Loop bugs

301.10 Iteration

Iteration — Note 4 of 4

Fix infinite + off-by-one loops.

What you need to know

An infinite loop runs forever because the condition never becomes False. An off-by-one error means the loop runs one too many or one too few times. Both are common bugs.

What to know

Infinite loop: condition never becomes False
Fix: ensure the loop variable changes each iteration
Off-by-one: range(1, 5) gives 1-4, not 1-5
Use range(1, 6) to include 5
Test with small values to verify loop count

Code examples

Off-by-one fix

python

# Bug: prints 1 to 4 (misses 5)
for i in range(1, 5):
    print(i)

# Fix: prints 1 to 5
for i in range(1, 6):
    print(i)

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Specification notes for programming are in Paper 2.