GATE CS 2019 Q.37 (CS Q.27): Functions, Recursion, Scope and Storage Classes

Consider the following C program:

#include <stdio.h>
int r(){
    static int num = 7;
    return num--;
}

int main(){
    for (r(); r(); r())
        printf("%d", r());
    return 0;
}

Which one of the following values will be displayed on execution of the programs?

630

Official answer and solution

Official answer: B
From: Official GATE 2019 answer key

Solution

Trace the execution order of the for loop components and track the static variable.

The function r() defines a static variable num initialized to 7:

int r(){
    static int num = 7;
    return num--;
}

Because num is static, its value persists across calls. The post-decrement num-- returns the current value and then decrements num by 1.

The loop structure is:

for (r(); r(); r())
    printf("%d", r());

Let us trace each call to r() in exact execution order:

Iteration 1:

Initialization: r() is called. Returns $7$ , num becomes $6$ .
Condition check: r() is called. Returns $6$ (non-zero / true), num becomes $5$ .
Loop body: printf("%d", r()) calls r(). Returns $5$ , num becomes $4$ . Output: 5.
Step / Increment: r() is called. Returns $4$ , num becomes $3$ .

Iteration 2:
5. Condition check: r() is called. Returns $3$ (non-zero / true), num becomes $2$ .
6. Loop body: printf("%d", r()) calls r(). Returns $2$ , num becomes $1$ . Output: 2.
7. Step / Increment: r() is called. Returns $1$ , num becomes $0$ .

Termination:
8. Condition check: r() is called. Returns $0$ (zero / false). The loop terminates!
(num becomes $-1$ ).

The values printed by the program are 52.

Final answer

Common trap

Missing one of the calls to r() in the for loop header (initialization, condition, and increment each call r()).

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

Source

Paper: GATE 2019 Computer Science and Information Technology (CS)
Question: 37 of 65, printed as CS Q.27
Section: Computer Science and Information Technology

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