Control Structures

Control Structures define which way a program flows. Lua provides a simple set of generic control structures seen in most languages today:
if, then, else, elseif
while
repeat
for
break
return
Each of these control structures determines the way your code flows in your program. Each one besides return, break, and repeat are terminated by the end keyword.
if, then, else, elseif
An if statement tests the condition given and determines whether or not to move forward, if the condition is truthy it executes the code given in the first if block. Otherwise, if there is an elseif statement, that condition is also tested and has the exact same behavior as an if statement. If none of those conditions evaluate to true, the else block is executed. Each if statement should be accompanied by an end to terminate the control structure, you should not place them for any else or elseifs you may place. Each statement may only contain one if, any number of elseifs, one else, and finally one end.
A general if statement:
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if condition then
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    print('1st condition was true')
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end
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If our condition was truthy (any value that isn't false or nil), the print will be executed. Otherwise, nothing will happen.
An example using all keywords:
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local x = 50
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local y = 10
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if (x - y) < 10 then
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    print('1st condition was true')
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elseif (x - y) < 20 then
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    print('2nd condition was true')
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else
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    print('none of the conditions were true')
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end
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In this example, all conditions evaluate to false because x - y is 40, which is not less than 10 nor 20, resulting in the last print being executed, since the else keyword is a fallback in the case that no other conditions are truthy.
while
A while loop will continuously execute its body as long as the condition provided is truthy. The condition is evaluated each time before executing the body again.
Structure of a while loop:
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while condition do
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    -- body
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end
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More information & examples are covered in the Loops page.
repeat
A repeat loop will also continuously execute its body as long as the condition provided is truthy. The difference here is that the condition is evaluated after the body runs, not before. So any repeat loop will always execute at least once.
Structure of a repeat loop:
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repeat
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    -- body
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until condition
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More information & examples are covered in the Loops page.
for
There are two different kinds of for loops, a numeric for, or a generic for. These also execute their bodies until the loop is terminated.
A numeric for is ran for a specific amount of times, the loop variable starts at the number provided for start, and stops at the number provided for stop. The loop variable gets incremented by the stepeach iteration, the default being 1.
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for var = start,stop,step do
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    -- body
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end
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A generic for loop traverses all values returned by an iterator (a function which provides a way to traverse a container, or a specific set of values)
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for <loop vars> in iterator(<args>) do
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    -- body
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end
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More information & examples are covered in the Loops page.
break
A break will instantly halt execution and break any loop, and will not be able to return back to the loop.
More information & examples are covered in the Loops page.
return
A return gives back any number of values from a function, it works like a break in the sense that it halts execution of the function and can't continue onward at any point.
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function abc()
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    return 1, 2, 3
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end
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​
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local a, b, c = abc() -- a = 1, b = 2, c = 3
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In this single return, 3 values are returned and can be unpacked into variables, or a table if you have no idea how many values will be returned, like this:
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function abc(arg)
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    if arg then
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        return 1, 2, 3
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    end
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    return 1, 2, 3, 4, 5
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end
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​
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local args = {abc(true)} -- {1, 2, 3}
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args = {abc(false)} -- {1, 2, 3, 4, 5}
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Here, the default return value is 1, 2, 3, 4, 5, if the condition to the if statement is false, the default is used. However, if arg evaluates to true in the if statement, 1, 2, 3 will be returned and not 1, 2, 3, 4, 5, since return breaks out of the function, the rest of the code will not be executed.
Since Lua automatically assigns indexes like previously discussed, unpacking it like this creates an array the same way you do it manually, which lets you use a different way to get all returned values of a function if you don't specifically know how many or what values it's returning.
If you still use variables to unpack the return values into, they are unpacked in order, so even if you have 3 variables and 5 return values, it will use the first 3 return values to assign those variables in sequence:
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function abc(arg)
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    if arg then
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        return 1, 2, 3
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    end
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    return 1, 2, 3, 4, 5
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end
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​
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local x, y, z = abc(false) -- returns 1, 2, 3, 4, 5 | x,y,z are 1,2,3 respectively
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if, then, else, elseif