret word is #{len} characters long"  now run the script: $ func1.rb
Hello, Giraffe
My secret word is 7 characters long


Methods are defined with the def keyword, followed by the function name. As with variables, local and class methods should start with a lower case letter.

In this example, the function takes one argument (a) and returns a value. Note that the input arguments aren't typed (i.e. a need not be a string) ... this allows for great flexibility but can also cause a lot of trouble. The function also returns a single value with the return keyword. Technically this isn't necessary -- the value of the last line executed in the function is used as the return value -- but more often than not using return explicitly makes things clearer.

As with other languages, Ruby supports both default values for arguments and variable-length argument lists, both of which will be covered in due time. There's also support for code blocks, as discussed below.

# Blocks

One very important concept in Ruby is the code block. It's actually not a particularly revolutionary concept -- any time you've written if ... { ... } in C or Perl you've defined a code block, but in Ruby a code block has some hidden secret powers...

Code blocks in Ruby are defined either with the keywords do..end or the curly brackets {..}

do
print "I like "
print "code blocks!"
end

{
print "Me too!"
}


One very powerful usage of code blocks is that methods can take one as a parameter and execute it along the way.

[ed note: the Pragmatic Programmers actually want to point out that it's not very useful to describe it this way. Instead, the block of code behaves like a 'partner' to which the function occasionally hands over control]

The concept can be hard to get the first time it's explained to you. Here's an example:

$irb --simple-prompt >> 3.times { puts "Hi!" } Hi! Hi! Hi! => 3  Surprise! You always thought 3 was just a number, but it's actually an object (of type Fixnum) As it's an object, it has a member function times which takes a block as a parameter. The function runs the block 3 times. Blocks can actually receive parameters, using a special notation |..|. In this case, a quick check of the documentation for times shows it will pass a single parameter into the block, indicating which loop it's on: $ irb --simple-prompt
>> 4.times { |x| puts "Loop number #{x}" }
Loop number 0
Loop number 1
Loop number 2
Loop number 3
=> 4


The times function passes a number into the block. The block gets that number in the variable x (as set by the |x|), then prints out the result.

Functions interact with blocks through the yield. Every time the function invokes yield control passes to the block. It only comes back to the function when the block finishes. Here's a simple example:

# Script block2.rb

def simpleFunction
yield
yield
end

simpleFunction { puts "Hello!" }

$block2.rb Hello! Hello!  The simpleFunction simply yields to the block twice -- so the block is run twice and we get two times the output. Here's an example where the function passes a parameter to the block: # Script block1.rb def animals yield "Tiger" yield "Giraffe" end animals { |x| puts "Hello, #{x}" }  $ block1.rb
Hello, Tiger
Hello, Giraffe


It might take a couple of reads through to figure out what's going on here. We've defined the function "animals" -- it expects a code block. When executed, the function calls the code block twice, first with the parameter "Tiger" then again with the parameter "Giraffe". In this example, we've written a simple code block which just prints out a greeting to the animals. We could write a different block, for example:

animals { |x| puts "It's #{x.length} characters long!" }


which would give:

It's 5 characters long!
It's 7 characters long!


Two completely different results from running the same function with two different blocks.

There are many powerful uses of blocks. One of the first you'll come across is the each function for arrays -- it runs a code block once for each element in the array -- it's great for iterating over lists.

# Ruby is really, really object-oriented

Ruby is very object oriented. Everything is an object -- even things you might consider constants. This also means that the vast majority of what you might consider "standard functions" aren't floating around in some library somewhere, but are instead methods of a given variable.

Here's one example we've already seen:

3.times { puts "Hi!" }


Even though 3 might seem like just a constant number, it's in fact an instance of the class Fixnum (which inherits from the class Numeric which inherits from the class Object). The method times comes from Fixnum and does just what it claims to do.

