Example: qsort Integers Ascending and Descending in C

// qsort Integers Ascending and Descending
#include <stdio.h>
#include <stdlib.h> // For qsort

// Comparison function for ascending order (a - b)
int compareAscending(const void *a, const void *b) {
    int int_a = *(const int *)a; // Cast void* to int* and dereference
    int int_b = *(const int *)b; // Cast void* to int* and dereference
    return int_a - int_b;
}

// Comparison function for descending order (b - a)
int compareDescending(const void *a, const void *b) {
    int int_a = *(const int *)a;
    int int_b = *(const int *)b;
    return int_b - int_a;
}

void printArray(const char *msg, int arr[], int n) {
    printf("%s: ", msg);
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
}

int main() {
    // Step 1: Initialize an integer array
    int arr[] = {50, 20, 80, 10, 90, 30, 70, 40, 60};
    int n = sizeof(arr) / sizeof(arr[0]);

    printArray("Original array", arr, n);

    // Step 2: Sort in ascending order
    qsort(arr, n, sizeof(int), compareAscending);
    printArray("Sorted (Ascending)", arr, n);

    // Step 3: Sort in descending order
    // Re-initialize array for clear demonstration
    int arr_desc[] = {50, 20, 80, 10, 90, 30, 70, 40, 60};
    qsort(arr_desc, n, sizeof(int), compareDescending);
    printArray("Sorted (Descending)", arr_desc, n);

    return 0;
}