ITPEC FE Subject A April 2024 Question 37

24, 25, 49, 50 — test each edge of the valid range together with the nearest invalid value just outside it.

For integers, 25 ≤ x < 50 means the valid values are 25 to 49.

• •Lower edge: 25 is the smallest valid value and 24 the largest invalid value below it
• •Upper edge: 49 is the largest valid value and 50 the smallest invalid value above it

Answer (a)

Why not others:

• •(b) 24, 25, 50, 51 — treats 50 as if it were valid; since x < 50 excludes 50, the valid edge is 49 and 51 is not next to any boundary
• •(c) 25, 26, 49, 50 — misses 24, the invalid value just below the lower edge; 26 is an interior value
• •(d) 25, 26, 50, 51 — misses both invalid neighbours on the lower side and the valid edge 49 on the upper side

Key rule: Turn the condition into the actual first and last valid values before choosing boundary tests: with a strict inequality, the last valid integer is one less than the limit.

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