Here are some other examples

$irb --simple-prompt >> 3.abs => 3 >> -3.abs => 3 >> "giraffe".length => 7 >> a = "giraffe" => "giraffe" >> a.reverse => "effarig"  There will be lots of time to consider how object-oriented design filters through Ruby in the coming chapters. # Basic Ruby - Data types ## Ruby Data Types As mentioned in the previous chapter, everything in Ruby is an object. Everything has a class. Don't believe me? Try running this bit of code: h = {"hash?" => "yep, it\'s a hash!", "the answer to everything" => 42, :linux => "fun for coders."} puts "Stringy string McString!".class puts 1.class puts nil.class puts h.class puts :symbol.class  displays String Fixnum NilClass Hash Symbol  See? Everything is an object. Every object has a method called class that returns that object's class. You can call methods on pretty much anything. Earlier you saw an example of this in the form of 3.times. (Technically when you call a method you're sending a message to the object, but I'll leave the significance of that for later.) Something that makes this extreme object oriented-ness very fun for me is the fact that all classes are open, meaning you can add variables and methods to a class at any time during the execution of your code. This, however, is a discussion of datatypes. ## Constants We'll start off with constants because they're simple. Two things to remember about constants: 1. Constants start with capital letters. Constant is a constant. constant is not a constant. 2. You can change the values of constants, but Ruby will give you a warning. (Silly, I know... but what can you do?) Congrats. Now you're an expert on Ruby constants. ## Symbols So did you notice something weird about that first code listing? "What the heck was that colon thingy about?" Well, it just so happens that Ruby's object oriented ways have a cost: lots of objects make for slow code. Every time you type a string, Ruby makes a new object. Regardless of whether two strings are identical, Ruby treats every instance as a new object. You could have "live long and prosper" in your code once and then again later on and Ruby wouldn't even realize that they're pretty much the same thing. Here is a sample irb session which demonstrates this fact: irb> "live long and prosper".object_id => -507772268 irb> "live long and prosper".object_id => -507776538 Notice that the object ID returned by irb Ruby is different even for the same two strings. To get around this memory hoggishness, Ruby has provided "symbols." Symbols are lightweight objects best used for comparisons and internal logic. If the user doesn't ever see it, why not use a symbol rather than a string? Your code will thank you for it. Let us try running the above code using symbols instead of strings: irb> :my_symbol.object_id => 150808 irb> :my_symbol.object_id => 150808 Symbols are denoted by the colon sitting out in front of them, like so: :symbol_name ## Hashes Hashes are like dictionaries, in a sense. You have a key, a reference, and you look it up to find the associated object, the definition. The best way to illustrate this, I think, is with a quick demonstration: 1. hash = { :leia => "Princess from Alderaan", :han => "Rebel without a cause", :luke => "Farmboy turned Jedi"}  2. puts hash[:leia]  3. puts hash[:han]  4. puts hash[:luke]  displays Princess from Alderaan Rebel without a cause Farmboy turned Jedi  I could have also written this like so: 1. hash = { :leia => "Princess from Alderaan", :han => "Rebel without a cause", :luke => "Farmboy turned Jedi"}  2. hash.each do |key, value|  3.  puts value  4. end  This code cycles through each element in the hash, putting the key in the key variable and the value in the value variable, which is then displayed Princess of Alderaan Rebel without a cause Farmboy turned Jedi  I could have been more verbose about defining my hash; I could have written it like this: 1. hash = Hash.[](:leia => "Princess from Alderaan", :han => "Rebel without a cause", :luke => "Farmboy turned Jedi")  2. hash.each do |key, value|  3.  puts value  4. end  If I felt like offing Luke, I could do something like this: 1. hash.delete(:luke)  Now Luke's no longer in the hash. Or lets say I just had a vendetta against farmboys in general. I could do this: 1. hash.delete_if {|key, value| value.downcase.match("farmboy")}  This iterates through each key-value pair and deletes it, but only if the block of code following it returns true. In the block I made the value lowercase (in case the farmboys decided to start doing stuff like "FaRmBoY!1!") and then checked to see if "farmboy" matched anything in its contents. I could have used a regular expression, but that's another story entirely. I could add Lando into the mix by assigning a new value to the hash: 1. hash[:lando] = "Dashing and debonair city administrator."  I can measure the hash with hash.length. I can look at only keys with the hash.inspect method, which returns the hash's keys as an Array. Speaking of which... ## Arrays Arrays are a lot like Hashes, except that the keys are always consecutive numbers, and always starts at 0. In an Array with five items, the last element would be found at array[4] and the first element would be found at array[0]. In addition, all the methods that you just learned with Hashes can also be applied to Arrays. Here are two ways to create an Array: 1. array1 = ["hello", "this", "is", "an", "array!"]  2. array2 = []  3. array2 << "This" # index 0  4. array2 << "is" # index 1  5. array2 << "also" # index 2  6. array2 << "an" # index 3  7. array2 << "array!" # index 4  As you may have guessed, the << operator pushes values onto the end of an Array. If I were to write puts array2[4] after declaring those two Arrays the output would be array!. Of course, if I felt like simultaneously getting array! and deleting it from the array, I could just Array.pop it off. The Array.pop method returns the last element in an array and then immediately removes it from that array: 1. string = array2.pop  Then string would hold array! and array2 would be an element shorter. If I kept doing this, array2 wouldn't hold any elements. I can check for this condition by calling the Array.empty? method. For example, the following bit of code moves all the elements from one Array to another: 1. array1 << array2.pop until array2.empty?  Here's something that really excites me: Arrays can be subtracted from, and added to, each other. I can't vouch for every language that's out there, but I know that Java, C++, C# and perl would all look at me like I was a crazy person if I tried to execute the following bit of code: 1. array3 = array1 - array2  2. array4 = array1 + array2  After that code is evaluated, all of the following are true: • array3 contains all of the elements that array1 did, except the ones that were also in array2. • All the elements of array1, minus the elements of array2, are now contained within array3. • array4 now contains all the elements of both array1 and array2. You may search for a particular value in variable array1 with the Array.include? method: array1.include?("Is this in here?") If you just wanted to turn the whole Array into a String, you could: 1. string = array2.join(" ")  If array2 had the value that we declared in the last example, then string's value would be This is also an array!  We could have called the Array.join method without any arguments: 1. string = array2.join  string's value would now be Thisisalsoanarray!  ## Strings I would recommend reading the chapters on strings and alternate quotes now if you haven't already. This chapter is going to cover some pretty spiffy things with Strings and just assume that you already know the information in these two chapters. In Ruby, there are some pretty cool built-in functions where Strings are concerned. For example, you can multiply them: "Danger, Will Robinson!" * 5  yields Danger, Will Robinson!Danger, Will Robinson!Danger, Will Robinson!Danger, Will Robinson!Danger, Will Robinson!  Strings may also be compared: "a" < "b"  yields true  The preceding evaluation is actually comparing the ASCII values of the characters. But what, I hear you ask, is the ASCII value of an given character? With ruby versions prior to 1.9 you can find the ASCII value of a character with: 1. puts ?A  However, With Ruby version 1.9 or later that no longer works. Instead, you can try the String.ord method: 1. puts "A".ord  Either approach will display 65  which is the ASCII value of A. Simply replace A with whichever character you wish to inquire about. To perform the opposite conversion (from 65 to A, for instance), use the Integer.chr method: 1. puts 65.chr  displays A  Concatenation works the same as most other languages: putting a + character between two Strings will yield a new String whose value is the same as the others, one after another: 1. "Hi, this is " + "a concatenated string!"  yields Hi, this is a concatenated string!  For handling pesky String variables without using the concatenate operator, you can use interpolation. In the following chunk of code string1, string2, and string3 are identical: 1. thing1 = "Red fish, "  2. thing2 = "blue fish."  3. string1 = thing1 + thing2 + " And so on and so forth."  4. string2 = "#{thing1 + thing2} And so on and so forth."  5. string3 = "#{thing1}#{thing2} And so on and so forth."  If you need to iterate through (that is, step through each of) the letters in a String object, you can use the String.scan method: 1. thing = "Red fish"  2. thing.scan(/./) {|letter| puts letter}  Displays each letter in thing (puts automatically adds a newline after each call): R e d f i s h  But what's with that weird "/./" thing in the parameter? That, my friend, is called a regular expression. They're helpful little buggers, quite powerful, but outside the scope of this discussion. All you need to know for now is that /./ is "regex" speak for "any one character." If we had used /../ then ruby would have iterated over each group of two characters, and missed the last one since there's an odd number of characters! Another use for regular expressions can be found with the =~ operator. You can check to see if a String matches a regular expression using the match operator, =~: 1. puts "Yeah, there's a number in this one." if "C3-P0, human-cyborg relations" =~ /[0-9]/  displays Yeah, there's a number in this one.  The String.match method works much the same way, except it can accept a String as a parameter as well. This is helpful if you're getting regular expressions from a source outside the code. Here's what it looks like in action: 1. puts "Yep, they mentioned Jabba in this one." if "Jabba the Hutt".match("Jabba")  Alright, that's enough about regular expressions. Even though you can use regular expressions with the next two examples, we'll just use regular old Strings. Lets pretend you work at the Ministry of Truth and you need to replace a word in a String with another word. You can try something like: 1. string1 = "2 + 2 = 4"  2. string2 = string1.sub("4", "5")  Now string2 contains 2 + 2 = 5. But what if the String contains lots of lies like the one you just corrected? String.sub only replaces the first occurrence of a word! I guess you could iterate through the String using String.match method and a while loop, but there's a much more efficient way to accomplish this: 1. winston = %q{ Down with Big Brother!  2.  Down with Big Brother!  3.  Down with Big Brother!  4.  Down with Big Brother!  5.  Down with Big Brother!}  6. winston.gsub("Down with", "Long live")  Big Brother would be so proud! String.gsub is the "global substitute" function. Every occurrence of "Down with" has now been replaced with "Long live" so now winston is only proclaiming its love for Big Brother, not its disdain thereof. On that happy note, lets move on to Integers and Floats. If you want to learn more about methods in the String class, look at the end of this chapter for a quick reference table. ## Numbers (Integers and Floats) You can skip this paragraph if you know all the standard number operators. For those who don't, here's a crash course. + adds two numbers together. - subtracts them. / divides. * multiplies. % returns the remainder of two divided numbers. Alright, integers are numbers with no decimal place. Floats are numbers with decimal places. 10 / 3 yields 3 because dividing two integers yields an integer. Since integers have no decimal places all you get is 3. If you tried 10.0 / 3 you would get 3.33333... If you have even one float in the mix you get a float back. Capice? Alright, let's get down to the fun part. Everything in Ruby is an object, let me reiterate. That means that pretty much everything has at least one method. Integers and floats are no exception. First I'll show you some integer methods. Here we have the venerable times method. Use it whenever you want to do something more than once. Examples: puts "I will now count to 99..." 100.times {|number| puts number} 5.times {puts "Guess what?"} puts "I'm done!"  This will print out the numbers 0 through 99, print out Guess what? five times, then say I'm done! It's basically a simplified for loop. It's a little slower than a for loop by a few hundredths of a second or so; keep that in mind if you're ever writing Ruby code for NASA. ;-) Alright, we're nearly done, six more methods to go. Here are three of them: # First a visit from The Count... 1.upto(10) {|number| puts "#{number} Ruby loops, ah-ah-ah!"} # Then a quick stop at NASA... puts "T-minus..." 10.downto(1) {|x| puts x} puts "Blast-off!" # Finally we'll settle down with an obscure Schoolhouse Rock video... 5.step(50, 5) {|x| puts x}  Alright, that should make sense. In case it didn't, upto counts up from the number it's called from to the number passed in its parameter. downto does the same, except it counts down instead of up. Finally, step counts from the number its called from to the first number in its parameters by the second number in its parameters. So 5.step(25, 5) {|x| puts x} would output every multiple of five starting with five and ending at twenty-five. Time for the last three: string1 = 451.to_s string2 = 98.6.to_s int = 4.5.to_i float = 5.to_f  to_s converts floats and integers to strings. to_i converts floats to integers. to_f converts integers to floats. There you have it. All the data types of Ruby in a nutshell. Now here's that quick reference table for string methods I promised you. ## Additional String Methods # Outputs 1585761545 "Mary J".hash # Outputs "concatenate" "concat" + "enate" # Outputs "Washington" "washington".capitalize # Outputs "uppercase" "UPPERCASE".downcase # Outputs "LOWERCASE" "lowercase".upcase # Outputs "Henry VII" "Henry VIII".chop # Outputs "rorriM" "Mirror".reverse # Outputs 810 "All Fears".sum # Outputs cRaZyWaTeRs "CrAzYwAtErS".swapcase # Outputs "Nexu" (next advances the word up one value, as if it were a number.) "Next".next # After this, nxt == "Neyn" (to help you understand the trippiness of next) nxt = "Next" 20.times {nxt = nxt.next}  # Basic Ruby - Writing methods ## Defining Methods Methods are defined using the def keyword and ended with the end keyword. Some programmers find the Methods defined in Ruby very similar to those in Python.  def myMethod end  To define a method that takes in a value, you can put the local variable name in parentheses after the method definition. The variable used can only be accessed from inside the method scope.  def myMethod(msg) puts msg end  If multiple variables need to be used in the method, they can be separated with a comma.  def myMethod(msg, person) puts "Hi, my name is " + person + ". Some information about myself: " + msg end  Any object can be passed through using methods.  def myMethod(myObject) if(myObject.is_a?(Integer)) puts "Your Object is an Integer" end #Check to see if it defined as an Object that we created #You will learn how to define Objects in a later section if(myObject.is_a?(MyObject)) puts "Your Object is a MyObject" end end  The return keyword can be used to specify that you will be returning a value from the method defined.  def myMethod return "Hello" end  It is also worth noting that ruby will return the last expression evaluated, so this is functionally equivalent to the previous method.  def myMethod "Hello" end  Some of the Basic Operators can be overridden using the def keyword and the operator that you wish to override.  def ==(oVal) if oVal.is_a?(Integer) #@value is a variable defined in the class where this method is defined #This will be covered in a later section when dealing with Classes if(oVal == @value) return true else return false end end end  # Basic Ruby - Classes and objects ## Ruby Classes As stated before, everything in Ruby is an object. Every object has a class. To find the class of an object, simply call that object's class method. For example, try this:  puts "This is a string".class puts 9.class puts ["this","is","an","array"].class puts {:this => "is", :a => "hash"}.class puts :symbol.class  Anyhow, you should already know this. What you don't know however, is how to make your own classes and extend Ruby's classes. ## Creating Instances of a Class An instance of a class is an object that has that class. For example, "chocolate" is an instance of the String class. You already know that you can create strings, arrays, hashes, numbers, and other built-in types by simply using quotes, brackets, curly braces, etc., but you can also create them via the new method. For example, my_string = "" is the same as my_string = String.new. Every class has a new method: arrays, hashes, integers, whatever. When you create your own classes, you'll use the new method to create instances. ## Creating Classes Classes represent a type of an object, such as a book, a whale, a grape, or chocolate. Everybody likes chocolate, so let's make a chocolate class:  class Chocolate def eat puts "That tasted great!" end end  Let's take a look at this. Classes are created via the class keyword. After that comes the name of the class. All class names must start with a Capital Letter. By convention, we use CamelCase for class name. So we would create classes like PieceOfChocolate, but not like Piece_of_Chocolate. The next section defines a class method. A class method is a method that is defined for a particular class. For example, the String class has the length method:  # outputs "5" puts "hello".length  To call the eat method of an instance of the Chocolate class, we would use this code:  my_chocolate = Chocolate.new my_chocolate.eat # outputs "That tasted great!"  You can also call a method by using send  "hello".send(:length) # outputs "5" my_chocolate.send(:eat) # outputs "That tasted great!"  However, using send is rare unless you need to create a dynamic behavior, as we do not need to specify the name of the method as a literal - it can be a variable. ## Self Inside a method of a class, the pseudo-variable self (a pseudo-variable is one that cannot be changed) refers to the current instance. For example:  class Integer def more return self + 1 end end 3.more # -> 4 7.more # -> 8  ## Class Methods You can also create methods that are called on a class rather than an instance. For example:  class Strawberry def Strawberry.color return "red" end def self.size return "kinda small" end class << self def shape return "strawberry-ish" end end end Strawberry.color # -> "red" Strawberry.size # -> "kinda small" Strawberry.shape # -> "strawberry-ish"  Note the three different constructions: ClassName.method_name and self.method_name are essentially the same - outside of a method definition in a class block, self refers to the class itself. The latter is preferred, as it makes changing the name of the class much easier. The last construction, class << self, puts us in the context of the class's "meta-class" (sometimes called the "eigenclass"). The meta-class is a special class that the class itself belongs to. However, at this point, you don't need to worry about it. All this construct does is allow us to define methods without the self. prefix. # Basic Ruby - Exceptions There is no exceptions chapter at present. Instead, here is a link to a chapter about exceptions from Yukihiro Matsumoto's book, Programming Ruby: The Pragmatic Programmer's Guide [1] More detail and simpler examples about exceptions, by Satish Talim, maybe found in a tutorial at RubyLearning.com [2] # Syntax - Lexicology ## Identifiers An identifier is a name used to identify a variable, method, or class. As with most languages, valid identifiers consist of alphanumeric characters (A-Za-z0-9) and underscores (_), but may not begin with a digit (0-9). Additionally, identifiers that are method names may end with a question mark (?), exclamation point (!), or equals sign (=). There are no arbitrary restrictions to the length of an identifier (i.e. it may be as long as you like, limited only by your computer's memory). Finally, there are reserved words which may not be used as identifiers. Examples: foobar ruby_is_simple  ## Comments Line comments run from a bare '#' character to the end of the line. Code commenting and documentation is best implemented with Ruby Embedded Documentation. http://www.ruby-doc.org/docs/ProgrammingRuby/html/rdtool.html Examples: # this line does nothing; print "Hello" # this line prints "Hello"  ## Embedded Documentation Example: =begin Everything between a line beginning with =begin' down to one beginning with =end' will be skipped by the interpreter. These reserved words must begin in column 1. =end  ## Reserved Words The following words are reserved in Ruby: __FILE__ and def end in or self unless __LINE__ begin defined? ensure module redo super until BEGIN break do false next rescue then when END case else for nil retry true while alias class elsif if not return undef yield  You can find some examples of using them here. ## Expressions Example: true (1 + 2) * 3 foo() if test then okay else not_good end  All variables, literals, control structures, etcetera are expressions. Using these together is called a program. You can divide expressions with newlines or semicolons (;) — however, a newline with a preceding backslash (\) is continued to the following line. Since in Ruby control structures are expressions as well, one can do the following:  foo = case 1 when 1 true else false end  The above equivalent in a language such as C would generate a syntax error since control structures are not expressions in the C language. # Syntax - Variables and Constants A variable in Ruby can be distinguished by the characters at the start of its name. There's no restriction to the length of a variable's name (with the exception of the heap size). ## Local Variables Example: foobar  A variable whose name begins with a lowercase letter (a-z) or underscore (_) is a local variable or method invocation. A local variable is only accessible from within the block of its initialization. For example: i0 = 1 loop { i1 = 2 puts defined?(i0) # true; "i0" was initialized in the ascendant block puts defined?(i1) # true; "i1" was initialized in this block break } puts defined?(i0) # true; "i0 was initialized in this block puts defined?(i1) # false; "i1" was initialized in the loop  ## Instance Variables Example: @foobar  A variable whose name begins with '@' is an instance variable of self. An instance variable belongs to the object itself. Uninitialized instance variables have a value of nil. ## Class Variables A class variable is shared by all instances of a class and begins with '@@'. Example: @@foobar  An important note is that the class variable is shared by all the descendants of the class. Example: class Parent @@foo = "Parent" end class Thing1 < Parent @@foo = "Thing1" end class Thing2 < Parent @@foo = "Thing2" end >>Parent.class_eval("@@foo") =>"Thing2" >>Thing1.class_eval("@@foo") =>"Thing2" >>Thing2.class_eval("@@foo") =>"Thing2" >>Thing2.class_variables =>[] Parent.class_variables =>[:@@foo]   This shows us that all our classes were changing the same variable. Class variables behave like global variables which are visible only in the inheritance tree. Because Ruby resolves variables by looking up the inheritance tree *first*, this can cause problems if two subclasses both add a class variable with the same name. ## Global Variables Example: $foobar


