ITPEC FE Subject B April 2026 Question 13

The blank must read decrease i from atIndex - 1 to index.

This loop collects the name part, which lies to the left of the "@", and it builds the string with prepend:

• •start one position before the "@", at atIndex - 1, because the "@" itself is not in validChars
• •walk backwards toward index, the point where the previous email ended
• •each valid character is prepended, so scanning right to left restores the original order
• •the first invalid character, such as a space, ends the name

For "Contact us at [email protected]" the walk starts at the "e" of doe and stops at the space before john.

Answer (a)

Why not others:
- (b) — starts at atIndex, which is the "@" itself; it is not in validChars, so the loop exits at once and candidate stays empty, meaning no email is ever collected
- (c) and (d) — walk forwards while still prepending each character, which reverses the name; they also stop at the first invalid character encountered from the left, cutting the name short or missing it entirely

Key rule: Match the direction of travel to the accumulation. prepend while moving backwards preserves order; append while moving forwards does the same. Mixing them reverses the result.

AI-generated — may contain errors