control flow~12 minRoute 01 6 of 12

Loops

How a computer does the same thing again, and again, and again.

Story — the courier on Main Street

Imagine a courier with a bag of letters and a row of five mailboxes, numbered 0 to 4. Their job is simple: walk to mailbox 0, drop off a letter, walk to mailbox 1, drop off a letter, and so on until every mailbox has been visited once.

That is what a loop is. The courier's position is the loop variable; the mailbox count is how many times the loop runs; the action of dropping a letter is the loop body.

Courier says

The trick is that I do not write five copies of "deliver letter." I write it once, and I do it for each mailbox.

See it — drive the loop

Press Step to advance one tick, or Play to watch it run. Notice how i changes between iterations and how the mailboxes light up one at a time.

total

Check: i (0) < N (5)? Yes — enter the body.

Speed1x

Text-only trace

N = 5, i starts at 0, total starts at 0.

Try it — fix the order

The four lines below build a working loop, but they are out of order. Move them so the program is correct.

Drag the pieces — or tab to one and use the ↑/↓ buttons — to put the program in order.

total += i
total = 0
for i in range(5):
print(total)

Hint: Initialise total before the loop. Indented lines belong inside the loop.

Code it — five languages, one idea

The same loop in five popular languages. Notice what is the same (the three jobs: start, condition, step) and what is just punctuation.

total = 0
for i in range(5):
    total += i
print(total)  # 10

Walk through it line by line

Each click of Next executes one line. Watch the variable panel on the right to see what the computer sees.

Step 1 / 13python

1total = 0
2for i in range(5):
3    total += i
4print(total)

Quiz it — make it stick

Question 1
How many times does the body of `for i in range(5)` run?
456It depends on what is inside the body
Predict — Question 2
What does this print?
```
total = 0
for i in range(3):
    total += 2
print(total)
```
Your guess
True or false — Question 3
A for loop must always run at least once.

No Dumb Questions

Real questions other learners asked on this page.

Why do programmers start counting at 0?
Habit, mostly — but a useful habit. When `i` is 0 you can use it as a direct offset into a list (the first item is at offset 0). Languages that start at 1 exist (Lua, MATLAB), and they trade arithmetic clarity for human intuition.
What is the difference between a for loop and a while loop?
A for loop is a while loop in fancy clothes. Use a for loop when you know how many times you want to run something; use a while loop when you want to keep going until some condition flips.
What happens if I never increment `i` inside a while loop?
The condition stays true forever and your program hangs. This is called an infinite loop. Most editors and runtimes will let you `Ctrl-C` your way out of one — but a deployed server will not.