Ruby Programming/Syntax/Control Structures
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[edit] Control Structures
[edit] 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. For instance, the following if expression evaluates to 3:
if true 3 end
Ruby's conditional branches are explained below.
[edit] 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
This is equal to:
a = 5 a = 7 if a == 4 print a # prints 5 since the if-block isn't executed
[edit] 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 if-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 if-block is executed
The "one-liners" are handy when the code executed in the block is one line only.
[edit] 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
[edit] short-if expression
The "short-if" statement provides you with a space-saving way of evaluating an expression and returning a value. 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) + "."
This [? (expr) : (expr)] is also known as the ternary operator. 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"
[edit] 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} less than 5"
when a == 5 then puts "#{a} equals 5"
when a > 5 then puts "#{a} greater than 5"
end
Note that, as with if, the comparison operator is ==. The assignment operator is =. Although 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.
Another equivalent syntax for case is to use ":" instead of "then":
when a < 5 then puts "#{a} less than 5"
when a < 5 : puts "#{a} less than 5"
A more concise syntax for case is to imply the comparision:
case a
when 0..4 then puts "#{a} less than 5"
when 5 then puts "#{a} equals 5"
when 5..10 then puts "#{a} greater than 5"
else puts "unexpected value #{a} " #just in case "a" is bigger then 10 or negative
end
Note: because the ranges are explicitly stated, it is good coding practise to handle unexpected values of a. This concise syntax is perhaps most useful when we know in advance what the values to expect. For example:
a = "apple" puts case a when "vanilla" then "a spice" when "spinach" then "a vegetable" when "apple" then "a fruit" else "an unexpected value" end
If entered in the irb this gives:
a fruit =>nil
Other ways to use case and variations on its syntax maybe seen at Linuxtopia Ruby Programming [1]
[edit] Loops
[edit] 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 = inf.readline 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.
[edit] Keywords
[edit] return
return value causes the method in which it appears to exit at that point and return the value specified
