Lua programming/Statements

From Wikibooks, open books for an open world
Jump to: navigation, search

Statements are pieces of code that can be executed and that contain an instruction and expressions to use with it. Some statements will also contain code inside of themselves that may, for example, be ran under certain conditions. Dissimilarly to expressions, they can be put directly in code and will execute. Lua has few instructions, but these instructions, combined with other instructions and with complex expressions, give a good amount of control and flexibility to the user.

Assignment[edit]

Programmers frequently need to be able to store values in the memory to be able to use them later. This is done using variables. Variables are references to a value which is stored in the computer's memory. They can be used to access a number later after storing it in the memory. Assignment is the instruction that is used to assign a value to a variable. It consists of the name of the variable the value should be stored in, an equal sign, and the value that should be stored in the variable:

variable = 43
print(variable) --> 43

As demonstrated in the above code, the value of a variable can be accessed by putting the variable's name where the value should be accessed. It is possible to assign many variables at once like demonstrated in the next example:

variable1, variable2 = 54, 87

Moreover, unlike most programming languages Lua also enables reassignment of variables' values through permutation. For example:

variable1, variable2 = 54, 87
variable1, variable2 = variable2, variable1
print(variable1, variable2) --> 87 54

Identifiers[edit]

Identifiers, in Lua, are also called names. They can be any text composed of letters, digits, and underscores and not beginning with a digit. They are used to name variables and table fields, which will be covered in the chapter about tables.

Here are some valid names:

  • name
  • hello
  • _
  • _tomatoes
  • me41
  • __
  • _thisIs_StillaValid23name

Here are some invalid names:

  • 2hello ; starts with a digit
  • th$i ; contains a character that isn't a letter, a digit or an underscore
  • hel!o ; contains a character that isn't a letter, a digit or an underscore
  • 563text ; starts with a digit
  • 82_something ; starts with a digit

Also, the following keywords are reserved by Lua and can not be used as names: and, end, in, repeat, break, false, local, return, do, for, nil, then, else, function, not, true, elseif, if, or, until, while

When naming a variable or a table field, you must choose a valid name for it. It must therefore start with a letter or an underscore and only contain letters, underscores and digits. Note that Lua is case sensitive. This means that Hello and hello are two different names.

Scope[edit]

The scope of a variable is the region of the code of the program where that variable is meaningful. The examples of variables you have seen before are all examples of global variables, variables which can be accessed from anywhere in the program. Local variables, on the other hand, can only be used from the region of the program in which they were defined and in regions of the program that are located inside that region of the program. They are created exactly in the same way as global variables, but they must be prefixed with the local keyword.

The do statement will be used to describe them. The do statement is a statement that has no other purpose than to create a new block of code, and therefore a new scope. It ends with the end keyword:

local variable = 13 -- This defines a local variable that can be accessed from anywhere in the script since it was defined in the main region.
do
	-- This statement creates a new block and also a new scope.
	variable = variable + 5 -- This adds 5 to the variable, which now equals 18.
	local variable = 17 -- This creates a variable with the same name as the previous variable, but this one is local to the scope created by the do statement.
	variable = variable - 1 -- This subtracts 1 from the local variable, which now equals 16.
	print(variable) --> 16
end
print(variable) --> 18

When a scope ends, all the variables in it are gotten rid of. Regions of code can use variables defined in regions of code they are included in, but if they "overwrite" them by defining a local variable with the same name, that local variable will be used instead of the one defined in the other region of code. This is why the first call to the print function prints 16 while the second, which is outside the scope created by the do statement, prints 18.

In practice, only local variables should be used because they can be defined and accessed faster than global variables, since they are stored in registers instead of being stored in the environment of the current function, like global variables. Registers are areas that Lua uses to store local variables to access them quickly, and can only usually contain up to 200 local variables. The processor, an important component of all computers, also has registers, but these are not related to Lua's registers. Each function (including the main thread, the core of the program, which is also a function) also has its own environment, which is a table that uses indices for the variable names and stores the values of these variables in the values that correspond to these indices.

Conditional statement[edit]

Conditional statements are instructions that check whether an expression is true and execute a certain piece of code if it is. If the expression is not true, they just skip over that piece of code and the program continues. In Lua, the only conditional statement uses the if instruction. False and nil are both considered as false, while everything else is considered as true.

local number = 6
 
if number < 10 then
	print("The number " .. number .. " is smaller than ten.")
end
 
-- Other code can be here and it will execute regardless of whether the code in the conditional statement executed.

