Iteration: FOR, WHILE and REPEAT loops

Iteration means repeating a set of instructions. It's what makes computers useful: they never get bored doing the same thing a million times.

There are three kinds of loop. In the exam you must write them in pseudocode; in real programs you'll use Python. This lesson shows both side by side.

1. Count-controlled loop: FOR

Use it when you know how many times to repeat before the loop starts.

FOR Count ← 1 TO 5
    OUTPUT Count * Count
NEXT Count

for count in range(1, 6):
    print(count * count)

Both print 1, 4, 9, 16, 25. Watch out: Python's range(1, 6) stops before 6.

You can count in steps, and backwards:

FOR Count ← 10 TO 0 STEP -2
    OUTPUT Count
NEXT Count

2. Pre-condition loop: WHILE

The condition is checked before each repeat. If it's FALSE at the start, the loop body never runs.

Total ← 0
INPUT Number
WHILE Number <> -1 DO
    Total ← Total + Number
    INPUT Number
ENDWHILE
OUTPUT Total

total = 0
number = int(input())
while number != -1:
    total = total + number
    number = int(input())
print(total)

This keeps adding numbers until the user types −1 (a rogue value that means "stop").

3. Post-condition loop: REPEAT … UNTIL

The condition is checked after each repeat, so the body always runs at least once. The loop stops when the condition becomes TRUE.

REPEAT
    OUTPUT "Enter your PIN"
    INPUT PIN
UNTIL PIN = 1234

Python has no REPEAT … UNTIL, so we use while True: with break:

while True:
    pin = int(input("Enter your PIN: "))
    if pin == 1234:
        break

Which loop should I use?

WHILE repeats while the condition is TRUE. UNTIL repeats until the condition is TRUE. They're opposites, so the condition flips when you swap one for the other.

Key terms