Example: Even-Odd Sort using qsort in C

// Even-Odd Sort using qsort
#include <stdio.h>
#include <stdlib.h> // For qsort

// Comparison function for qsort
int compareEvenOdd(const void *a, const void *b) {
    int num1 = *(const int *)a;
    int num2 = *(const int *)b;

    // Prioritize even numbers over odd numbers
    if (num1 % 2 == 0 && num2 % 2 != 0) {
        return -1; // num1 (even) comes before num2 (odd)
    }
    if (num1 % 2 != 0 && num2 % 2 == 0) {
        return 1;  // num1 (odd) comes after num2 (even)
    }

    // If both are even or both are odd, their relative order is not strictly defined
    // by this comparison for stability. qsort might not preserve it.
    // For simple even/odd separation, 0 (equal) is sufficient here.
    return 0;
}

void evenOddSortQsort(int arr[], int n) {
    // Step 1: Call qsort with the custom comparison function
    qsort(arr, n, sizeof(int), compareEvenOdd);
}

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

int main() {
    int arr[] = {12, 34, 45, 9, 8, 90, 3};
    int n = sizeof(arr) / sizeof(arr[0]);

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

    evenOddSortQsort(arr, n);

    printf("Sorted array (qsort): ");
    printArray(arr, n);

    int arr2[] = {7, 2, 10, 5, 1, 8, 4};
    n = sizeof(arr2) / sizeof(arr2[0]);

    printf("Original array: ");
    printArray(arr2, n);

    evenOddSortQsort(arr2, n);

    printf("Sorted array (qsort): ");
    printArray(arr2, n);

    return 0;
}