Loops: do something again and again

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What a loop is

Most code runs once, top to bottom, like reading a page. A loop breaks that rule. It runs a chosen block of code again and again before moving on. You decide how many times, or you decide a condition that says when to stop.

Think of a chorus in a song. The verses play once each, but the chorus repeats. A loop is the chorus of your program: one block of lines, sung over and over, until you say enough.

The numeric for loop

The most common loop in FiveM code is the numeric for loop. It counts from a start number to a stop number and runs the block once for each value.

for i = 1, 5 do
    print(i)
end

Read it left to right. for opens the loop. i = 1 sets a counter called i and starts it at 1. The 5 is where it stops. do marks the start of the block, and end marks the finish. On each pass, i holds the current number, then Lua bumps it up by one and runs the block again. When i would pass 5, the loop ends.

There is an optional third number, called the step. It controls how much the counter jumps each time. Leave it out and the step is 1.

for i = 0, 10, 2 do
    print(i)
end

That counts 0, 2, 4, 6, 8, 10, stepping by 2. A negative step counts downward:

for i = 5, 1, -1 do
    print(i)
end

That counts 5, 4, 3, 2, 1. So the shape is for counter = start, stop, step do, and the step is the only part you are allowed to drop.

See it run

No server needed. Edit the Lua below and press RUN to execute it in your browser on real Lua 5.4. Change the start and stop numbers and watch the output change.

Press RUN to execute.

The .. joins two strings end to end, so "Count: " .. i glues the word Count: onto whatever number i holds right now. You should see this:

Five lines, one per iteration. You wrote one print and got five outputs. That is the loop earning its keep.

The while loop

A for loop is for when you know the count up front: one to five, ten down to one. Sometimes you do not know the count. You only know a condition that should keep the loop going. That is the while loop.

while condition do
    -- code that runs while condition is true
end

while checks the condition first. If it is true, it runs the block, then checks again. The moment the condition is false, it stops and the program moves on. The key responsibility is yours: something inside the block has to eventually make that condition false, or the loop never stops.

Here is a counter that climbs to 3 and then quits. Press RUN.

Press RUN to execute.

Trace it. count starts at 1. The condition count <= 3 is true, so it prints Tick 1, then count = count + 1 makes count 2. True again, prints Tick 2, count becomes 3. Still true, prints Tick 3, count becomes 4. Now 4 <= 3 is false, so the loop stops and Done prints. The line count = count + 1 is what makes the condition eventually fail. Delete it and the loop would run forever.

You can also stop a loop early from the inside with break. It quits the loop immediately, no matter what the condition says.

local count = 1
while true do
    print(count)
    if count >= 3 then
        break
    end
    count = count + 1
end

Here the condition is literally true, which would normally never stop. The break inside the if is the real exit. When count reaches 3, break fires and the loop ends. A while true loop is only safe when something inside it is guaranteed to break or, on a server, to pause.

The one rule that matters later: never loop forever without a pause

This is the most important idea in the lesson, so read it twice. A loop with no exit, like while true do with no break and no pause, will run forever. The computer does nothing else while a loop is running. It is too busy spinning through the block to draw the screen, answer the network, or react to anything. So an endless loop with no pause freezes the whole program.

In the browser sandbox, an endless loop just hangs the tab. Annoying, but you can close it. On a real FiveM server it is far worse: an endless loop in your script freezes the entire server for every player connected to it.

The fix on a real server is to give the loop a short pause each time around so the server can breathe and do its other work between passes. You do not need to write this yet, but here is what it looks like, so it is not a surprise later:

Citizen.CreateThread(function()
    while true do
        -- do a little work
        Citizen.Wait(1000) -- pause for 1000 ms, then loop again
    end
end)

The Citizen.Wait(1000) line tells the server "rest for one second, then come back to me". That single pause is the difference between a smooth server and a frozen one. You will learn exactly how this works in Track B. For now, just lock in the rule.

A quick tool you will need: the remainder

Before the exercise, one small operator. The % sign gives you the remainder left over after a division. 10 % 2 is 0, because 10 splits into 2 with nothing left over. 7 % 2 is 1, because 7 splits into two threes with 1 left over. The handy trick: a number is even exactly when dividing it by 2 leaves nothing behind, so n % 2 == 0 is true for even numbers and false for odd ones. You will use that check in a moment.

Common beginner mistakes

Try it yourself

What you can do now

• Explain what a loop is and why it beats copy and paste: one block, run many times.
• Write a numeric for loop with a start, a stop, and an optional step, and know the stop value is included.
• Write a while loop that repeats on a condition, and make sure something inside it eventually makes the condition false.
• Use break to leave a loop early, and read why a while true loop needs a break or a pause.
• State the one rule: never loop forever without an exit or a pause, because an endless loop freezes everything.

You can now make code repeat, and you know the trap that turns a useful loop into a frozen server. Next you will learn to package code so you can name it once and reuse it anywhere, instead of rewriting the same lines in every file. That is the next lesson, "Functions: reuse your code".

REFERENCES

Lua 5.4 reference manual ↗