qsort() Function in C

Last Updated : 27 Nov, 2024

The qsort() in C is a library function used to sort an array of items in ascending order or descending order. It stands for "quick sort," as it implements the quicksort algorithm for sorting which is one of the fastest and most efficient algorithms to sort the array.

Let's take a look at an example that sorts an array of integers:

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

int comp(const void *a, const void *b) {
    return (*(int *)a - *(int *)b);
}

int main() {
    int arr[] = {5, 2, 3, 1, 4};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Sort the array arr
    qsort(arr, n, sizeof(arr[0]), comp);

    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    return 0;
}

Output

1 2 3 4 5

This article covers the syntax, examples, and rules for defining comparator for qsort() function in C:

Table of Content

Syntax of qsort()
Rules for Defining Comparision Function
Examples of qsort()

Sort Array of Integers in Descending Order
Sort Array of Strings Lexicographically
Sort an Array of Structures

Syntax of qsort()

The qsort() function is defined inside <stdlib.h> header file.

qsort(arr, n, size, comp);

Parameters:

arr: Pointer to the first element of the array.
n: Number of elements in the array.
size: Size of each element.
comp: Function pointer to a comparison function/comparator.

Return Value:

This function does not return any value.

Rules for Defining Comparision Function

The most important part of the qsort() is the comparator function. To provide universal compatibility, the comparator should follow these rules:

It should accept two const void * arguments.
It should return integer value:
- Less than zero (<0): If the first argument should be placed before the second argument.
- Zero (0): If both arguments are equal.
- Greater than zero (>0): If the first argument should be placed after the second argument.

Note: We can use qsort() to sort arrays of any data type, integers, strings and complex structures by typecasting the arguments to the relevant type.

Examples of qsort()

The below program demonstrates how we can sort a given array using qsort() function in C.

Sort Array of Integers in Descending Order

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

// Comparator to sort in descending order
int comp(const void *a, const void *b) {
    return (*(int *)b - *(int *)a);
}

int main() {
    int arr[] = {5, 2, 3, 1, 4};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Sort the array
    qsort(arr, n, sizeof(arr[0]), comp);

    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    return 0;
}

Output

5 4 3 2 1

Sort Array of Strings Lexicographically

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

// Comparison function to sort strings in ascending order
int compare(const void *a, const void *b) {
    return strcmp(*(const char **)a, *(const char **)b);
}

int main() {
    const char *arr[] = {"Welcome", "to", "Geeks", "for", "Geeks", };
    int n = sizeof(arr) / sizeof(arr[0]);

    // Sort the array of strings
    qsort(arr, n, sizeof(arr[0]), compare);

    for (int i = 0; i < n; i++)
        printf("%s ", arr[i]);
    return 0;
}

Output

Geeks Geeks Welcome for to

Sort an Array of Structures

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

typedef struct {
    int data;
} A;

// Comparison function to sort by data
int comp(const void *a, const void *b) {
    return ((A *)a)->data - ((A *)b)->data;
}

int main() {
    A arr[] = {5, 2, 3, 1, 4};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Sort the array of structures
    qsort(arr, n, sizeof(A), comp);

    for (int i = 0; i < n; i++)
        printf("%d ", arr[i].data);
    return 0;
}

Output

1 2 3 4 5

strrev() function in C

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Article Tags :

qsort() Function in C

Syntax of qsort()

Rules for Defining Comparision Function

Examples of qsort()

Sort Array of Integers in Descending Order

Sort Array of Strings Lexicographically

Sort an Array of Structures

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