In the code above, the variable number is assigned the number 6 with an assignment statement. Then, a conditional statement checks if the value stored in the variable number is smaller than ten, which is the case here. If it is, it prints "The number 6 is smaller than ten.".

It is also possible to execute a certain piece of code only if the expression was not true by using the else keyword and to chain conditional statements with the elseif keyword:

local number = 15
 
if number < 10 then
	print("The number is smaller than ten.")
elseif number < 100
	print("The number is bigger than ten, but smaller than one hundred.")
elseif number ~= 1000 and number < 3000
	print("The number is bigger than one hundred, smaller than three thousands and is not exactly one thousand.")
else
	print("The number is bigger than ten.")
end

Note that the else block must always be the last one. There cannot be an elseif block after the else block. The elseif blocks are only meaningful if none of the blocks that preceded them was executed.

Operators used to compare two values, some of which are used in the code above, are called relational operators. If the relation is true, they return the boolean value true. Otherwise, they return the boolean value false.

equal to not equal to greater than less than greater than or equal to less than or equal to
Mathematical notation = > <
Lua operator == ~= > < >= <=

The code above also demonstrates how the and keyword can be used to combine many boolean expressions in a conditional expression.

Loops[edit]

Frequently, programmers will need to run a certain piece of code or a similar piece of code many times, or to run a certain piece of code a number of times that may depend on user input. A loop is a sequence of statements which is specified once but which may be carried out several times in succession.

Condition-controlled loops[edit]

Condition-controlled loops are loops that are controlled by a condition. They are very similar to conditional statements, but instead of executing the code if the condition is true and skipping it otherwise, they will keep running it while the condition is true, or until the condition is false. Lua has two statements for condition-controlled loops: the while loop and the repeat loop. Such loops will run code, then check if the condition is true. If it is true, then they run the code again, and they repeat until the condition is false. When the condition is false, they stop repeating the code and the program flow continues. Each execution of the code is called an iteration. The difference between while and repeat loops is that repeat loops will check the condition at the end of the loop while while loops will check it at the start of the loop. This only makes a difference for the first iteration: repeat loops will always execute the code at least once, even if the condition is false at the first time the code is executed. This is not the case for while loops, which will only execute the code the first time if the condition is actually true.

local number = 0
 
while number < 10 do
	print(number)
	number = number + 1 -- Increase the value of the number by one.
end

The code above will print 0, then 1, then 2, then 3, and so on, until 9. After the tenth iteration, number will no longer be smaller than ten, and therefore the loop will stop executing. Sometimes, loops will be meant to run forever, in which case they are called infinite loops. Renderers, software processes that draw things on the screen, for example, will often loop constantly to redraw the screen to update the image that is shown to the user. This is frequently the case in video games, where the game view must be updated constantly to make sure what the user sees is kept up-to-date. However, cases where loops need to run forever are rare and such loops will often be the result of errors. Infinite loops can take a lot of computer resources, so it is important to make sure that loops will always end even if unexpected input is received from the user.

local number = 0
 
repeat
	print(number)
	number = number + 1
until number >= 10

The code above will do exactly the same thing as the code that used a while loop above. The main differences is that, unlike while loops, where the condition is put between the while keyword and the do keyword, the condition is put at the end of the loop, after the until keyword. The repeat loop is the only statement in Lua that creates a block and that is not closed by the end keyword.

Count-controlled loops[edit]

Incrementing a variable is increasing its value by steps, especially by steps of one. The two loops in the previous section incremented the variable number and used it to run the code a certain number of times. This kind of loop is so common that most languages, including Lua, have a built-in control structure for it. This control structure is called a count-controlled loop, and, in Lua and most languages, is defined by the for statement. The variable used in such loops is called the loop counter.

for number = 0, 9, 1 do
	print(number)
end

The code above does exactly the same thing as the two loops presented in the previous section, but the number variable can only be accessed from inside the loop because it is local to it. The first number following the variable name and the equality symbol is the initialization. It is the value the loop counter is initialized to. The second number is the number the loop stops at. It will increment the variable and repeat the code until the variable reaches this number. Finally, the third number is the increment: it is the value the loop counter is increased of at each iteration. If the increment is not given, it will be assumed to be 1 by Lua. The code below would therefore print 1, 1.1, 1.2, 1.3, 1.4 and 1.5.

