GATE CS 2015 Set 3 Q.36 (CS Q.26): Pointers, Arrays and Strings

Consider the following C program.

#include <stdio.h>
int main()
{
    static int a[] = {10, 20, 30, 40, 50};
    static int *p[] = {a, a + 3, a + 4, a + 1, a + 2};
    int **ptr = p;
    ptr++;
    printf("%d%d", ptr - p, **ptr);
}

The output of the program is _____.

Your answer

Official answer and solution

Official answer: 140
From: Official GATE 2015 answer key

Solution

Trace pointer initialization, pointer arithmetic, and indirection step-by-step.

Analyze the arrays:

a = {10, 20, 30, 40, 50} has base address &a[0]:
- a points to a[0] = 10
- a + 1 points to a[1] = 20
- a + 2 points to a[2] = 30
- a + 3 points to a[3] = 40
- a + 4 points to a[4] = 50

p = {a, a + 3, a + 4, a + 1, a + 2} is an array of pointers to int:
- p[0] = &a[0]
- p[1] = &a[3]
- p[2] = &a[4]
- p[3] = &a[1]
- p[4] = &a[2]

int **ptr = p; initializes ptr to &p[0].
ptr++; advances ptr by one element, so ptr now points to p[1].
Compute expressions in printf("%d%d", ptr - p, **ptr):
- ptr - p: pointer subtraction between &p[1] and &p[0] evaluates to the index difference, which is $1$ .
- *ptr evaluates to p[1], which is a + 3 (pointing to a[3]).
- **ptr evaluates to *(a + 3) = a[3] = 40.

The format string "%d%d" outputs 1 immediately followed by 40, resulting in 140.

Final answer

140

Common trap

Inserting an unintended space between the two values, or confusing pointer difference ptr - p (which is in units of sizeof(*p), yielding 1) with byte offset.

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

Source

Paper: GATE 2015 Computer Science and Information Technology (CS) Session 3
Question: 36 of 65, printed as CS Q.26
Section: Computer Science and Information Technology

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