ITPEC FE Subject B October 2025 Question 12

The function slides a window of length seqlen across str. For each starting position i, it compares characters one by one using index j from 1 to seqlen.

• •Blank A — `str[i + j - 1]`: Since both arrays are 1-indexed and j starts at 1, the corresponding position in str is i + j - 1. At i=1, j=1 this gives str[1], correctly aligning the first character.
• •Blank B — `j = seqlen + 1`: The while loop runs while j ≤ seqlen. If all characters match, j increments past seqlen to seqlen + 1, and the loop exits naturally. If a mismatch occurs, exit while fires with j ≤ seqlen. So j = seqlen + 1 is the success condition.

Why not others:
- str[i + j] (options a–c) — off by one; skips the first character at each window position
- j = seqlen (options a, d) — j equals seqlen during the last comparison before incrementing, so this is true even mid-comparison, not a reliable success signal
- j = seqlen - 1 (options c, f) — too early; would trigger before all characters are compared

Key rule: In 1-indexed pseudocode, when iterating j from 1 inside a window starting at i, the mapped index is i + j - 1. A while loop's natural exit means the loop variable exceeded the bound — check for bound + 1, not bound.

AI-generated — may contain errors