qsort in C: solved exercise with custom comparator

qsort in C: solved exercise with custom comparator

This exercise is scheduled for daily publication and follows the same didactic structure used across the site: clear statement, compilable code, and expected output.

Problem statement

Implement a practical example of the topic and validate the output in the console.

C solution

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#include <stdio.h>
#include <stdlib.h>

int cmp_asc(const void *a, const void *b) {
    int x = *(const int *)a;
    int y = *(const int *)b;
    return (x > y) - (x < y);
}

int main(void) {
    int v[] = {9, 1, 5, 3, 7};
    int n = (int)(sizeof(v) / sizeof(v[0]));

    qsort(v, n, sizeof(int), cmp_asc);
    for (int i = 0; i < n; i++) printf("%d ", v[i]);
    printf("\n");
    return 0;
}

Expected output

1

1 3 5 7 9

Common mistakes

  • Not validating input and standard-library return values.
  • Ignoring edge cases (buffers, limits, null pointers).
  • Skipping basic compile/run verification.

Practical use

The C standard library qsort is the most used sorting function in real C code due to its flexibility with custom comparators.

Guided practice and full book

If you want a complete path with progressive difficulty:

FAQ

Is this exercise useful for C exams and technical interviews?

Yes. It targets patterns that commonly appear in practice assignments, technical interviews, and C programming exams.

Where can I keep practicing with more solved C exercises?

In Programming in C in 100 Solved Exercises and C Exercises. Kindle Unlimited: View on Amazon.

How should I practice this exercise type to improve faster?

Start with small inputs, run edge cases (empty, one item, max capacity), then rewrite the solution from scratch without copying.