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C GATE Practice Set

Mixed GATE-style questions on C programming. Predict the output of each program, then check your answer.

GATE tests C mainly by showing a short program and asking what it prints. These questions mix every topic from this track: operators, control flow, recursion, static variables, pointers, arrays, strings, structures and function pointers.

How to practise:

  1. Read the program carefully and trace it on paper, writing down the value of every variable as it changes.
  2. Type or choose your answer and press Check answer.
  3. Open the explanation even when you were right; it often shows a faster way to reason.

Every program on this page has been compiled and run with GCC, and its real output is the stated answer. The questions assume a typical 64-bit system with 4-byte int and 8-byte pointers.

GATE-styleNAT · numerical answer

[static, recursion] What does the following program print?

#include <stdio.h>
int f(int n) {
    static int i = 1;
    if (n >= 5) return n;
    n = n + i;
    i++;
    return f(n);
}
int main(void) {
    printf("%d\n", f(1));
    return 0;
}
Show answer and explanationAnswer: 7

i is static, so it keeps growing across the recursive calls. f(1): n = 1 + 1 = 2, i = 2. f(2): n = 2 + 2 = 4, i = 3. f(4): n = 4 + 3 = 7, i = 4. f(7): 7 ≥ 5, so it returns 7.

GATE-styleNAT · numerical answer

[2-D arrays and pointers] What does the following program print?

#include <stdio.h>
int main(void) {
    int a[3][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
    printf("%d\n", *(*(a + 2) + 1) - **(a + 1) + *(a[0] + 2));
    return 0;
}
Show answer and explanationAnswer: 7

*(*(a + 2) + 1) is a[2][1] = 8, **(a + 1) is a[1][0] = 4 and *(a[0] + 2) is a[0][2] = 3. So 8 − 4 + 3 = 7.

GATE-styleMCQ · one correct answer

[comma operator] What does the following program print?

#include <stdio.h>
int main(void) {
    int x = (1, 2, 3);
    int y = 0;
    y = x++, x += 10;
    printf("%d %d\n", x, y);
    return 0;
}
Show answer and explanationAnswer: A

In (1, 2, 3) the comma operator evaluates each operand and gives the last, so x = 3. In y = x++, x += 10; assignment binds tighter than the comma, so it means (y = x++), (x += 10): y gets 3, x becomes 4 and then 14.

GATE-styleMCQ · one correct answer

[arrays as parameters] What does the following program print?

#include <stdio.h>
int count(int a[]) {
    return sizeof(a) / sizeof(a[0]);
}
int main(void) {
    int a[10];
    printf("%zu %d\n", sizeof(a) / sizeof(a[0]), count(a));
    return 0;
}
Show answer and explanationAnswer: B

In main, a is a real array of 10 ints: 40 / 4 = 10. A parameter declared as int a[] is really a pointer, so inside count, sizeof(a) is the size of a pointer (8), and 8 / 4 = 2. This is why array functions need the length as a separate parameter.

GATE-styleMCQ · one correct answer

[type conversion] What does the following program print?

#include <stdio.h>
int main(void) {
    int a = -1;
    unsigned int b = 1;
    if (a < b) printf("less\n");
    else printf("not less\n");
    return 0;
}
Show answer and explanationAnswer: B

When an int is compared with an unsigned int, the int is converted to unsigned. −1 becomes the largest unsigned value (4294967295 with 32-bit unsigned), which is not less than 1.

GATE-styleNAT · numerical answer

[function pointers] What does the following program print?

#include <stdio.h>
int add(int a, int b) { return a + b; }
int sub(int a, int b) { return a - b; }
int mul(int a, int b) { return a * b; }
int main(void) {
    int (*ops[])(int, int) = {add, sub, mul};
    printf("%d\n", ops[2](ops[0](2, 3), ops[1](7, 4)));
    return 0;
}
Show answer and explanationAnswer: 15

ops[0] is add, ops[1] is sub and ops[2] is mul. The call is mul(add(2, 3), sub(7, 4)) = mul(5, 3) = 15.

GATE-styleMCQ · one correct answer

[do-while, post-decrement] What does the following program print?

