C Interview Questions and Output Puzzles

Campus tests and C interviews love "what is the output?" questions and short coding tasks. Each tests one concept from this course – integer division, sizeof, pointers, strings, operators. Here are the most common, with the real output from a compiler and the reason behind it. Cover the answer, predict, then check.

Operators and types

#include <stdio.h>

int main(void) {
    printf("1) %d\n", 5 / 2 * 2);              // integer division first
    printf("2) %d\n", 7 % -3);                 // the sign follows the left operand
    printf("3) %d\n", (3 > 2) > 1);            // what "3 > 2 > 1" really means
    printf("4) %d\n", 010 + 0x10 + 10);        // octal 8 + hex 16 + 10
    printf("5) %zu\n", sizeof(char) + sizeof('a'));   // 'a' is an int in C
    printf("6) %d\n", -1 >> 31 == -1);         // implementation-defined, usually arithmetic shift
    int x = 5;
    printf("7) %d\n", x > 3 ? x < 10 ? 1 : 2 : 3);   // nested ternary
    return 0;
}

1) 4
2) 1
3) 0
4) 34
5) 5
6) 1
7) 1

Loops and control flow

#include <stdio.h>

int main(void) {
    int i;
    for (i = 0; i < 5; i++);                   // note the semicolon: empty loop body
    printf("1) i = %d\n", i);

    int count = 0;
    for (int a = 1; a <= 10; a += 3) count++;
    printf("2) count = %d\n", count);

    int n = 0;
    while (n++ < 3) { }
    printf("3) n = %d\n", n);                  // the final failed test still increments

    switch (2) {
        case 1: printf("one ");
        case 2: printf("two ");
        case 3: printf("three ");               // fall-through: no breaks
        default: printf("default");
    }
    printf("\n");
    return 0;
}

1) i = 5
2) count = 4
3) n = 4
two three default

Pointers, arrays and strings

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

int main(void) {
    int a[] = {10, 20, 30, 40};
    int *p = a;
    printf("1) %d %d\n", *(p + 2), *p + 2);      // element 2 vs value of element 0 plus 2
    printf("2) %d\n", *p++);                     // value first, then pointer moves
    printf("3) %d\n", *p);
    printf("4) %td\n", &a[3] - &a[0]);           // elements, not bytes

    char s[] = "Hello";
    printf("5) %zu %zu\n", sizeof s, strlen(s)); // array size includes '\0'
    printf("6) %c\n", *(s + 1));
    printf("7) %s\n", s + 2);                    // a string starting at index 2

    char t[10] = "Hi";
    printf("8) %zu %zu\n", sizeof t, strlen(t));
    return 0;
}

1) 30 12
2) 10
3) 20
4) 3
5) 6 5
6) e
7) llo
8) 10 2

Functions and static

#include <stdio.h>

int counter(void) {
    static int c = 0;          // initialised once
    return ++c;
}

int change(int x, int *y) {
    x = 100;                   // changes only the copy
    *y = 100;                  // changes the caller's variable
    return x;
}

int f(int n) { return n <= 1 ? 1 : n + f(n - 1); }

int main(void) {
    counter(); counter();
    printf("1) %d\n", counter());

    int a = 1, b = 1;
    int copy = change(a, &b);
    printf("2) a=%d b=%d copy=%d\n", a, b, copy);

    printf("3) %d\n", f(4));   // 4 + 3 + 2 + 1
    return 0;
}

1) 3
2) a=1 b=100 copy=100
3) 10

Questions with no correct answer

Some popular "output" questions are traps: the code has undefined behaviour, so different compilers (and optimisation levels) print different things. The professional answer is to say so and explain why:

Theory questions to prepare

Classic coding tasks

#include <stdio.h>

int count_ones(unsigned int n) { int c = 0; while (n) { n &= n - 1; c++; } return c; }

int is_power_of_two(unsigned int n) { return n && !(n & (n - 1)); }

void swap_no_temp(int *a, int *b) { if (a != b) { *a ^= *b; *b ^= *a; *a ^= *b; } }

int missing_number(const int a[], int n) {     // numbers 1..n+1 with one missing
    int expected = (n + 1) * (n + 2) / 2, sum = 0;
    for (int i = 0; i < n; i++) sum += a[i];
    return expected - sum;
}

int main(void) {
    printf("ones in 29: %d\n", count_ones(29));
    printf("64 power of two? %d, 96? %d\n", is_power_of_two(64), is_power_of_two(96));
    int x = 3, y = 8;
    swap_no_temp(&x, &y);
    printf("after swap: x=%d y=%d\n", x, y);
    int nums[] = {1, 2, 4, 5, 6};
    printf("missing number: %d\n", missing_number(nums, 5));
    return 0;
}

ones in 29: 4
64 power of two? 1, 96? 0
after swap: x=8 y=3
missing number: 3

Keep practising

• The C language basics
• Variables and data storage
• Bits and bytes
• Pointers and functions
• Dynamic memory allocation
• C coding programs and output questions
• Output-based tricky questions
• Coding: arrays, strings and patterns

Summary