Control structures are an important feature of programming languages because they enable you to direct the flow of the program based on conditions that are often established dynamically as the program is running. Different languages provide different controls, and in Lua there's the while loop, for loop, and repeatuntil loop. This article covers the while and repeat until loops. Because of their flexibility, I cover for loops in a [separate article][1].
A condition is defined by an expression using an operator, which is a fancy term for symbols you may recognize from math classes. Valid operators in Lua are:
-`==` equal to
-`~=` not equal to
-`<` less than
-`>` greater than
-`⇐` less than or equal to
-`>=` greater than or equal to
Those are known as relational operators because they prompt an investigation of how two values relate to one another. There are also logical operators, which mean the same as they mean in English and can be incorporated into conditions to further describe the state you want to check for:
-`and`
-`or`
Here are some example conditions:
-`foo > 3`: Is the variable `foo` is greater than 3? The `foo` must be 4 or more to satisfy this condition.
-`foo >= 3`: Is `foo` greater than or equal to 3? The `foo` must be 3 or more to satisfy this condition.
-`foo > 3 and bar < 1`: Is `foo` greater than 3 while `bar` is less than 1? For this condition to be true, the `foo` variable must be 4 or more at the same moment that `bar` is 0.
-`foo > 3 or bar < 1`: Is `foo` greater than 3? Alternately, is `bar` less than 1? If `foo` is 4 or more, or `bar` is 0, then this condition is true. What happens if `foo` is 4 or more while `bar` is 0? The answer appears later in this article.
### While loop
A while loop executes instructions for as long as _some condition_ is satisfied. For example, suppose you're developing an application to monitor an ongoing zombie apocalypse. When there are no zombies remaining, then there is no more zombie apocalypse:
```
zombie = 1024
while (zombie > 0) do
print(zombie)
zombie = zombie-1
end
if zombie == 0 then
print("No more zombie apocalypse!")
end
```
Run the code to watch the zombies vanish:
```
$ lua ./while.lua
1024
1023
[...]
3
2
1
No more zombie apocalypse!
```
### Until loop
Lua also has a repeat until loop construct that's essentially a while loop with a "catch" statement. Suppose you've taken up gardening and you want to track what's left to harvest:
```
mytable = { "tomato", "lettuce", "brains" }
bc = 3
repeat
print(mytable[bc])
bc = bc - 1
until( bc == 0 )
```
Run the code:
```
$ lua ./until.lua
brains
lettuce
tomato
```
That's helpful!
### Infinite loops
An infinite loop has a condition that can never be satisfied, so it runs infinitely. This is often a bug caused by bad logic or an unexpected state in your program. For instance, at the start of this article, I posed a logic puzzle. If a loop is set to run until `foo > 3 or bar < 1`, then what happens when `foo` is 4 or more while `bar` is 0?
Here's the code to solve this puzzle, with a safety catch using the `break` statement just in case:
```
foo = 9
bar = 0
while ( foo > 3 or bar <1)do
print(foo)
foo = foo-1
-- safety catch
if foo <-800000then
break
end
end
```
You can safely run this code, but it does mimic an accidental infinite loop. The flawed logic is the `or` operator, which permits this loop to continue both when `foo` is greater than 3 and when `bar` is less than 1. The `and` operator has a different effect, but I leave that to you to explore.
Infinite loops actually do have their uses. Graphical applications use what are technically infinite loops to keep the application window open. There's no way of knowing how long your user intends to use the application, so the program runs infinitely until the user selects **Quit**. The simple condition used in these cases is one that is obviously always satisfied. Here's an example infinite loop, again with a safety catch built in for convenience:
```
n = 0
while true do
print(n)
n = n+1
if n > 100 then
break
end
end
```
The condition `while true` is always satisfied because `true` is always true. It's the terse way of writing `while 1 == 1` or something similarly eternally true.
### Loops in Lua
As you can tell from the sample code, although there are different implementations, loops all basically work toward the same goal. Choose the one that makes sense to you and that works best with the processing you need to perform. And just in case you need it: The keyboard shortcut to terminate a runaway loop is **Ctrl+C**.
