GATE CS 2017 Set 1 Q.53 (CS Q.53): Data Types, Operators and Control Flow

Consider the following C program.

#include <stdio.h>
#include <string.h>

void printlength(char *s, char *t) {
    unsigned int c = 0;
    int len = ((strlen(s) - strlen(t)) > c) ? strlen(s) : strlen(t);
    printf("%d\n", len);
}

void main() {
    char *x = "abc";
    char *y = "defgh";
    printlength(x, y);
}

Recall that strlen is defined in string.h as returning a value of type size_t, which is an unsigned int. The output of the program is ______.

Your answer

Official answer and solution

Official answer: 3
From: Official GATE 2017 answer key

Solution

Evaluate the subtraction of two size_t unsigned values and trace the condition in the ternary operator.

The strings passed to printlength are $s = \text{"abc"}$ and $t = \text{"defgh"}$ .
Their lengths are:

\text{strlen}(s) = 3

\text{strlen}(t) = 5

The function strlen returns a value of type size_t, which is an unsigned integer type.
Consider the expression:

\text{strlen}(s) - \text{strlen}(t) = 3 - 5

Because both operands are unsigned, the subtraction is performed in unsigned modular arithmetic. In 32-bit unsigned arithmetic:

3 - 5 = 2^{32} - 2 = 4294967294

Next, this large unsigned integer is compared with unsigned int c = 0:

((4294967294) > 0)

This comparison evaluates to TRUE (1).

The ternary operator selects the true branch:

\text{len} = \text{strlen}(s) = 3

Thus, printf("%d\n", len) prints 3.

Final answer

3.0

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 1
Question: 53 of 65, printed as CS Q.53
Section: Computer Science and Information Technology

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