A variable whose name begins with '$' has a global scope; meaning it can be accessed from anywhere within the program during runtime. ## Constants Usage: FOOBAR  A variable whose name begins with an uppercase letter (A-Z) is a constant. A constant can be reassigned a value after its initialization, but doing so will generate a warning. Every class is a constant. Trying to access an uninitialized constant raises the NameError exception. ### How constants are looked up Constants are looked up based on your scope or via the scope resolution operator (i.e. '::'). For example  class A A2 = 'a2' class B def go A2 end end end instance_of_b = A::B.new a2 = A::A2  Another example class Foo BAR = 123 end puts Foo::BAR # => 123  ## Pseudo Variables self Execution context of the current method. nil The sole-instance of the NilClass class. Expresses nothing. true The sole-instance of the TrueClass class. Expresses true. false The sole-instance of the FalseClass class. Expresses false. $1, $2 ...$9

These are contents of capturing groups for regular expression matches. They are local to the current thread and stack frame!

(nil also is considered to be false, and every other value is considered to be true in Ruby.) The value of a pseudo variable cannot be changed. Substitution to a pseudo variable causes an exception to be raised.

## Pre-defined Variables

Name Aliases Description
$! $ERROR_INFO[1] The exception information message set by the last 'raise' (last exception thrown).
$@ $ERROR_POSITION[1] Array of the backtrace of the last exception thrown.
$& $MATCH[1] The string matched by the last successful pattern match in this scope.
$ $PREMATCH[1] The string to the left of the last successful match.
$' $POSTMATCH[1] The string to the right of the last successful match.
$+ $LAST_PAREN_MATCH[1] The last bracket matched by the last successful match.
$1 to $9 The Nth group of the last successful regexp match.
$~ $LAST_MATCH_INFO[1] The information about the last match in the current scope.
$= $IGNORECASE[1] The flag for case insensitive, nil by default (deprecated).
$/ $INPUT_RECORD_SEPARATOR[1], $RS[1] or $-0 The input record separator, newline by default.
$\ $OUTPUT_RECORD_SEPARATOR[1] or $ORS[1] The output record separator for the print and IO#write. Default is nil. $, $OUTPUT_FIELD_SEPARATOR[1] or $OFS[1] The output field separator for the print and Array#join.
$; $FIELD_SEPARATOR[1], $FS[1] or $-F The default separator for String#split.
$. $INPUT_LINE_NUMBER[1] or $NR[1] The current input line number of the last file that was read. $< $DEFAULT_INPUT[1] An object that provides access to the concatenation of the contents of all the files given as command-line arguments, or$stdin (in the case where there are no arguments). Read only.
$FILENAME Current input file from$<. Same as $<.filename. $> $DEFAULT_OUTPUT[1] The destination of output for Kernel.print and Kernel.printf. The default value is$stdout.
$_ $LAST_READ_LINE[1] The last input line of string by gets or readline.
$0 Contains the name of the script being executed. May be assignable. $* ARGV[1] Command line arguments given for the script. Also known as ARGV
$$ PROCESS_ID[1], PID[1] or Process.pid The process number of the Ruby running this script. ? CHILD_STATUS[1] The status of the last executed child process. : LOAD_PATH Load path for scripts and binary modules by load or require. " LOADED_FEATURES or -I The array contains the module names loaded by require. stderr The current standard error output. stdin The current standard input. stdout The current standard output. -d DEBUG The status of the -d switch. Assignable. -K KCODE Character encoding of the source code. -v VERBOSE The verbose flag, which is set by the -v switch. -a True if option -a ("autosplit" mode) is set. Read-only variable. -i If in-place-edit mode is set, this variable holds the extension, otherwise nil. -l True if option -l is set ("line-ending processing" is on). Read-only variable. -p True if option -p is set ("loop" mode is on). Read-only variable. -w True if option -w is set. The use of cryptic two-character ? expressions is a thing that people will frequently complain about, dismissing Ruby as just another perl-ish line-noise language. Keep this chart handy. Note, a lot of these are useful when working with regexp code. Part of the standard library is "English" which defines longer names to replace the two-character variable names to make code more readable. The defined names are also listed in the table. To include these names, just require the English library as follows.[1] Without ‘English’:  \ = ' -- ' "waterbuffalo" =~ /buff/ print ", ',$$, "\n"


With English:

   require "English"

$OUTPUT_FIELD_SEPARATOR = ' -- ' "waterbuffalo" =~ /buff/ print$LOADED_FEATURES, $POSTMATCH,$PID, "\n"


## Pre-defined Constants

Note that there are some pre-defined constants at parse time, as well, namely

 __FILE__ (current file)
__LINE__ (current line)


and

 __dir__ (current directory)


(new in Ruby 2.0)

# Syntax - Literals

## Numerics

123                       # Fixnum
-123                      # Fixnum (signed)
1_123                     # Fixnum (underscore is ignored)
-543                      # Negative Fixnum
123_456_789_123_456_789   # Bignum
123.45                    # Float
1.2e-3                    # Float
0377                      # (Octal) Fixnum
-0b1010                   # (Binary [negated]) Fixnum
0b001_001                 # (Binary) Fixnum
?a                        # ASCII character code for 'a' (97)
?\C-a                     # Control-a (1)
?\M-a                     # Meta-a (225)
?\M-\C-a                  # Meta-Control-a (129)


Note: the meaning of "?x" notation has been changed. In ruby 1.9 this means not an ASCII numeric code but a string i.e. ?a == "a"

## Strings

Examples:

"this is a string"
=> "this is a string"

"three plus three is #{3+3}"
=> "three plus three is 6"

foobar = "blah"
"the value of foobar is #{foobar}"
=> "the value of foobar is blah"

'the value of foobar is #{foobar}'
=> "the value of foobar is \#{foobar}"


A string expression begins and ends with a double or single-quote mark. Double-quoted string expressions are subject to backslash notation and interpolation. A single-quoted string expression isn't; except for \' and \\.

### Backslash Notation

Also called escape characters or escape sequences, they are used to insert special characters in a string.

Example:

"this is a\ntwo line string"
"this string has \"quotes\" in it"

 Escape Sequence Meaning \n newline (0x0a) \s space (0x20) \r carriage return (0x0d) \t tab (0x09) \v vertical tab (0x0b) \f formfeed (0x0c) \b backspace (0x08) \a bell/alert (0x07) \e escape (0x1b) \nnn character with octal value nnn \xnn character with hexadecimal value nn \unnnn Unicode code point U+nnnn (Ruby 1.9 and later) \cx control-x \C-x control-x \M-x meta-x \M-\C-x meta-control-x \x character x itself (\" a single quote, for example)

For characters with decimal values, you can do this:

"" << 197 # add decimal value 304 to a string

or embed them thus:

"#{197.chr}"

### Interpolation

Interpolation allows Ruby code to appear within a string. The result of evaluating that code is inserted into the string:

 "1 + 2 = #{1 + 2}"    #=> "1 + 2 = 3"

 #{expression}


The expression can be just about any Ruby code. Ruby is pretty smart about handling string delimiters that appear in the code and it generally does what you want it to do. The code will have the same side effects as it would outside the string, including any errors:

"the meaning of life is #{1/0}"
=> divided by 0 (ZeroDivisionError)


### The % Notation

There is also a Perl-inspired way to quote strings: by using % (percent character) and specifying a delimiting character, for example:

%{78% of statistics are "made up" on the spot}
=> "78% of statistics are \"made up\" on the spot"


Any single non-alpha-numeric character can be used as the delimiter, %[including these], %?or these?, %~or even these things~. By using this notation, the usual string delimiters " and ' can appear in the string unescaped, but of course the new delimiter you've chosen does need to be escaped. However, if you use %(parentheses), %[square brackets], %{curly brackets} or %<pointy brackets> as delimiters then those same delimiters can appear unescaped in the string as long as they are in balanced pairs:

%(string (syntax) is pretty flexible)
=> "string (syntax) is pretty flexible"


A modifier character can appear after the %, as in %q[], %Q[], %x[] - these determine how the string is interpolated and what type of object is produced:

 Modifier Meaning %q[ ] Non-interpolated String (except for \\ $and$) %Q[ ] Interpolated String (default) %r[ ] Interpolated Regexp (flags can appear after the closing delimiter) %i[ ] Non-interpolated Array of symbols, separated by whitespace (after Ruby 2.0) %I[ ] Interpolated Array of symbols, separated by whitespace (after Ruby 2.0) %w[ ] Non-interpolated Array of words, separated by whitespace %W[ ] Interpolated Array of words, separated by whitespace %x[ ] Interpolated shell command

Here are some more examples:

%Q{one\ntwo\n#{ 1 + 2 }}
=> "one\ntwo\n3"

%q{one\ntwo\n#{ 1 + 2 }}
=> "one\\ntwo\\n#{ 1 + 2 }"

%r/#{name}/i
=> /nemo/i

%w{one two three}
=> ["one", "two", "three"]

%i{one two three} # after Ruby 2.0
=> [:one, :two, :three]

=> "ruby - Copyright (C) 1993-2009 Yukihiro Matsumoto\n"


### "Here document" notation

There is yet another way to make a string, known as a 'here document', where the delimiter itself can be any string:

string = <<END
on the one ton temple bell
a moon-moth, folded into sleep,
sits still.
END


The syntax begins with << and is followed immediately by the delimiter. To end the string, the delimiter appears alone on a line.

