Loops

Contents

[edit] Loops

[edit] What is a Loop?

A loop is a chunk of code that is executed multiple times. There are three types of loops: for, while, and repeat. Each type loops a block of code, but in different ways. You will see the differences between each of the loops later on. The reason why one would use a loop is that loops are great ways to not repeat the same code over and over. Let's dive in!

Avoid using infinite loops that do not have a delay between iterations. This kind of loop can freeze your game.

[edit] While

The while loop will evaluate the condition to see if it is true or false. If it is false, the loop will end. If it is true, the body of the loop after the 'do' statement will be executed, and the true/false condition will be reevaluated afterward.

Whileflowchart.JPG

Example
local i = 1
while i < 10 do
   print(i.." < 10")
   i = i + 1
end
print (i.." = 10")
1 < 10
2 < 10
3 < 10
4 < 10
5 < 10
6 < 10
7 < 10
8 < 10
9 < 10
10 = 10


As you can see in the above script, as long as i is less than 10, it will print the statement that i < 10. Once i has been incremented to a value equal to 10 (namely, 10), the while loop will end, and the final line will print that i=10.

[edit] For

The for loop is a way of running a command or set of commands a set number of times. The basic syntax is as following:

for iterator_variable = start value, end value, increment do

However, there is also a more complicated style of the for loop called the Generic For Loop, but for now we will just discuss the basics.

Keep in mind that as long as the iterator is between the start and end values, the code following the 'do' statement and before the 'end' will be executed.

ForFlowchart.jpg

For example, if you know you want to print "Hello Mom!" ten times, then you can use the for loop.

Example
for i=1, 10 do   	
   print("Hello Mom!")
end
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!
Hello Mom!


In the for loop, you see two numbers: 1, which is the starting value, and 10, which is the ending value. The loop will run from 1 to 10, and print "Hello Mom!" once per each number between 1 and 10 -- 10 times.

Lua will assume you are going to be adding positive numbers. If you want to get fancy, such as subtracting numbers, or adding decimals, you have to specify this as follows:

Example
for i=10,1, -1 do   	
   print(i)
end
10
9
8
7
6
5
4
3
2
1


Notice that we have specified that we want to count downwards from 10 to 1, and we are subtracting 1 number every time.

Another example, but with decimals:

Example
for i=1,10, .5 do   	
   print(i)
end
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10


This will count upwards from 1 to 10 by halves.

[edit] Repeat

A repeat ... until statement will repeat until a certain condition is met. The body is executed at least once, because the test is performed after the body (i.e., "the process is preceding the decision").

Repeatloop.JPG

Example
local i = 1
repeat
   print(i,"< 10")
   i = i + 1
until i==10
print (i, "=10")
1	< 10
2	< 10
3	< 10
4	< 10
5	< 10
6	< 10
7	< 10
8	< 10
9	< 10
10	=10


This will print i < 10 until i has reached the value of 10, at which point it will print that i = 10.

[edit] Break

If you have a while, a for, or a repeat loop that otherwise won't end, you can program it to end with the break command, so you can continue with the next part of code:

Example
while wait() do
   print("hi mom!")
   break -- this forces the endless loop to end
end 
 
for i = 1, math.huge do -- math.huge is infinite or we can use the number 1000000000
   print("hi mom!")
   wait()
   break -- this forces the ridiculously long loop to end
end


These loops only run once because of the break command, and print "Hi mom" once.

Example
local i = 1
repeat
   print(i,"< 10")
   i = i - 1
   if i == -5 then break end -- this forces the otherwise neverending loop to end
until i==10
print (1, "minus one will always be less than 10")


Notice in these three loops, something is wrong -- the loops would either take too long to end, or are neverending. We have to force them to end with break.

[edit] See Also

Programming in Lua: While

Programming in Lua: Repeat

Programming in Lua: For

[edit] Practice

Exercise

Instructions

Using what you learned in this tutorial, make a script that outputs the song “99 Bottles of Soda” to the console.

Solution
Select
--this function is optional - it just makes it so that it's "2 bottles" and "1 bottle"
function bottleString(number)
	if number == 1 then
		return "bottle"
	else
		return "bottles"
	end
end
 
 
for bottles = 99, 1, -1 do
	local noun = bottleString(bottles)
	songLine = bottles .. " " .. noun .. " of soda on the wall, " .. bottles .. " " .. noun .. " of soda!"
	print(songLine)
 
	bottles = bottles - 1
	noun = bottleString(bottles)
 
	songLine = "Take one down, pass it around, " .. bottles .. " " .. noun .. " of soda on the wall!"
	print(songLine)
	print("") -- adds whitespace to separate each verse.
end