Example: Counting Sort Implementation in C

// Counting Sort Implementation
#include <stdio.h>
#include <stdlib.h> // For malloc and free

// Function to find the maximum element in an array
int findMax(int arr[], int n) {
    int max = arr[0];
    for (int i = 1; i < n; i++) {
        if (arr[i] > max) {
            max = arr[i];
        }
    }
    return max;
}

// Function to implement Counting Sort
void countingSort(int arr[], int n) {
    // Step 1: Find the maximum element in the input array
    int max = findMax(arr, n);

    // Step 2: Create a count array to store the frequency of each element
    // The size of count array will be max + 1 (for 0 to max)
    int *count = (int *)calloc(max + 1, sizeof(int));
    if (count == NULL) {
        printf("Memory allocation failed for count array.\n");
        return;
    }

    // Step 3: Create an output array to store the sorted elements
    int *output = (int *)malloc(n * sizeof(int));
    if (output == NULL) {
        printf("Memory allocation failed for output array.\n");
        free(count); // Clean up already allocated memory
        return;
    }

    // Step 4: Store the count of each element
    for (int i = 0; i < n; i++) {
        count[arr[i]]++;
    }

    // Step 5: Modify the count array to store the actual position
    // of each element in the output array.
    // This accumulates the counts, so count[i] now stores the number
    // of elements less than or equal to i.
    for (int i = 1; i <= max; i++) {
        count[i] += count[i - 1];
    }

    // Step 6: Build the output array
    // Iterate from the end of the input array to maintain stability
    // (if elements are equal, their relative order is preserved).
    for (int i = n - 1; i >= 0; i--) {
        output[count[arr[i]] - 1] = arr[i];
        count[arr[i]]--; // Decrement the count for the current element
    }

    // Step 7: Copy the sorted elements from the output array back to the original array
    for (int i = 0; i < n; i++) {
        arr[i] = output[i];
    }

    // Step 8: Free the dynamically allocated memory
    free(count);
    free(output);
}

// Utility function to print an array
void printArray(int arr[], int n) {
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
}

int main() {
    // Example array to be sorted
    int arr[] = {4, 2, 2, 8, 3, 3, 1, 0, 9, 5};
    int n = sizeof(arr) / sizeof(arr[0]);

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

    // Apply Counting Sort
    countingSort(arr, n);

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

    return 0;
}