There is a slightly nicer way to write a here document which allows the ending delimiter to be indented by whitespace:

string = <<-FIN
on the one-ton temple bell
a moon-moth, folded into sleep
sits still.

--Taniguchi Buson, 18th century; translated by X. J. Kennedy
FIN


To use non-alpha-numeric characters in the delimiter, it can be quoted:

string = <<-"."
Orchid - breathing
incense into
butterfly's wings.

--Matsuo Basho; translated by Lucien Stryk
.


Here documents are interpolated, unless you use single quotes around the delimiter.

The rest of the line after the opening delimiter is not interpreted as part of the string, which means you can do this:

strings = [<<END, "short", "strings"]
a long string
END

=> ["a long string\n", "short", "strings"]


You can even "stack" multiple here documents:

string = [<<ONE, <<TWO, <<THREE]
the first thing
ONE
the second thing
TWO
and the third thing
THREE

=> ["the first thing\n", "the second thing\n", "and the third thing\n"]


## Arrays

An array is a collection of objects indexed by a non-negative integer. You can create an array object by writing Array.new, by writing an optional comma-separated list of values inside square brackets, or if the array will only contain string objects, a space-delimited string preceded by %w.

array_one   = Array.new
array_two   = []              # shorthand for Array.new
array_three = ["a", "b", "c"] # array_three contains "a", "b" and "c"
array_four  = %w[a b c]       # array_four also contains "a", "b" and "c"

array_three[0]                # => "a"
array_three[2]                # => "c"
array_four[0]                 # => "a"
#negative indices are counted back from the end
array_four[-2]                # => "b"
#[start, count] indexing returns an array of count objects beginning at index start
array_four[1,2]               # => ["b", "c"]
#using ranges. The end position is included with two periods but not with three
array_four[0..1]              # => ["a", "b"]
array_four[0...1]             # => ["a"]


The last method, using %w, is in essence shorthand for the String method split when the substrings are separated by whitespace only. In the following example, the first two ways of creating an array of strings are functionally identical while the last two create very different (though both valid) arrays.

 array_one   = %w'apple orange pear'            # => ["apple", "orange", "pear"]
array_two   = 'apple orange pear'.split        # => ["apple", "orange", "pear"]
array_one   == array_two                       # => true

array_three = %w'dog:cat:bird'                 # => ["dog:cat:bird"]
array_four  = 'dog:cat:bird'.split(':')        # => ["dog", "cat", "bird"]
array_three == array_four                      # => false


## Hashes

Hashes are basically the same as arrays, except that a hash not only contains values, but also keys pointing to those values. Each key can occur only once in a hash. A hash object is created by writing Hash.new or by writing an optional list of comma-separated key => value pairs inside curly braces.

hash_one   = Hash.new
hash_two   = {}                             # shorthand for Hash.new
hash_three = {"a" => 1, "b" => 2, "c" => 3} #=> {"a"=>1, "b"=>2, "c"=>3}


Usually Symbols are used for Hash keys (allows for quicker access), so you will see hashes declared like this:

hash_sym   = { :a => 1, :b => 2, :c => 3}   #=> {:b=>2, :c=>3, :a=>1}
hash_sym   = { a: 1, b: 2, c: 3}            #=> {:b=>2, :c=>3, :a=>1}


The latter form was introduced in Ruby 1.9.

### Hash ordering

Note that with 1.8, iterating over hashes will iterate over the key value pairs in a "random" order. Beginning with 1.9, it will iterate over them in the order they were inserted. Note however, that if you reinsert a key (or change an existing key's value), that does not change that keys' order in the iteration.

>> a = {:a => 1, :b => 2, :c => 3}
=> {:a=>1, :b=>2, :c=>3}
>> a.keys # iterate over, show me the keys
=> [:a, :b, :c]
>> a[:b] = 2
> a.keys
=> [:a, :b, :c] # same order
>> a.delete(:b)
>> a[:b] = 4 # re insert now
=> 4
>> a.keys
=> [:a, :c, :b] # different order


## Ranges

A range represents a subset of all possible values of a type, to be more precise, all possible values between a start value and an end value.

This may be:

• All integers between 0 and 5.
• All numbers (including non-integers) between 0 and 1, excluding 1.
• All characters between 't' and 'y'.

In Ruby, these ranges are expressed by:

0..5
0.0...1.0
't'..'y'


Therefore, ranges consist of a start value, an end value, and whether the end value is included or not (in this short syntax, using two . for including and three . for excluding).

A range represents a set of values, not a sequence. Therefore,

5..0


though syntactically correct, produces a range of length zero.

Ranges can only be formed from instances of the same class or subclasses of a common parent, which must be Comparable (implementing <=>).

Ranges are instances of the Range class, and have certain methods, for example, to determine whether a value is inside a range:

r = 0..5
puts r === 4  # => true
puts r === 7  # => false


For detailed information of all Range methods, consult the Range class reference.

Here is a tutorial on their use.

# Syntax - Operators

## Operators

### 1. Assignment

Assignment in Ruby is done using the equal operator "=". This is both for variables and objects, but since strings, floats, and integers are actually objects in Ruby, you're always assigning objects.

Examples:

1.   myvar = 'myvar is now this string'

2.   var = 321

3.   dbconn = Mysql::new('localhost','root','password')


Self assignment

1.   x = 1           #=>1

2.   x += x          #=>2

3.   x -= x          #=>0

4.   x += 4          #=>x was 0 so x= + 4 # x is positive 4

5.   x *= x          #=>16

6.   x **= x         #=>18446744073709551616 # Raise to the power

7.   x /= x          #=>1


A frequent question from C and C++ types is "How do you increment a variable? Where are ++ and -- operators?" In Ruby, one should use x+=1 and x-=1 to increment or decrement a variable.

1.   x = 'a'

2.   x.succ!         #=>"b" : succ! method is defined for String, but not for Integer types


Multiple assignments

Examples:

1.   var1, var2, var3 = 10, 20, 30

2.   puts var1           #=>var1 is now 10

3.   puts var2           #=>var2 is now 20,var3...etc

4. 

5.   myArray=%w(John Michel Fran Doug) # %w() can be used as syntactic sugar to simplify array creation

6.   var1,var2,var3,var4=*myArray

7.   puts var1           #=>John

8.   puts var4           #=>Doug

9. 

10.   names,school=myArray,'St. Whatever'

11.   names               #=>["John", "Michel", "Fran", "Doug"]

12.   school              #=>"St. Whatever"


Conditional assignment

1.   x = find_something() #=>nil

2.   x ||= "default"      #=>"default" : value of x will be replaced with "default", but only if x is nil or false

3.   x ||= "other"        #=>"default" : value of x is not replaced if it already is other than nil or false


Operator ||= is a shorthand form of the expression:

1.  x = x || "default"


Operator ||= can be shorthand for code like:

1.   x = "(some fallback value)" unless respond_to? :x or x


In same way &&= operator works:

1.   x = get_node() #=>nil

2.   x &&= x.next_node #=> nil : x will be set to x.next_node, but only if x is NOT nil or false

3.   x = get_node() #=>Some Node

4.   x &&= x.next_node #=>Next Node


Operator &&= is a shorthand form of the expression:

1.  x = x && x.get_node()


## Scope

In Ruby there's a local scope, a global scope, an instance scope, and a class scope.

### Local Scope

Example:

 var=2
4.times do |x|
puts x=x*var
end
#=>0,2,4,6
puts x #=>undefined local variable or method x' for main:Object (NameError)


This error appears because this x(toplevel) is not the x(local) inside the do..end block the x(local) is a local variable to the block, whereas when trying the puts x(toplevel) we're calling a x variable that is in the top level scope, and since there's not one, Ruby protests.

### Global scope

 4.times do |var|
$global=$global*var
end  #=>0,2,4,6 last assignment of $global is 6 puts$global
#=> 6


This works because prefixing a variable with a dollar sign makes the variable a global.

### Instance scope

Within methods of a class, you can share variables by prefixing them with an @.

 class A
def setup
@instvar = 1
end
def go
@instvar = @instvar*2
puts @instvar
end
end
instance = A.new
instance.setup
instance.go
#=> 2
instance.go
#=> 4


### Class scope

A class variable is one that is like a "static" variable in Java. It is shared by all instances of a class.

 class A
@@classvar = 1
def go
@@classvar = @@classvar*2
puts @@classvar
end
end
instance = A.new
instance.go
#=> 2
instance = A.new
instance.go
#=> 4 -- variable is shared across instances


Here's a demo showing the various types:

