Documentation › Tutorials › Lua Scripting

Loops

Loops allow you to execute the same code without rewriting it over and over.  This helps structure your program.  It's important to understand how while and for loops work in Lua, so let's jump right in. Hold onto your hats because things are about to get...loopy!

While Loops


The while loop is a statement that keeps on looping over its code until a certain condition is met. The syntax of a while loop is as follows:
while condition do
--code to be executed
end

The example below can be pasted into your "Main.lua" script file to see how while loops work.  When you run this example (select the Game > Debug menu item in the script editor menu) it will keep looping until the space key is pressed.  Even clicking on the game window won't close it because you have to press Space to exit the loop:

--Create a window
window = Window:Create()
--Create a rendering context
context = Context:Create(window)
--While loop
while window:KeyHit(Key.Escape)==false do
        --Set the drawing color and clear the screen
        context:SetColor(0,0,0)
        context:Clear()
        --Set the drawing color and blend mode
        context:SetColor(1,1,1)
        context:SetBlendMode(Blend.Alpha)
        --Draw some text onscreen
        context:DrawText("PRESS ESCAPE TO END THE PROGRAM.",2,2)
        --Update the screen
        context:Sync()
end

For Loops


The for loop looks a lot like the while loop, but it's a little fancier.  A for loop has a defined starting and ending value.  The loop will be performed a certain number of times until the ending value is reached.. If you want to do something ten times you can use a for loop instead of writing the same code over and over again.

A for loop has the following syntax:

for startValue, EndValue, [increments] do
	--code to execute
end

The increments value is optional.  If it isn't defined, it is assumed to be "1", meaning that the value gets one added to it at the end of each loop.  Paste the code below into the "Main.lua" and run the game to see how it works:

--For loop
for n=1,10 do
        --Print out a string
        print("Loop iteration: "..n)
end

When run, the program above will print out the following text:
Quote
Loop iteration: 1
Loop iteration: 2
Loop iteration: 3
Loop iteration: 4
Loop iteration: 5
Loop iteration: 6
Loop iteration: 7
Loop iteration: 8
Loop iteration: 9
Loop iteration: 10

If we add an increment value we can customize the way the loop is incremented.  By setting the increment value to "2" we can print every other number from 1 to 10:

--For loop
for n=1,10,2 do
	--Print out a string
	System:Print("Loop iteration: "..n)
end

Here is the printed output of the program:
Quote
Loop iteration: 1
Loop iteration: 3
Loop iteration: 5
Loop iteration: 7
Loop iteration: 9

We can even use the increment value to decrease the starting value each loop.  In that case, we would want to start value to be greater than the ending value, or else the loop would continue forever.  The following code will iterate backwards from 10 to one, subtracting one from the n variable each iteration:
--For loop

for n=10,1,-1 do
	--Print out a string
	System:Print("Loop iteration: "..n)
end

Here is the printed output of the above program:
Quote
Loop iteration: 10
Loop iteration: 9
Loop iteration: 8
Loop iteration: 7
Loop iteration: 6
Loop iteration: 5
Loop iteration: 4
Loop iteration: 3
Loop iteration: 2
Loop iteration: 1

Break


You can exit a loop early at any time with the break keyword.  The example below will iterate through a loop until the fifth iteration, at which point it will encounter the break command and exit the loop early.

--For loop
for n=1,10 do
	--Exit loop when 5 is reached
	if n==5 then
		break
	end
	--Print out a string
	print("Loop iteration: "..n)
end

The printed output of this program looks like this:
Quote
Loop iteration: 1
Loop iteration: 2
Loop iteration: 3
Loop iteration: 4

Conclusion


Loops are an important language used to structure code.  In fact your game itself is running in one big loop inside the script interpreter.  Every programming language in existence uses some variation of this idea.