GATE CS 2021 Set 1 Q.47 (CS Q.37): Data Types, Operators and Control Flow

Consider the following ANSI C program.

#include <stdio.h>
int main()
{
    int i, j, count;
    count = 0;
    i = 0;
    for (j = -3; j <= 3; j++)
    {
        if ((j >= 0) && (i++))
            count = count + j;
    }
    count = count + i;
    printf("%d", count);
    return 0;
}

Which one of the following options is correct?

The program will not compile successfully.

The program will compile successfully and output 10 when executed.

The program will compile successfully and output 8 when executed.

The program will compile successfully and output 13 when executed.

Official answer and solution

Official answer: B
From: Official GATE 2021 answer key

Solution

Trace the loop while carefully applying short-circuit evaluation of the logical AND operator and post-increment side effects.

Initially, count = 0 and i = 0. The loop runs for $j = -3, -2, -1, 0, 1, 2, 3$ .

In C, (j >= 0) && (i++) evaluates left-to-right with short-circuiting:

$j = -3$ : j >= 0 is false (0). The right operand (i++) is not evaluated. i remains 0.
$j = -2$ : j >= 0 is false. i remains 0.
$j = -1$ : j >= 0 is false. i remains 0.
$j = 0$ : j >= 0 is true (1). Right operand (i++) evaluates to the current value of i, which is 0 (false), and then i increments to 1. Because the condition evaluates to false, count = count + j does not execute.

$j = 1$ : j >= 0 is true. (i++) evaluates to 1 (true) and i becomes 2. Since condition is true, count = count + j = 0 + 1 = 1.
$j = 2$ : j >= 0 is true. (i++) evaluates to 2 (true) and i becomes 3. count = count + j = 1 + 2 = 3.
$j = 3$ : j >= 0 is true. (i++) evaluates to 3 (true) and i becomes 4. count = count + j = 3 + 3 = 6.

After the loop finishes, count = count + i = 6 + 4 = 10. The statement printf("%d", count); outputs 10, matching option B.

Final answer

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

Source

Paper: GATE 2021 Computer Science and Information Technology (CS, Set-1)
Question: 47 of 65, printed as CS Q.37
Section: Computer Science and Information Technology

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