$variable class Test def initialize(arg1='kiwi') @instvar=arg1 @@classvar=@instvar+' told you so!!' localvar=@instvar end def print_instvar puts @instvar end def print_localvar puts @@classvar puts localvar end end var=Test.new var.print_instvar #=>"kiwi", it works because a @instance_var can be accesed inside the class var.print_localvar #=>undefined local variable or method 'localvar' for #<Test:0x2b36208 @instvar="kiwi"> (NameError).  This will print the two lines "kiwi" and "kiwi told you so!!", then fail with a undefined local variable or method 'localvar' for #<Test:0x2b36208 @instvar="kiwi"> (NameError). Why? Well, in the scope of the method print_localvar there doesn't exists localvar, it exists in method initialize(until GC kicks it out). On the other hand, class variables '@@classvar' and '@instvar' are in scope across the entire class and, in the case of @@class variables, across the children classes.  class SubTest < Test def print_classvar puts @@classvar end end newvar=SubTest.new #newvar is created and it has @@classvar with the same value as the var instance of Test!! newvar.print_classvar #=>kiwi told you so!!  Class variables have the scope of parent class AND children, these variables can live across classes, and can be affected by the children actions ;-)  class SubSubTest < Test def print_classvar puts @@classvar end def modify_classvar @@classvar='kiwi kiwi waaai!!' end end subtest=SubSubTest.new subtest.modify_classvar #lets add a method that modifies the contents of @@classvar in SubSubTest subtest.print_classvar  This new child of Test also has @@classvar with the original value newvar.print_classvar. The value of @@classvar has been changed to 'kiwi kiwi waaai!!' This shows that @@classvar is "shared" across parent and child classes. ## Default scope When you don't enclose your code in any scope specifier, ex:  @a = 33  it affects the default scope, which is an object called "main". For example, if you had one script that says @a = 33 require 'other_script.rb'  and other_script.rb says puts @a #=> 33  They could share variables. Note however, that the two scripts don't share local variables. ## Local scope gotchas Typically when you are within a class, you can do as you'd like for definitions, like. class A a = 3 if a == 3 def go 3 end else def go 4 end end end  And also, procs "bind" to their surrounding scope, like  a = 3 b = proc { a } b.call # 3 -- it remembered what a was  However, the "class" and "def" keywords cause an *entirely new* scope. class A a = 3 def go return a # this won't work! end end  You can get around this limitation by using define_method, which takes a block and thus keeps the outer scope (note that you can use any block you want, to, too, but here's an example). class A a = 3 define_method(:go) { a } end  Here's using an arbitrary block a = 3 PROC = proc { a } # gotta use something besides a local # variable because that "class" makes us lose scope. class A define_method(:go, &PROC) end  or here  class A end a = 3 A.class_eval do define_method(:go) do puts a end end  ## Logical And The binary "and" operator will return the logical conjunction of its two operands. It is the same as "&&" but with a lower precedence. Example: a = 1 b = 2 c = nil puts "yay all my arguments are true" if a and b puts "oh no, one of my argument is false" if a and c  ## Logical Or The binary "or" operator will return the logical disjunction of its two operands. It is the same as "||" but with a lower precedence. Example: a = nil b = "foo" c = a || b # c is set to "foo" it's the same as saying c = (a || b) c = a or b # c is set to nil it's the same as saying (c = a) || b which is not what you want.  # Syntax - Control Structures ## Control Structures ### Conditional Branches Ruby can control the execution of code using Conditional branches. A conditional Branch takes the result of a test expression and executes a block of code depending whether the test expression is true or false. If the test expression evaluates to the constant false or nil, the test is false; otherwise, it is true. Note that the number zero is considered true, whereas many other programming languages consider it false. In many popular programming languages, conditional branches are statements. They can affect which code is executed, but they do not result in values themselves. In Ruby, however, conditional branches are expressions. They evaluate to values, just like other expressions do. An if expression, for example, not only determines whether a subordinate block of code will execute, but also results in a value itself. If no block is executed, the if expression results in nil. For instance, the following if expression evaluates to 3:  if true 3 end  Ruby's conditional branches are explained below. #### if expression Examples:  a = 5 if a == 4 a = 7 end print a # prints 5 since the if-block isn't executed  You can also put the test expression and code block on the same line if you use then:  if a == 4 then a = 7 end #or if a == 4: a = 7 end #Note that the ":" syntax for if one line blocks do not work anymore in ruby 1.9. Ternary statements still work  This is equal to:  a = 5 a = 7 if a == 4 print a # prints 5 since the if-block isn't executed  #### unless expression The unless-expression is the opposite of the if-expression, the code-block it contains will only be executed if the test expression is false. Examples:  a = 5 unless a == 4 a = 7 end print a # prints 7 since the unless-block is executed  The unless expression is almost exactly like a negated if expression:  if !expression # is equal to using unless expression  The difference is that the unless does not permit a following elsif. And there is no elsunless. Like the if-expression you can also write:  a = 5 a = 7 unless a == 4 print a # prints 7 since the unless-block is executed  The "one-liners" are handy when the code executed in the block is one line only. #### if-elsif-else expression The elsif (note that it's elsif and not elseif) and else blocks give you further control of your scripts by providing the option to accommodate additional tests. The elsif and else blocks are considered only if the if test is false. You can have any number of elsif blocks but only one if and one else block. Syntax:  if expression ...code block... elsif another expression ...code block... elsif another expression ...code block... else ...code block... end  #### short-if expression (aka ternary operator) The "short-if" statement provides you with a space-saving way of evaluating an expression and returning a value. The format is:  result = (condition) ? (expression-if-true) : (expression-if-false)  It is also known as the ternary operator and it is suggested to only use this syntax for minor tasks, such as string formatting, because of poor code readability that may result.  irb(main):037:0> true ? 't' : 'f' => "t" irb(main):038:0> false ? 't' : 'f' => "f"  This is very useful when doing string concatenation among other things. Example:  a = 5 plus_or_minus = '+' print "The number #{a}#{plus_or_minus}1 is: #{plus_or_minus == '+' ? (a+1).to_s : (a-1).to_s}."  Also, this can be written as:  result = if condition then expression-1 else expression-2 end  Assignments can be made as:  result = (value-1 if expression-1) || (value-2 if expression-2)  #### case expression An alternative to the if-elsif-else expression (above) is the case expression. Case in Ruby supports a number of syntaxes. For example, suppose we want to determine the relationship of a number (given by the variable a) to 5. We could say:  a = 1 case when a < 5 then puts "#{a} is less than 5" when a == 5 then puts "#{a} equals 5" when a > 5 then puts "#{a} is greater than 5" end  Note that, as with if, the comparison operator is ==. The assignment operator is =. Though Ruby will accept the assignment operator:  when a = 5 then puts "#{a} equals 5" # WARNING! This code CHANGES the value of a!  this is not want we want! Here, we want the comparison operator. A more concise syntax for case is to imply the comparison:  case a when 0..4 then puts "#{a} is less than 5" when 5 then puts "#{a} equals 5" when 5..10 then puts "#{a} is greater than 5" else puts "unexpected value #{a} " # Just in case "a" is bigger than 10 or negative. end  Note: because the ranges are explicitly stated, it is a good coding practice to handle unexpected values of a. This concise syntax is perhaps most useful when we know in advance what values to expect. For example:  a = "apple" case a when "vanilla" then "a spice" when "spinach" then "a vegetable" when "apple" then "a fruit" else "an unexpected value" end  If entered into IRB this gives:  => "a fruit"  Other ways to use case and variations on its syntax may be seen at Linuxtopia Ruby Programming [3] ### Loops #### while The while statement in Ruby is very similar to if and to other languages' while (syntactically): while <expression> <...code block...> end  The code block will be executed again and again, as long as the expression evaluates to true. Also, like if and unless, the following is possible: <...code...> while <expression>  Note the following strange case works...  line = readline.chomp while line != "what I'm looking for"  So if local variable line has no existence prior to this line, on seeing it for the first time it has the value nil when the loop expression is first evaluated. #### until The until statement is similar to the while statement in functionality. Unlike the while statement, the code block for the until loop will execute as long as the expression evaluates to false. until <expression> <...code block...> end  ### Keywords #### return return value causes the method in which it appears to exit at that point and return the value specified Note that if no return of a value is specified in a method the value of the last value set is implicitly returned as the return value of the method. # Syntax - Method Calls A method in Ruby is a set of expressions that returns a value. With methods, one can organize their code into subroutines that can be easily invoked from other areas of their program. Other languages sometimes refer to this as a function. A method may be defined as a part of a class or separately. ## Method Calls Methods are called using the following syntax: method_name(parameter1, parameter2,…)  If the method has no parameters the parentheses can usually be omitted as in the following: method_name  If you don't have code that needs to use method result immediately, Ruby allows to specify parameters omitting parentheses: results = method_name parameter1, parameter2 # calling method, not using parentheses # You need to use parentheses if you want to work with the result immediately. # e.g., if a method returns an array and we want to reverse element order: results = method_name(parameter1, parameter2).reverse  ## Method Definitions Methods are defined using the keyword def followed by the method name. Method parameters are specified between parentheses following the method name. The method body is enclosed by this definition on the top and the word end on the bottom. By convention method names that consist of multiple words have each word separated by an underscore. Example: def output_something(value) puts value end  ### Return Values Methods return the value of the last statement executed. The following code returns the value x+y. def calculate_value(x,y) x + y end  An explicit return statement can also be used to return from function with a value, prior to the end of the function declaration. This is useful when you want to terminate a loop or return from a function as the result of a conditional expression. Note, if you use "return" within a block, you actually will jump out from the function, probably not what you want. To terminate block, use break. You can pass a value to break which will be returned as the result of the block: six = (1..10).each {|i| break i if i > 5}  In this case, six will have the value 6. ### Default Values A default parameter value can be specified during method definition to replace the value of a parameter if it is not passed into the method. def some_method(value='default', arr=[]) puts value puts arr.length end some_method('something')  The method call above will output:  something 0  The following is a syntax error in Ruby 1.8  def foo( i = 7, j ) # Syntax error in Ruby 1.8.7 Unexpected ')', expecting '=' return i + j end  The above code will work in 1.9.2 and will be logically equivalent to the snippet below  def foo( j, i = 7) return i + j end  ### Variable Length Argument List, Asterisk Operator The last parameter of a method may be preceded by an asterisk(*), which is sometimes called the 'splat' operator. This indicates that more parameters may be passed to the function. Those parameters are collected up and an array is created.  def calculate_value(x,y,*otherValues) puts otherValues end calculate_value(1,2,'a','b','c')  In the example above the output would be ['a', 'b', 'c']. The asterisk operator may also precede an Array argument in a method call. In this case the Array will be expanded and the values passed in as if they were separated by commas.  arr = ['a','b','c'] calculate_value(*arr)  has the same result as:  calculate_value('a','b','c')  Another technique that Ruby allows is to give a Hash when invoking a function, and that gives you best of all worlds: named parameters, and variable argument length.  def accepts_hash( var ) print "got: ", var.inspect # will print out what it received end accepts_hash :arg1 => 'giving arg1', :argN => 'giving argN' # => got: {:argN=>"giving argN", :arg1=>"giving arg1"}  You see, the arguments for accepts_hash got rolled up into one hash variable. This technique is heavily used in the Ruby On Rails API. Also note the missing parenthesis around the arguments for the accepts_hash function call, and notice that there is no { } Hash declaration syntax around the :arg1 => '...' code, either. The above code is equivalent to the more verbose:  accepts_hash( :arg1 => 'giving arg1', :argN => 'giving argN' ) # argument list enclosed in parens accepts_hash( { :arg1 => 'giving arg1', :argN => 'giving argN' } ) # hash is explicitly created  Now, if you are going to pass a code block to function, you need parentheses.  