Programonic

Before you start

Lua variables.

Count with a numeric for and repeat with while.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A numeric for runs from a start value to an end value, both inclusive: for i = 1, 3 visits 1, 2, and 3. The loop variable is local to the loop.

while checks its condition before each pass. Starting at 5 and subtracting 2 gives 3, then 1; at 1 the condition n > 2 is false, so the loop stops and 1 is printed.

Code example

Lua

for i = 1, 3 do
  print(i)
end

local n = 5
while n > 2 do
  n = n - 2
end
print(n)

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1for i = 1, 3 do

2 print(i)

3end

5local n = 5

6while n > 2 do

7 n = n - 2

8end

9print(n)

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
The counter runs 1, 2, 3.	`—`	—
Print each counter value.	`i = 1 → 2 → 3`	1 2 3
n = 5: the condition holds, so the body runs and n becomes 3. n = 3 holds again and n becomes 1.	`n = 5 → 3 → 1`	1 2 3
At n = 1 the condition is false, so the loop ends.	`n = 1`	1 2 3 1

Behind the scenes

How Lua processes this code

The Lua interpreter executes the chunk and resolves local values as it runs.

Cost and common mistakes

The first loop is O(3) and the second takes O(n/2) steps.

A while loop that never changes its condition runs forever.
Changing i inside a numeric for does not affect the iteration.

Try it yourself

What is printed for n if it starts at 6?

Reveal answer and reasoning

6 → 4 → 2; at 2 the condition is false, so it prints 2.

Lua Loops