for n = 1, 2, 0.1 do
	print(n)
	if n >= 1.5 then
		break -- Terminate the loop instantly and do not repeat.
	end
end

The reason the code above does not go up to 2 and only up to 1.5 is because of the break statement, which instantly terminates the loop. This statement can be used with any loop, including while loops and repeat loops. Note that the >= operator was used here, although the == operator would theoretically have done the job as well. This is because of decimal precision errors. Lua represents numbers with the double-precision floating-point format, which stores numbers in the memory as an approximation of their actual value. In some cases, the approximation will match the number exactly, but in some cases, it will only be an approximation. Usually, these approximations will be close enough to the number for it to not make a difference, but this system can cause errors when using the equality operator. This is why it is generally safer when working with decimal numbers to avoid using the equality operator. In this specific case, the code would not have worked if the equality operator had been used[1] (it would have continued going up until 1.9), but it works with the >= operator.

  1. http://codepad.org/kYHPSvqx

Next Page: Functions | Previous Page: Expressions

Home: Lua programming

Blocks[edit]

A block is a list of statements that are executed sequentially. These statements can include empty statements, that do not contain any instruction. Empty statements can be used to start a block with a semicolon or write two semicolons in sequence.

Function calls and assignments may start with a parenthesis, which can lead to an ambiguity. This fragment is an example of this:

a = b + c
(print or io.write)('done')

This code could be interpreted in two ways:

a = b + c(print or io.write)('done')
a = b + c; (print or io.write)('done')

The current parser always sees such constructions in the first way, interpreting the opening parenthesis as the start of the arguments to a call. To avoid this ambiguity, it is a good practice to always precede with a semicolon statements that start with a parenthesis:

;(print or io.write)('done')

Chunks[edit]

The unit of compilation of Lua is called a chunk. A chunk can be stored in a file or in a string inside the host program. To execute a chunk, Lua first precompiles the chunk into instructions for a virtual machine, and then it executes the compiled code with an interpreter for the virtual machine. Chunks can also be precompiled into binary form (bytecode) using luac, the compilation program that comes with Lua, or the string.dump function, which returns a string containing a binary representation of the function it is given.

The load function can be used to load a chunk. If the first parameter given to the load function is a string, the chunk is that string. In this case, the string may be either Lua code or Lua bytecode. If the first parameter is a function, load will call that function repeatedly to get the pieces of the chunk, each piece being a string that will be concatenated with the previous strings. It is then considered that the chunk is complete when nothing or the empty string is returned.

The load function will return the compiled chunk as a function is there is no syntactic error. Otherwise, it will return nil and the error message.

The second parameter of the load function is used to set the source of the chunk. All chunks keep a copy of their source within them, in order to be able to give appropriate error messages and debugging information. By default, that copy of their source will be the code given to load (if code was given; if a function was given instead, it will be "=(load)"). This parameter can be used to change it. This is mostly useful when compiling code to prevent people from getting the original source back. It is then necessary to remove the source included with the binary representation because otherwise the original code can be obtained there.

The third parameter of the load function can be used to set the environment of the generated function and the fourth parameter controls whether the chunk can be in text or binary. It may be the string "b" (only binary chunks), "t" (only text chunks), or "bt" (both binary and text). The default is "bt".

There is also a loadfile function that works exactly like load, but instead gets the code from a file. The first parameter is the name of the file from which to get the code. There is no parameter to modify the source stored in the binary representation, and the third and fourth parameters of the load function correspond to the second and third parameters of this function. The loadfile function can also be used to load code from the standard input, which will be done if no file name is given.

The dofile function is similar to the loadfile function, but instead of loading the code in a file as a function, it immediately executes the code contained in a source code file as a Lua chunk. Its only parameter is used to specify the name of the file it should execute the contents of; if no argument is given, it will execute the contents of the standard input. If the chunk returns values, they will be provided by the call to the dofile function. Because dofile does not run in protected mode, all errors in chunks executed through it will propagate.

← Expressions Statements Functions →