Example: Replace Array Elements by Rank in C Program

// Replace Array Elements by Rank
#include <stdio.h>
#include <stdlib.h> // Required for qsort

// Structure to hold an element's value and its original index
typedef struct {
    int value;
    int original_index;
} Element;

// Comparison function for qsort: sorts Element structs by their 'value'
int compareElements(const void *a, const void *b) {
    return ((Element *)a)->value - ((Element *)b)->value;
}

// Function to replace array elements by their ranks
void replaceWithRanks(int arr[], int n) {
    // Step 1: Create an array of Element structs
    Element *elements = (Element *)malloc(n * sizeof(Element));
    if (elements == NULL) {
        fprintf(stderr, "Memory allocation failed!\n");
        return;
    }

    // Step 2: Populate the elements array with values and original indices
    for (int i = 0; i < n; i++) {
        elements[i].value = arr[i];
        elements[i].original_index = i;
    }

    // Step 3: Sort the elements array based on their values
    qsort(elements, n, sizeof(Element), compareElements);

    // Step 4: Create a temporary array to store ranks based on original indices
    int *ranks = (int *)malloc(n * sizeof(int));
    if (ranks == NULL) {
        fprintf(stderr, "Memory allocation failed!\n");
        free(elements);
        return;
    }

    // Step 5: Assign ranks to the original positions
    int current_rank = 1;
    for (int i = 0; i < n; i++) {
        // If it's not the first element and current value is different from previous
        if (i > 0 && elements[i].value != elements[i-1].value) {
            current_rank++;
        }
        // Store the current_rank at the original position of this element
        ranks[elements[i].original_index] = current_rank;
    }

    // Step 6: Replace the original array elements with their calculated ranks
    for (int i = 0; i < n; i++) {
        arr[i] = ranks[i];
    }

    // Step 7: Free dynamically allocated memory
    free(elements);
    free(ranks);
}

int main() {
    // Original array
    int arr[] = {10, 20, 10, 30, 5, 20};
    int n = sizeof(arr) / sizeof(arr[0]);

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

    // Replace elements with their ranks
    replaceWithRanks(arr, n);

    printf("Array with ranks: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");

    // Another example
    int arr2[] = {7, 3, 7, 1, 9, 3};
    int n2 = sizeof(arr2) / sizeof(arr2[0]);

    printf("Original array 2: ");
    for (int i = 0; i < n2; i++) {
        printf("%d ", arr2[i]);
    }
    printf("\n");

    replaceWithRanks(arr2, n2);

    printf("Array 2 with ranks: ");
    for (int i = 0; i < n2; i++) {
        printf("%d ", arr2[i]);
    }
    printf("\n");

    return 0;
}