Lua/Loops

Lua modules based on the Scribunto/Lua extension are stored in resource pages using the Module: namespace. Each module uses a table to hold functions and variables, and that containing table is returned at the end of the module code. Loops are code structures used to repeat statements in scripts. This lesson will show you how to use loops in your scripts.

Prerequisites

This lesson assumes you have already completed the Conditions lesson.

Create a Lua Script with Loops

To create a Lua script with loops:

1. Navigate to Module:Sandbox.
2. Clear all existing code.

   It's a sandbox. Everyone is free to play in the sandbox. But if you find another user is actively editing the sandbox at the same time, you may also use Module:Sandbox/Username, where Username is your Wikiversity username.

3. Add the following code and save the page:

local p = {}
 
function p.forloop()
    local i
    local result
 
    result = ';for\n'
    for i = 2, 10, 2 do
        result = result .. ":i = " .. i .. '\n'
    end
 
    return result
end
 
function p.whileloop()
    local i
    local result
 
    result = ';while\n'
    i = 2
    while i <= 10 do 
        result = result .. ":i = " .. i .. '\n'
        i = i + 2
    end
 
    return result
end
 
function p.repeatloop()
    local i
    local result
 
    result = ';repeat\n'
    i = 2
    repeat 
        result = result .. ":i = " .. i .. '\n'
        i = i + 2
    until i > 10
 
    return result
end
 
return p

Test Your Lua Script

To test your Lua script:

1. Navigate to either the Module_talk:Sandbox page, the Wikiversity:Sandbox page, or your own user or sandbox page.
2. Add the following code and save the page:

{{#invoke:Sandbox|forloop}}
{{#invoke:Sandbox|whileloop}}
{{#invoke:Sandbox|repeatloop}}

The result should be:

for

i = 2

i = 4

i = 6

i = 8

i = 10

while

i = 2

i = 4

i = 6

i = 8

i = 10

repeat

i = 2

i = 4

i = 6

i = 8

i = 10

Understand Your Lua Script

To understand your Lua script for loop:

1. local and the following code defines the variables i and result. Both are nil.
2. result = ';for\n' assigns a string literal value to the variable result.

   This could also have been written with the initial variable declaration as local result = ';for\n'. There is no difference in functionality, only in the author's preference for coding style. Some programming languages require variables to be declared before they are used, resulting in some developers preferring to see variables declared before they are used.

3. for i = 2, 10, 2 do creates a loop code block that will vary the value of the variable i from 2 to 10 by 2.
   • For loops are used when the number of loops desired can be easily determined before the loop is entered. They are typically counting loops of some type.
   • For loops can be sequenced either in a positive or a negative direction. for i = 10, 2, -2 do would count down from 10 to 2 by 2.
   • If the increment value is not provided, the default increment is 1. for i = 1, 10 do would count from 1 to 10 by 1.
4. result = result .. ":i = " .. i .. '\n' concatenates a string literal and the current value of the variable i to result.
5. end ends the loop.

   The value of the loop variable, i in this case, will be one increment greater than the stop value when the loop ends. In this example, i will be equal to 12 after the loop ends.

6. return result returns the current value of result as the result of the function.

To understand your Lua script while loop:

1. local and the following code defines the variables i and result. Both are nil.
2. result = ';while\n' assigns a string literal value to the variable result.
3. i = 2 assigns the value 2 to the variable i.

   This could also have been written with the initial variable declaration as local i = 2.

4. while i <= 10 do creates a loop code block that will loop while the value of i is less than 10.
   • While loops are used when the number of loops desired is determined within the loop, such as repeating until the end of a string is reached. A while loop is used here to demonstrate the comparison in code structure between for, while, and repeat loops.
   • If the while loop condition is not met, the loop code block will be skipped.
   • If the while loop condition is met, the while loop condition must be altered inside the loop, or the loop will repeat forever.
5. result = result .. ":i = " .. i .. '\n' concatenates a string literal and the current value of the variable i to result.
6. i = i + 2 takes the current value of i, adds 2, and stores the result in i.
7. end ends the loop.
8. return result returns the current value of result as the result of the function.

To understand your Lua script repeat loop:

1. local and the following code defines the variables i and result. Both are nil.
2. result = ';repeat\n' assigns a string literal value to the variable result.
3. i = 2 assigns the value 2 to the variable i.
4. repeat creates a loop code block that will loop until the ending condition is true.
   • Repeat loops are used when the number of loops desired is determined within the loop, such as repeating until the end of a string is reached.
   • The difference between a while loop and a repeat loop is that the while loop may not execute if the condition is not met. A repeat loop always executes at least once.
   • If the repeat loop condition is met, the repeat loop condition must be altered inside the loop, or the loop will repeat forever.
5. result = result .. ":i = " .. i .. '\n' concatenates a string literal and the current value of the variable i to result.
6. i = i + 2 takes the current value of i, adds 2, and stores the result in i.
7. until i > 10 sets the condition that is tested to end the loop. If the condition is false, the loop repeats. If the condition is true, the loop terminates.
8. return result returns the current value of result as the result of the function.

Conclusion

Congratulations! You've now created, tested, and understood a Lua script with loops. Continue on to the Functions lesson.

See Also

• Wikipedia: For loop
• Wikipedia: While loop
• Wikipedia: Do while loop

References

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