accepts_hash( :arg1 => 'giving arg1', :argN => 'giving argN' ) { |s| puts s } accepts_hash( { :arg1 => 'giving arg1', :argN => 'giving argN' } ) { |s| puts s } # second line is more verbose, hash explicitly created, but essentially the same as above  ### The Ampersand Operator Much like the asterisk, the ampersand(&) may precede the last parameter of a function declaration. This indicates that the function expects a code block to be passed in. A Proc object will be created and assigned to the parameter containing the block passed in. Also similar to the ampersand operator, a Proc object preceded by an ampersand during a method call will be replaced by the block that it contains. Yield may then be used.  def method_call yield end method_call(&someBlock)  ### Understanding blocks, Procs and methods #### Introduction Ruby provides the programmer with a set of very powerful features borrowed from the domain of functional programming, namely closures, higher-order functions, and first-class functions [1]. These features are implemented in Ruby by means of code blocks, Proc objects, and methods (that are also objects)—concepts that are closely related and yet differ in subtle ways. In fact, I found myself quite confused about this topic, having difficulty understanding the difference between blocks, procs, and methods as well as unsure about the best practices of using them. Additionally, having some background in Lisp and years of Perl experience, I was unsure of how the Ruby concepts map to similar idioms from other programming languages like Lisp’s functions and Perl’s subroutines. Sifting through hundreds of newsgroup posts, I saw that I am not the only one with this problem and, in fact, quite a lot of “Ruby Nubies” struggle with the same ideas. In this article I lay out my understanding of this facet of Ruby, which comes as a result of extensive research of Ruby books, documentation, and comp.lang.ruby, in sincere hope that other people will find it useful as well. #### Procs Shamelessly ripping from the Ruby documentation, Procs are defined as follows: Proc objects are blocks of code that have been bound to a set of local variables. Once bound, the code may be called in different contexts and still access those variables. A useful example is also provided:  def gen_times(factor) return Proc.new {|n| n*factor } end times3 = gen_times(3) # 'factor' is replaced with 3 times5 = gen_times(5) times3.call(12) #=> 36 times5.call(5) #=> 25 times3.call(times5.call(4)) #=> 60  Procs play the role of functions in Ruby. It is more accurate to call them function objects, since like everything in Ruby they are objects. Such objects have a name in the folklore - functors. A functor is defined as an object to be invoked or called as if it were an ordinary function, usually with the same syntax, which is exactly what a Proc is. From the example and the definition above, it is obvious that Ruby Procs can also act as closures. On Wikipedia, a closure is defined as a function that refers to free variables in its lexical context. Note how closely it maps to the Ruby definition blocks of code that have been bound to a set of local variables. #### More on Procs Procs in Ruby are first-class objects, since they can be created during runtime, stored in data structures, passed as arguments to other functions and returned as the value of other functions. Actually, the gen_times example demonstrates all of these criteria, except for “passed as arguments to other functions”. This one can be presented as follows:  def foo (a, b) a.call(b) end putser = Proc.new {|x| puts x} foo(putser, 34)  There is also a shorthand notation for creating Procs - the Kernel method lambda [2] (we’ll come to methods shortly, but for now assume that a Kernel method is something akin to a global function, which can be called from anywhere in the code). Using lambda the Proc object creation from the previous example can be rewritten as:  putser = lambda {|x| puts x}  Actually, there are two slight differences between lambda and Proc.new. First, argument checking. The Ruby documentation for lambda states: Equivalent to Proc.new, except the resulting Proc objects check the number of parameters passed when called. Here is an example to demonstrate this:  pnew = Proc.new {|x, y| puts x + y} lamb = lambda {|x, y| puts x + y} # works fine, printing 6 pnew.call(2, 4, 11) # throws an ArgumentError lamb.call(2, 4, 11)  Second, there is a difference in the way returns are handled from the Proc. A return from Proc.new returns from the enclosing method (acting just like a return from a block, more on this later):  def try_ret_procnew ret = Proc.new { return "Baaam" } ret.call "This is not reached" end # prints "Baaam" puts try_ret_procnew  While return from lambda acts more conventionally, returning to its caller:  def try_ret_lambda ret = lambda { return "Baaam" } ret.call "This is printed" end # prints "This is printed" puts try_ret_lambda  With this in light, I would recommend using lambda instead of Proc.new, unless the behavior of the latter is strictly required. In addition to being a whopping two characters shorter, its behavior is less surprising. #### Methods Simply put, a method is also a block of code. However, unlike Procs, methods are not bound to the local variables around them. Rather, they are bound to some object and have access to its instance variables [3]:  class Boogy def initialize @dix = 15 end def arbo puts "#{@dix} ha\n" end end # initializes an instance of Boogy b = Boogy.new # prints "15 ha" b.arbo  A useful idiom when thinking about methods is that you are sending messages to the object that the method is defined for. Given a receiver - an object that has some method defined - we send it a message, which contains the name of the method, and optionally provides the arguments that the method would receive. In the example above, calling the method arbo without any arguments, is akin to sending a message with just “arbo” as the argument. Ruby supports the message sending idiom more directly, by including the send method in class Object (which is the parent of all objects in Ruby). So the following three lines are equivalent to the arbo method call:  # method is called on the object, with no arguments b.arbo # method/message name is given as a string b.send("arbo") # method/message name is given as a symbol b.send(:arbo)  Note that methods can also be defined in the so-called “top-level” scope, which is not inside any user-defined class. For example:  def say (something) puts something end say "Hello"  While it seems that the method say is “free-standing”, it is not - Ruby silently tucks it into the Object class, which represents the scope of your application:  def say (something) puts something end say "Hello" Object.send(:say, "Hello") # this will be the same as the above line  But this doesn’t really matter, and for all practical purposes say can be seen as an independent method. Which is, by the way, just what’s called a “function” in some languages (like C and Perl). The following Proc is, in many ways similar:  say = lambda {|something| puts something} say.call("Hello") # same effect say["Hello"]  The [] construct is a synonym to call in the context of Proc [4]. Methods, however, are more versatile than procs and support a very important feature of Ruby, which I will present right after explaining what blocks are. #### Blocks Blocks are so powerfully related to Procs that it gives many newbies a headache trying to decipher how they actually differ. I will try to ease on comprehension with a (hopefully not too corny) metaphor. Blocks, as I see them, are unborn Procs. Blocks are the larval, Procs are the insects. A block does not live on its own - it prepares the code for when it will actually become alive, and only when it is bound and converted to a Proc, it starts living:  # a naked block can't live in Ruby # this is a compilation error ! {puts "hello"} # now it's alive, having been converted # to a Proc ! pr = lambda {puts "hello"} pr.call  Is that it, is that what all the fuss is about, then ? No, not at all. The designer of Ruby, Matz saw that while passing Procs to methods (and other Procs) is nice and allows high-level functions and all kinds of fancy functional stuff, there is one common case that stands high above all other cases - passing a single block of code to a method that makes something useful out of it, for example iteration. And as a very talented designer, Matz decided that it is worthwhile to emphasize this special case, and make it both simpler and more efficient. #### Passing a block to a method No doubt that any programmer who has spent at least a couple of hours with Ruby has been shown the following examples of Ruby glory (or something very similar):  10.times do |i| print "#{i} " end numbers = [1, 2, 5, 6, 9, 21] numbers.each do |x| puts "#{x} is " + (x >= 3 ? "many" : "few") end squares = numbers.map {|x| x * x}  (Note that do |x| ... end is equivalent to { |x| ... }.) Such code is, in my opinion, the part of what makes Ruby the clean, readable, and wonderful language it is. What happens here behind the scenes is quite simple, or at least may be depicted in a very simple way. Perhaps Ruby may not implement it exactly the way I am going to describe it, since there are optimization considerations surely playing their role, but it is definitely close enough to the truth to serve as a metaphor for understanding. Whenever a block is appended to a method call, Ruby automatically converts it to a Proc object but one without an explicit name. The method, however, has a way to access this Proc, by means of the yield statement. See the following example for clarification:  def do_twice yield yield end do_twice {puts "Hola"}  The method do_twice is defined and called with an attached block. Although the method did not explicitly ask for the block in its arguments list, the yield can call the block. This can be implemented in a more explicit way using a Proc argument:  def do_twice(what) what.call what.call end do_twice lambda {puts "Hola"}  This is equivalent to the previous example but using blocks with yield is cleaner and better optimized since only one block is passed to the method. Using the Proc approach, any amount of code blocks can be passed:  def do_twice(what1, what2, what3) 2.times do what1.call what2.call what3.call end end do_twice( lambda {print "Hola, "}, lambda {print "querido "}, lambda {print "amigo\n"})  This is important to note that many people frown at passing blocks and prefer explicit Procs instead. Their rationale is that a block argument is implicit and one has to look through the whole code of the method to see if there are any calls to yield there, while a Proc is explicit and can be immediately spotted in the argument list. While this is simply a matter of taste, understanding both approaches is vital. #### The ampersand (&) The ampersand operator can be used to explicitly convert between blocks and Procs in a couple of cases. It is worthy to understand how these work. Remember how I said that although an attached block is converted to a Proc under the hood, it is not accessible as a Proc from inside the method ? Well, if an ampersand is prepended to the last argument in the argument list of a method, the block attached to this method is converted to a Proc object and gets assigned to that last argument:  def contrived(a, &f) # the block can be accessed through f f.call(a) # but yield also works ! yield(a) end # this works contrived(25) {|x| puts x} # this raises ArgumentError, because &f # isn't really an argument - it's only there # to convert a block contrived(25, lambda {|x| puts x})  Another (IMHO far more efficacious) use of the ampersand is the other-way conversion - converting a Proc into a block. This is very useful because many of Ruby’s great built-ins, and especially the iterators, expect to receive a block as an argument, and sometimes it’s much more convenient to pass them a Proc. The following example is taken right from the excellent “Programming Ruby” book by the pragmatic programmers:  print "(t)imes or (p)lus: " times = gets print "number: " number = Integer(gets) if times =~ /^t/ calc = lambda {|n| n*number } else calc = lambda {|n| n+number } end puts((1..10).collect(&calc).join(", "))  The collect method expects a block, but in this case it is very convenient to provide it with a Proc, since the Proc is constructed using knowledge gained from the user. The ampersand preceding calc makes sure that the Proc object calc is turned into a code block and is passed to collect as an attached block. The ampersand also allows the implementation of a very common idiom among Ruby programmers: passing method names into iterators. Assume that I want to convert all words in an Array to upper case. I could do it like this:  words = %w(Jane, aara, multiko) upcase_words = words.map {|x| x.upcase} p upcase_words  This is nice, and it works, but I feel it’s a little bit too verbose. The upcase method itself should be given to map, without the need for a separate block and the apparently superfluous x argument. Fortunately, as we saw before, Ruby supports the idiom of sending messages to objects, and methods can be referred to by their names, which are implemented as Ruby Symbols. For example:  p "Erik".send(:upcase)  This, quite literally, says send the message/method upcase to the object “Erik”. This feature can be utilized to implement the map {|x| x.upcase} in an elegant manner, and we’re going to use the ampersand for this ! As I said, when the ampersand is prepended to some Proc in a method call, it converts the Proc to a block. But what if we prepend it not to a Proc, but to another object ? Then, Ruby’s implicit type conversion rules kick in, and the to_proc method is called on the object to try and make a Proc out of it. We can use this to implement to_proc for Symbol and achieve what we want:  class Symbol # A generalized conversion of a method name # to a proc that runs this method. # def to_proc lambda {|x, *args| x.send(self, *args)} end end # Voilà ! words = %w(Jane, aara, multiko) upcase_words = words.map(&:upcase)  ## Dynamic methods You can define a method on "just one object" in Ruby.  a = 'b' def a.some_method 'within a singleton method just for a' end >> a.some_method => 'within a singleton method just for a'  Or you can use define_(singleton_)method, which preserves the scope around the definition, as well.  a = 'b' a.define_singleton_method(:some_method) { 'within a block method' } a.some_method  ## Special methods Ruby has a number of special methods that are called by the interpreter. For example:  class Chameleon alias __inspect__ inspect def method_missing(method, *arg) if (method.to_s)[0..2] == "to_" @identity = __inspect__.sub("Chameleon", method.to_s.sub('to_','').capitalize) def inspect @identity end self else super #method_missing overrides the default Kernel.method_missing #pass on anything we weren't looking for so the Chameleon stays unnoticed and uneaten ;) end end end mrlizard = Chameleon.new mrlizard.to_rock  This does something silly but method_missing is an important part of meta-programming in Ruby. In Ruby on Rails it is used extensively to create methods dynamically. Another special methods is initialize that ruby calls whenever a class instance is created, but that belongs in the next chapter: Classes. ## Conclusion Ruby doesn’t really have functions. Rather, it has two slightly different concepts - methods and Procs (which are, as we have seen, simply what other languages call function objects, or functors). Both are blocks of code - methods are bound to Objects, and Procs are bound to the local variables in scope. Their uses are quite different. Methods are the cornerstone of object-oriented programming, and since Ruby is a pure-OO language (everything is an object), methods are inherent to the nature of Ruby. Methods are the actions Ruby objects do - the messages they receive, if you prefer the message sending idiom. Procs make powerful functional programming paradigms possible, turning code into a first-class object of Ruby allowing to implement high-order functions. They are very close kin to Lisp’s lambda forms (there’s little doubt about the origin of Ruby’s Proc constructor lambda) The construct of a block may at first be confusing, but it turns out to be quite simple. A block is, as my metaphor goes, an unborn Proc - it is a Proc in an intermediate state, not bound to anything yet. I think that the simplest way to think about blocks in Ruby, without losing any comprehension, would be to think that blocks are really a form of Procs, and not a separate concept. The only time when we have to think of blocks as slightly different from Procs is the special case when they are passed as the last argument to a method which may then access them using yield. That’s about it, I guess. I know for sure that the research I conducted for this article cleared many misunderstandings I had about the concepts presented here. I hope others will learn from it as well. If you see anything you don’t agree with - from glaring errors to small inaccuracies, feel free to amend the book. #### Notes [1] It seems that in the pure, theoretical interpretation what Ruby has isn’t first-class functions per se. However, as this article demonstrates, Ruby is perfectly capable of fulfilling most of the requirements for first-class functions, namely that functions can be created during the execution of a program, stored in data structures, passed as arguments to other functions, and returned as the values of other functions. [2] lambda has a synonym - proc, which is considered ‘mildly deprecated’ (mainly because proc and Proc.new are slightly different, which is confusing). In other words, just use lambda. [3] These are ‘instance methods’. Ruby also supports ‘class methods’, and ‘class variables’, but that is not what this article is about. [4] Or more accurately, call and [] both refer to the same method of class Proc. Yes, Proc objects themselves have methods ! # Syntax - Classes Classes are the basic template from which