GATE CS 2017 Set 2 Q.55 (CS Q.55): Pointers, Arrays and Strings

Consider the following C program.

#include <stdio.h>
#include <string.h>
int main() {
    char* c = "GATECSIT2017";
    char* p = c;
    printf("%d", (int) strlen(c + 2[p] - 6[p] - 1));
    return 0;
}

The output of the program is

Your answer

Official answer and solution

Official answer: 2
From: Official GATE 2017 answer key

Solution

Apply pointer arithmetic and the array subscript identity i[p] == *(p + i) == p[i] in C to determine the target string passed to strlen.

Given string literal c = "GATECSIT2017" and pointer p = c.
The characters and their indices in c are:

c[0] = 'G'
c[1] = 'A'
c[2] = 'T'
c[3] = 'E'
c[4] = 'C'
c[5] = 'S'
c[6] = 'I'
c[7] = 'T'
c[8] = '2'
c[9] = '0'
c[10] = '1'
c[11] = '7'

Evaluate the pointer expression c + 2[p] - 6[p] - 1:

By the commutative property of array indexing in C, 2[p] is equivalent to p[2], which is the character 'T'.
Similarly, 6[p] is equivalent to p[6], which is the character 'I'.
Using standard ASCII values: 'T' = 84 and 'I' = 73.
The difference is: $2[p] - 6[p] = 'T' - 'I' = 84 - 73 = 11$
Thus, the offset expression is: $2[p] - 6[p] - 1 = 11 - 1 = 10$
The pointer passed to strlen is c + 10, which points to the substring "17".
strlen("17") counts the two characters '1' and '7', returning 2.

Final answer

Common trap

Assuming 2[p] is invalid syntax in C; in standard C, E1[E2] is defined identically as *((E1) + (E2)).

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

Source

Paper: GATE 2017 Computer Science and Information Technology (CS) Session 2
Question: 55 of 65, printed as CS Q.55
Section: Computer Science and Information Technology

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