ITPEC FE Subject B October 2025 Question 12

Topic: String Processing

From the answer group below, select the correct combination of answers to be inserted into A and B in the program. Here, the array index starts at 1.

The function containsSubstring receives two-character arrays str and seq as arguments, and returns a boolean value indicating whether the character array str contains the sequence seq as a substring. The character array str contains seq as a substring if every sequence of consecutive characters in seq exists somewhere in the sequence of characters in str, in the same order. For example, containsSubstring({"a", "p", "p", "l", "e"}, {"p", "l", "e"}) returns true, and containsSubstring({"a", "p", "p", "l", "e"}, {"a", "e"}) returns false. Assume that the number of elements in str and seq is one or more.

[Program]
○ boolean: containsSubstring(character []: str, character []: seq)
integer: strlen, seqlen, len, i, j
strlen ← the number of elements in str
seqlen ← the number of elements in seq
if (seqlen > strlen)
return false
endif
len ← strlen - seqlen + 1
for (increase i from 1 to len by 1)
j ← 1
while (j ≤ seqlen)
if (A ≠ seq[j])
exit the while block
endif
j ← j + 1
endwhile
if (B)
return true
endif
endfor
return false

Answer group

OptionAB

Explanation

A: str[i + j - 1], B: j = seqlen + 1 — line up character j of seq with position i + j - 1 of str, and report a match only when the while loop ran past its last character.

i is the position in str where a candidate substring starts, from 1 to len = strlen - seqlen + 1, the last start that still fits. Character j of seq must therefore be compared with str at i + j - 1: for j = 1 that is str[i] itself, and at the last start with j = seqlen it is str[strlen], the final element.

The while loop ends in one of two ways. A mismatch exits the block with j still ≤ seqlen. If every character matches, j is incremented once more after the last comparison, and the loop ends with j = seqlen + 1. Only that value proves a full match.

Trace containsSubstring({"a", "p", "p", "l", "e"}, {"p", "l", "e"}), where len = 3. At i = 1, "a" ≠ "p", exit with j = 1. At i = 2, "p" matches, then str[3] = "p" ≠ "l", exit with j = 2. At i = 3, "p", "l" and "e" all match, j reaches 4 = seqlen + 1, and the function returns true.

Answer (e)

Why not others:

  • •(a), (b) and (c) — str[i + j] is shifted one place right: str[1] is never compared, and at i = len, j = seqlen it reads str[strlen + 1], past the end of the array
  • •(d) — j = seqlen is also what a mismatch on the last character leaves. For seq = {"a", "e"}: at i = 1, "a" matches, then "p" ≠ "e" exits with j = 2 = seqlen, so it returns true where the question says false
  • •(f) — j = seqlen - 1 is left by a mismatch on the second-to-last character, and a full match never satisfies it. For seq = {"a", "e"} it returns true at i = 2, where "p" ≠ "a" exits with j = 1

Key rule: When a loop can end either by a break or by its own condition, test the loop variable for the value only the natural exit leaves (bound + 1), and check window indexes at both ends of their range.

String Processing · AI-written, may contain errors

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