object instances are created. A class is made up of a collection of variables representing internal state and methods providing behaviours that operate on that state. ## Class Definition Classes are defined in Ruby using the class keyword followed by a name. The name must begin with a capital letter and by convention names that contain more than one word are run together with each word capitalized and no separating characters (CamelCase). The class definition may contain method, class variable, and instance variable declarations as well as calls to methods that execute in the class context at read time, such as attr_accessor. The class declaration is terminated by the end keyword. Example:  class MyClass def some_method end end  ### Instance Variables Instance variables are created for each class instance and are accessible only within that instance. They are accessed using the @ operator. Outside of the class definition, the value of an instance variable can only be read or modified via that instance's public methods. Example:  class MyClass @one = 1 def do_something @one = 2 end def output puts @one end end instance = MyClass.new instance.output instance.do_something instance.output  Surprisingly, this outputs:  nil 2  This happens (nil in the first output line) because @one defined below class MyClass is an instance variable belonging to the class object (note this is not the same as a class variable and could not be referred to as @@one), whereas @one defined inside the do_something method is an instance variable belonging to instances of MyClass. They are two distinct variables and the first is accessible only in a class method. ### Accessor Methods As noted in the previous section, an instance variable can only be directly accessed or modified within an instance method definition. If you want to provide access to it from outside, you need to define public accessor methods, for example  class MyClass def initialize @foo = 28 end def foo return @foo end def foo=(value) @foo = value end end instance = MyClass.new puts instance.foo instance.foo = 496 puts instance.foo  Note that ruby provides a bit of syntactic sugar to make it look like you are getting and setting a variable directly; under the hood  a = instance.foo instance.foo = b  are calls to the foo and foo= methods  a = instance.foo() instance.foo=(b)  Since this is such a common use case, there is also a convenience method to autogenerate these getters and setters:  class MyClass attr_accessor :foo def initialize @foo = 28 end end instance = MyClass.new puts instance.foo instance.foo = 496 puts instance.foo  does the same thing as the above program. The attr_accessor method is run at read time, when ruby is constructing the class object, and it generates the foo and foo= methods. However, there is no requirement for the accessor methods to simply transparently access the instance variable. For example, we could ensure that all values are rounded before being stored in foo:  class MyClass def initialize @foo = 28 end def foo return @foo end def foo=(value) @foo = value.round end end instance = MyClass.new puts instance.foo instance.foo = 496.2 puts instance.foo #=> 496  ### Class Variables Class variables are accessed using the @@ operator. These variables are associated with the class hierarchy rather than any object instance of the class and are the same across all object instances. (These are similar to class "static" variables in Java or C++). Example:  class MyClass @@value = 1 def add_one @@value= @@value + 1 end def value @@value end end instanceOne = MyClass.new instanceTwo = MyClass.new puts instanceOne.value instanceOne.add_one puts instanceOne.value puts instanceTwo.value  Outputs:  1 2 2  ### Class Instance Variables Classes can have instance variables. This gives each class a variable that is not shared by other classes in the inheritance chain.  class Employee class << self; attr_accessor :instances; end def store self.class.instances ||= [] self.class.instances << self end def initialize name @name = name end end class Overhead < Employee; end class Programmer < Employee; end Overhead.new('Martin').store Overhead.new('Roy').store Programmer.new('Erik').store puts Overhead.instances.size # => 2 puts Programmer.instances.size # => 1  For more details, see MF Bliki: ClassInstanceVariables ### Class Methods Class methods are declared the same way as normal methods, except that they are prefixed by self, or the class name, followed by a period. These methods are executed at the Class level and may be called without an object instance. They cannot access instance variables but do have access to class variables. Example:  class MyClass def self.some_method puts 'something' end end MyClass.some_method  Outputs:  something  ### Instantiation An object instance is created from a class through the a process called instantiation. In Ruby this takes place through the Class method new. Example:  anObject = MyClass.new(parameters)  This function sets up the object in memory and then delegates control to the initialize function of the class if it is present. Parameters passed to the new function are passed into the initialize function.  class MyClass def initialize(parameters) end end  ## Declaring Visibility By default, all methods in Ruby classes are public - accessible by anyone. There are, nonetheless, only two exceptions for this rule: the global methods defined under the Object class, and the initialize method for any class. Both of them are implicitly private. If desired, the access for methods can be restricted by public, private, protected object methods. It is interesting that these are not actually keywords, but actual methods that operate on the class, dynamically altering the visibility of the methods, and as a result, these 'keywords' influence the visibility of all following declarations until a new visibility is set or the end of the declaration-body is reached. ### Private Simple example:  class Example def methodA end private # all methods that follow will be made private: not accessible for outside objects def methodP end end  If private is invoked without arguments, it sets access to private for all subsequent methods. It can also be invoked with named arguments. Named private method example:  class Example def methodA end def methodP end private :methodP end  Here private was invoked with an argument, altering the visibility of methodP to private. Note for class methods (those that are declared using def ClassName.method_name), you need to use another function: private_class_method Common usage of private_class_method is to make the "new" method (constructor) inaccessible, to force access to an object through some getter function. A typical Singleton implementation is an obvious example.  class SingletonLike private_class_method :new def SingletonLike.create(*args, &block) @@inst = new(*args, &block) unless @@inst return @@inst end end  Note : another popular way to code the same declaration  class SingletonLike private_class_method :new def SingletonLike.create(*args, &block) @@inst ||= new(*args, &block) end end  More info about the difference between C++ and Ruby private/protected: http://lylejohnson.name/blog/?p=5 One person summed up the distinctions by saying that in C++, “private” means “private to this class”, while in Ruby it means “private to this instance”. What this means, in C++ from code in class A, you can access any private method for any other object of type A. In Ruby, you can not: you can only access private methods for your instance of object, and not for any other object instance (of class A). Ruby folks keep saying "private means you cannot specify the receiver". What they are saying, if method is private, in your code you can say:  class AccessPrivate def a end private :a # a is private method def accessing_private a # sure! self.a # nope! private methods cannot be called with an explicit receiver at all, even if that receiver is "self" other_object.a # nope, a is private, you can't get it (but if it was protected, you could!) end end  Here, "other_object" is the "receiver" that method "a" is invoked on. For private methods, it does not work. However, that is what "protected" visibility will allow. ### Public Public is default accessibility level for class methods. I am not sure why this is specified - maybe for completeness, maybe so that you could dynamically make some method private at some point, and later - public. In Ruby, visibility is completely dynamic. You can change method visibility at runtime! ### Protected Now, “protected” deserves more discussion. Those of you coming from Java or C++ have learned that in those languages, if a method is “private”, its visibility is restricted to the declaring class, and if the method is “protected”, it will be accessible to children of the class (classes that inherit from parent) or other classes in that package. In Ruby, “private” visibility is similar to what “protected” is in Java. Private methods in Ruby are accessible from children. You can’t have truly private methods in Ruby; you can’t completely hide a method. The difference between protected and private is subtle. If a method is protected, it may be called by any instance of the defining class or its subclasses. If a method is private, it may be called only within the context of the calling object---it is never possible to access another object instance's private methods directly, even if the object is of the same class as the caller. For protected methods, they are accessible from objects of the same class (or children). So, from within an object "a1" (an instance of Class A), you can call private methods only for instance of "a1" (self). And you can not call private methods of object "a2" (that also is of class A) - they are private to a2. But you can call protected methods of object "a2" since objects a1 and a2 are both of class A. Ruby FAQ gives following example - implementing an operator that compares one internal variable with a variable from the same class (for purposes of comparing the objects):  def <=>(other) self.age <=> other.age end  If age is private, this method will not work, because other.age is not accessible. If "age" is protected, this will work fine, because self and other are of same class, and can access each other's protected methods. To think of this, protected actually reminds me of the "internal" accessibility modifier in C# or "default" accessibility in Java (when no accessibility keword is set on method or variable): method is accessible just as "public", but only for classes inside the same package. ### Instance Variables Note that object instance variables are not really private, you just can't see them. To access an instance variable, you need to create a getter and setter. Like this (no, don't do this by hand! See below):  class GotAccessor def initialize(size) @size = size end def size @size end def size=(val) @size = val end end # you could the access @size variable as # a = GotAccessor.new(5) # x = a.size # a.size = y  Luckily, we have special functions to do just that: attr_accessor, attr_reader, attr_writer. attr_accessor will give you get/set functionality, reader will give only getter and writer will give only setter. Now reduced to:  class GotAccessor def initialize(size) @size = size end attr_accessor :size end # attr_accessor generates variable @size accessor methods automatically: # a = GotAccessor.new(5) # x = a.size # a.size = y  ## Inheritance The super keyword only accessing the direct parents method. There is a workaround though. A class can inherit functionality and variables from a superclass, sometimes referred to as a parent class or base class. Ruby does not support multiple inheritance and so a class in Ruby can have only one superclass. The syntax is as follows:  class ParentClass def a_method puts 'b' end end class SomeClass < ParentClass # < means inherit (or "extends" if you are from Java background) def another_method puts 'a' end end instance = SomeClass.new instance.another_method instance.a_method  Outputs:  a b  All non-private variables and functions are inherited by the child class from the superclass. If your class overrides a method from parent class (superclass), you still can access the parent's method by using 'super' keyword.  class ParentClass def a_method puts 'b' end end class SomeClass < ParentClass def a_method super puts 'a' end end instance = SomeClass.new instance.a_method  Outputs:  b a  (because a_method also did invoke the method from parent class). If you have a deep inheritance line, and still want to access some parent class (superclass) methods directly, you can't. super only gets you a direct parent's method. But there is a workaround! When inheriting from a class, you can alias parent class method to a different name. Then you can access methods by alias.  class X def foo "hello" end end class Y < X alias xFoo foo def foo xFoo + "y" end end class Z < Y def foo xFoo + "z" end end puts X.new.foo puts Y.new.foo puts Z.new.foo  Outputs  hello helloy helloz  ## Mixing in Modules First, you need to read up on modules Ruby modules. Modules are a way of grouping together some functions and variables and classes, somewhat like classes, but more like namespaces. So a module is not really a class. You can't instantiate a Module, and thus it does not have self. This trait, however, allows us to include the module into a class. Mix it in, so to speak. module A def a1 puts 'a1 is called' end end module B def b1 puts 'b1 is called' end end module C def c1 puts 'c1 is called' end end class Test include A include B include C def display puts 'Modules are included' end end object=Test.new object.display object.a1 object.b1 object.c1  Outputs: Modules are included a1 is called b1 is called c1 is called  The code shows Multiple Inheritance using modules. ## Ruby Class Meta-Model In keeping with the Ruby principle that everything is an object, classes are themselves instances of the class Class. They are stored in constants under the scope of the module in which they are declared. A call to a method on an object instance is delegated to a variable inside the object that contains a reference to the class of that object. The method implementation exists on the Class instance object itself. Class methods are implemented on meta-classes that are linked to the existing class instance objects in the same way that those classes instances are linked to them. These meta-classes are hidden from most Ruby functions. # Syntax - Hooks Ruby provides callbacks, to, for example, know when a new method is defined (later) in a class. Here is a list of known callbacks. ## const_missing class Object def self.const_missing c p 'missing const was', c end end  or more class Object class << self alias :const_missing_old :const_missing def const_missing c p 'const missing is', c const_missing_old c end end end  See also some rdoc documentation on the various keywords. # References # Built-In Functions By default many methods are available. You can see the available ones by running methods in an irb session, ex: >> class A; end >> A.singleton_methods [:nil?, :===, :=~, :!~, :eql?, :class, :clone, :dup, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze, :frozen?, :to_s, :inspect, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :extend, :display, :method, :public_method, :define_singleton_method, :hash, :__id__, :object_id, :to_enum, :enum_for, :gem, :==, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__]  You can see where most of those methods are defined by inspecting the object hierarchy: >> A.ancestors => [A, Object, Kernel, BasicObject]  1.9 introduced a few more has __method__ (the current method name), as well as require_relative, which requires a file relative to the dir of the current file. To see where each methods was defined, you can run something like: >> A.instance_methods.map{|m| [m, A.instance_method(m).owner] } => [[:nil?, Kernel], ...  # Predefined Variables Ruby's predefined (built-in) variables affect the behavior of the entire program, so their use in libraries isn't recommended. The values in most predefined variables can be accessed by alternative means.$!

