ITPEC FE Subject B April 2024 Question 4

Identifying digit-extraction operations: num mod 10 gives the last digit, integer part of (num ÷ 10) removes it.

The function sumDigits adds up all digits of a non-negative integer. Each loop iteration must:

1. Extract the last digit using num mod 10 and add it to sum
2. Remove the last digit using integer part of (num ÷ 10)

Trace with num = 123:

Loop ends (num = 0), returns 6. Correct: 1 + 2 + 3 = 6.

So A = sum + num mod 10, B = integer part of (num ÷ 10) → (b).

Why not others:
- (a) — B is num mod 10, which takes the last digit instead of removing it — num never shrinks, infinite loop
- (c, d) — sum × 10 builds a number from digits (like reversing), not summing them
- (e, f) — A uses integer part of (num ÷ 10) which grabs all digits except the last — wrong extraction
- (g, h) — combine both errors: sum × 10 (number building) and integer part of (num ÷ 10) in A

Key rule: mod 10 extracts the last digit; integer part of (÷ 10) removes it — these two always work as a pair for digit-by-digit processing.

AI-generated — may contain errors