Example: Sort Array by Another Array's Order (qsort with Lookup)

// Sort Array by Another Array's Order (qsort with Lookup)
#include <stdio.h>
#include <stdlib.h> // For qsort
#include <limits.h> // For INT_MAX (to mark elements not in order array)
#include <string.h> // For memset

// A global array to store the ranks/priorities.
// We assume array elements are non-negative and within a reasonable range (e.g., 0-100).
// Adjust MAX_VAL based on the maximum possible value in your arrays.
#define MAX_VAL 101 // Assuming elements are 0-100
int rank_map[MAX_VAL];

// Comparison function for qsort
int compare(const void *a, const void *b) {
    int val1 = *(const int *)a;
    int val2 = *(const int *)b;

    int rank1 = (val1 >= 0 && val1 < MAX_VAL) ? rank_map[val1] : INT_MAX;
    int rank2 = (val2 >= 0 && val2 < MAX_VAL) ? rank_map[val2] : INT_MAX;

    // If ranks are different, sort by rank (lower rank first)
    if (rank1 != rank2) {
        return rank1 - rank2;
    } else {
        // If ranks are the same (either both are in order_array B and have same rank,
        // which means they are the same value, or both are NOT in order_array B),
        // then sort by their actual value in ascending order.
        return val1 - val2;
    }
}

int main() {
    int arr_A[] = {10, 30, 20, 50, 40, 20};
    int n_A = sizeof(arr_A) / sizeof(arr_A[0]);

    int order_B[] = {20, 10, 40};
    int n_B = sizeof(order_B) / sizeof(order_B[0]);

    // Step 1: Initialize rank_map.
    // Set all ranks to INT_MAX, indicating elements not in order_B.
    for (int i = 0; i < MAX_VAL; i++) {
        rank_map[i] = INT_MAX;
    }
    // Alternative: memset(rank_map, 0xFF, sizeof(rank_map)); // Sets to -1 if unsigned, or large positive if signed char

    // Step 2: Populate rank_map based on order_B.
    // Assign increasing ranks (0, 1, 2...) to elements in order_B.
    for (int i = 0; i < n_B; i++) {
        if (order_B[i] >= 0 && order_B[i] < MAX_VAL) {
            rank_map[order_B[i]] = i;
        }
    }

    // Step 3: Print original array for comparison.
    printf("Original Array A: [");
    for (int i = 0; i < n_A; i++) {
        printf("%d%s", arr_A[i], (i == n_A - 1) ? "" : ", ");
    }
    printf("]\n");

    // Step 4: Sort arr_A using qsort with the custom comparator.
    qsort(arr_A, n_A, sizeof(int), compare);

    // Step 5: Print sorted array.
    printf("Sorted Array A:   [");
    for (int i = 0; i < n_A; i++) {
        printf("%d%s", arr_A[i], (i == n_A - 1) ? "" : ", ");
    }
    printf("]\n");

    return 0;
}