• The last exception object raised. The exception object can also be accessed using => in rescue clause.

$@ • The stack backtrace for the last exception raised. The stack backtrace information can retrieved by Exception#backtrace method of the last exception.$/

• The input record separator (newline by default). gets, readline, etc., take their input record separator as optional argument.

$\ • The output record separator (nil by default).$,

• The output separator between the arguments to print and Array#join (nil by default). You can specify separator explicitly to Array#join.

$; • The default separator for split (nil by default). You can specify separator explicitly for String#split.$.

• The number of the last line read from the current input file. Equivalent to ARGF.lineno.

$< • Synonym for ARGF.$>

• Synonym for $defout.$0

• The name of the current Ruby program being executed.



• The process.pid of the current Ruby program being executed.

$? • The exit status of the last process terminated.$:

• Synonym for $LOAD_PATH.$DEBUG

• True if the -d or --debug command-line option is specified.

$defout • The destination output for print and printf ($stdout by default).

$F • The variable that receives the output from split when -a is specified. This variable is set if the -a command-line option is specified along with the -p or -n option.$FILENAME

• The name of the file currently being read from ARGF. Equivalent to ARGF.filename.

$LOAD_PATH • An array holding the directories to be searched when loading files with the load and require methods.$SAFE

• The security level.
    0   No checks are performed on externally supplied (tainted) data. (default)

1   Potentially dangerous operations using tainted data are forbidden.

2   Potentially dangerous operations on processes and files are forbidden.

3   All newly created objects are considered tainted.

4   Modification of global data is forbidden.


$stdin • Standard input (STDIN by default).$stdout

• Standard output (STDOUT by default).

$stderr • Standard error (STDERR by default).$VERBOSE

• True if the -v, -w, or --verbose command-line option is specified.

$- x • The value of interpreter option -x (x=0, a, d, F, i, K, l, p, v). The following are local variables:$_

• The last string read by gets or readline in the current scope.

$~ • MatchData relating to the last match. Regex#match method returns the last match information. The following variables hold values that change in accordance with the current value of$~ and can't receive assignment:

$n ($1, $2,$3...)

• The string matched in the nth group of the last pattern match. Equivalent to m[n], where m is a MatchData object.

$& • The string matched in the last pattern match. Equivalent to m[0], where m is a MatchData object.$

• The string preceding the match in the last pattern match. Equivalent to m.pre_match, where m is a MatchData object.

$' • The string following the match in the last pattern match. Equivalent to m.post_match, where m is a MatchData object.$+

• The string corresponding to the last successfully matched group in the last pattern match.

# Predefined Classes

In ruby even the base types (also predefined classes) can be hacked.[4] In the following example, 5 is an immediate,[5] a literal, an object, and an instance of Fixnum.

class Fixnum

alias other_s to_s
def to_s()
a = self + 5
return a.other_s
end

end

a = 5
puts a.class  ## prints Fixnum
puts a        ## prints 10 (adds 5 once)
puts 0        ## prints 5  (adds 5 once)
puts 5        ## prints 10 (adds 5 once)
puts 10 + 0   ## prints 15 (adds 5 once)

b = 5+5
puts b        ## puts 15 (adds 5 once)


## Footnotes

1. ^ Which means the 4 VALUE bytes are not a reference but the value itself. All 5 have the same object id (which could also be achieved in other ways).
2. ^ Might not always work as you would like, the base types don't have a constructor (def initialize), and can't have simpleton methods. There are some other minor exceptions.

# Objects

Object is the base class of all other classes created in Ruby. It provides the basic set of functions available to all classes, and each function can be explicitly overridden by the user.

This class provides a number of useful methods for all of the classes in Ruby.

Object inherits from BasicObject which allows creating alternate object hierarchies. Methods on object are available to all classes unless explicitly overridden.

In Ruby, everything is an Object including classes and modules. Object and BasicObject are the most low-level class.

## Constants

### ARGF

ARGF is a stream designed for use in scripts that process files given as command-line arguments or passed in via STDIN.

### ARGV

ARGV contains the command line arguments used to run ruby with the first value containing the name of the executable.

### DATA

DATA is a File that contains the data section of the executed file. To create a data section use __END__:

### ENV

ENV is a Hash-like accessor for environment variables.

### STDERR

Holds the original stderr

### STDIN

Holds the original stdin

### STDOUT

Holds the original stdout

# Array

Class Methods

 Method: [ ]        Signature: Array[ [anObject]* ] -> anArray


Creates a new array whose elements are given between [ and ].

 Array.[]( 1, 2, 3 ) -> [1,2,3]
Array[ 1, 2, 3 ] -> [1,2,3]
[1,2,3] -> [1,2,3]


# Class

Ruby Programming/Reference/Objects/Class

# Comparable

Ruby Programming/Reference/Objects/Comparable

# Encoding

Encoding is basically a new concept introduced in Ruby 1.9

Strings now have "some bytes" and "some encoding" associated with them.

In 1.8, all strings were just "some bytes" (so basically treated as what BINARY/ASCII-8BIT encoding is in 1.9). You had to use helper libraries to use any m18n style stuff.

By default, when you open a file and read from it, it will read as strings with an encoding set to

Encoding.default_external` (which you can change).

This also means that it double checks the strings "a bit" for correctness on their way in.

In windows, it also has to do conversion from "\r\n" to "\n" which means that when you read a file in non-binary mode in windows, it has to first analyze the incoming string for correctness, then (second pass) convert its line endings, so it is a bit slower.

Recommend 1.9.2 for windows users loading large files, as it isn't quite as slow. Or read them as binary (File.binread or a=File.open('name', 'rb').