Official PYQ · 2013Q51Arrays, Stacks and QueuesMCQ1 mark

GATE CS 2013 Q51 — Arrays, Stacks and Queues

The procedure given below is required to find and replace certain characters inside an input character string supplied in array $A$ . The characters to be replaced are supplied in array $oldc$ , while their respective replacement characters are supplied in array $newc$ . Array $A$ has a fixed length of five characters, while arrays $oldc$ and $newc$ contain three characters each. However, the procedure is flawed.

void find_and_replace (char *A, char *oldc, char *newc) {
    for (int i=0; i<5; i++)
        for (int j=0; j<3; j++)
            if (A[i] == oldc[j])
                A[i] = newc[j];
}

The procedure is tested with the following four test cases.

$oldc = “abc”, newc = “dab”$
$oldc = “cde”, newc = “bcd”$
$oldc = “bca”, newc = “cda”$
$oldc = “abc”, newc = “bac”$

If array $A$ is made to hold the string “ $abcde$ ”, which of the above four test cases will be successful in exposing the flaw in this procedure?

View worked solution

Answer: C

The flaw in the given procedure is that the inner loop continues to run even after a character replacement has occurred. If a newly replaced character at $A[i]$ matches a subsequent character in $oldc[j]$ (where $j$ is larger than the index of the first match), it undergoes a second replacement in the same pass.

Let's test the cases with the initial string $A = \text{"abcde"}$ :

Test Case 3: $oldc = \text{"bca"}$ $o l d c = "bca"$ , $newc = \text{"cda"}$ $n e w c = "cda"$
- Expected: $A[1] = \text{'b'}$ should be replaced by $\text{'c'}$ .
- Actual: At $i=1$ , $A[1] = \text{'b'}$ . For $j=0$ , $A[1]$ matches $oldc[0] = \text{'b'}$ and is replaced by $newc[0] = \text{'c'}$ . Next, for $j=1$ , the updated $A[1] = \text{'c'}$ matches $oldc[1] = \text{'c'}$ and is replaced again by $newc[1] = \text{'d'}$ . This results in an incorrect final character $\text{'d'}$ .
Test Case 4: $oldc = \text{"abc"}$ $o l d c = "abc"$ , $newc = \text{"bac"}$ $n e w c = "bac"$
- Expected: $A[0] = \text{'a'}$ should be replaced by $\text{'b'}$ .
- Actual: At $i=0$ , $A[0] = \text{'a'}$ . For $j=0$ , $A[0]$ matches $oldc[0] = \text{'a'}$ and becomes $\text{'b'}$ . For $j=1$ , the updated $A[0] = \text{'b'}$ matches $oldc[1] = \text{'b'}$ and is replaced again by $newc[1] = \text{'a'}$ . This results in an incorrect final character $\text{'a'}$ .

Both Test Cases 3 and 4 result in unintended multiple replacements, exposing the flaw in the procedure.

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Published 10 Oct 2026

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