GATE CS 2024 Set 2 Q.36: Pointers, Arrays and Strings

What is the output of the following C program?

#include <stdio.h>

int main() {
    double a[2]={20.0, 25.0}, *p, *q;
    p = a;
    q = p + 1;
    printf("%d,%d", (int)(q - p), (int)(*q - *p));
    return 0;}

4,8

1,5

8,5

1,8

Official answer and solution

Official answer: B
From: Official GATE 2024 answer key

Solution

Use C pointer arithmetic rules: subtracting two pointers to elements of the same array yields the index difference (not byte difference).

p = a points to a[0] (20.0), and q = p + 1 points to a[1] (25.0).
In C, subtracting two pointers of the same type (q - p) returns the number of array elements between them (scaled by sizeof(double)), so (int)(q - p) = 1.
Dereferencing gives *q - *p = 25.0 - 20.0 = 5.0, so (int)(*q - *p) = 5.

Thus printf outputs 1,5.

Final answer

Option (B)

Common trap

Multiplying q - p by sizeof(double) = 8 to get 8,5.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2024 Computer Science and Information Technology - Set 2 (CS2)
Question: 36 of 65
Section: Computer Science and Information Technology

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