#include <stdio.h>
int main(void) {
    int i = 3;
    do {
        printf("%d", i);
    } while (i-- > 1);
    printf(" %d\n", i);
    return 0;
}
Show answer and explanationAnswer: A

The body runs before the first test. It prints 3; the test 3 > 1 is true and i becomes 2. It prints 2; 2 > 1 is true and i becomes 1. It prints 1; 1 > 1 is false, but the post-decrement still makes i 0. So it prints 321 and then 0.

GATE-styleMCQ · one correct answer

[strings, pointers] What does the following program print?

#include <stdio.h>
void rev(char *s) {
    char *e = s;
    while (*e) e++;
    e--;
    while (s < e) {
        char t = *s;
        *s++ = *e;
        *e-- = t;
    }
}
int main(void) {
    char w[] = "GATECSE";
    rev(w);
    printf("%s\n", w);
    return 0;
}
Show answer and explanationAnswer: A

rev moves s forward and e backward from the two ends, swapping characters until they meet, which reverses the string in place.

GATE-styleMCQ · one correct answer

[static and global variables] What does the following program print?

#include <stdio.h>
int g = 10;
int f(int x) {
    static int s = 0;
    s += x;
    g -= x;
    return s + g;
}
int main(void) {
    int a = f(2);
    int b = f(3);
    printf("%d %d %d\n", a, b, g);
    return 0;
}
Show answer and explanationAnswer: A

f(2): s = 2, g = 8, returns 10. f(3): s = 2 + 3 = 5 (static, so it is kept), g = 8 − 3 = 5, returns 10. Finally g is 5.

GATE-styleMCQ · one correct answer

[pointer subtraction] What does the following program print?

#include <stdio.h>
int main(void) {
    int arr[] = {2, 4, 6, 8, 10};
    int *p = arr + 4, *q = arr;
    printf("%d %d\n", (int)(p - q), *p - *q);
    return 0;
}
Show answer and explanationAnswer: A

Subtracting two pointers into the same array gives the number of elements between them, not bytes: p - q = 4. *p - *q = 10 − 2 = 8.

GATE-styleNAT · numerical answer

[recursion] What does the following program print?

#include <stdio.h>
int fun(int n) {
    if (n == 0) return 0;
    return (n % 10) + fun(n / 10);
}
int main(void) {
    printf("%d\n", fun(fun(98765)));
    return 0;
}
Show answer and explanationAnswer: 8

fun returns the sum of the decimal digits. fun(98765) = 9 + 8 + 7 + 6 + 5 = 35, and fun(35) = 3 + 5 = 8.

GATE-styleMCQ · one correct answer

[string library] What does the following program print?

#include <stdio.h>
#include <string.h>
int main(void) {
    char a[20] = "GATE";
    char *b = "2026";
    strcat(a, b);
    printf("%s %zu %d\n", a, strlen(a), strcmp("abc", "abd") < 0);
    return 0;
}
Show answer and explanationAnswer: A

strcat appends "2026" to "GATE" (the array has room for 20 chars). strlen gives 8. strcmp("abc", "abd") is negative because 'c' < 'd', so the comparison < 0 is 1.

GATE-styleNAT · numerical answer

[pointer to pointer] What does the following program print?

#include <stdio.h>
int main(void) {
    int x = 5;
    int *p = &x, **pp = &p;
    **pp += 3;
    *p *= 2;
    printf("%d\n", x);
    return 0;
}
Show answer and explanationAnswer: 16

pp points to p, which points to x. **pp += 3 makes x = 8, then *p *= 2 makes x = 16.

GATE-styleNAT · numerical answer

[nested loops, break, continue] What does the following program print?

#include <stdio.h>
int main(void) {
    int i, j, c = 0;
    for (i = 1; i <= 4; i++)
        for (j = 1; j <= 4; j++) {
            if (j == i) continue;
            if (i + j > 5) break;
            c++;
        }
    printf("%d\n", c);
    return 0;
}
Show answer and explanationAnswer: 8

For each i, j runs from 1 upwards: j == i is skipped with continue, and the inner loop stops at the first j with i + j > 5. Counted pairs: i = 1: j = 2, 3, 4 (3); i = 2: j = 1, 3 (2, then j = 4 breaks); i = 3: j = 1, 2 (2, then j = 4 breaks); i = 4: j = 1 (1, then j = 2 breaks). Total 3 + 2 + 2 + 1 = 8.