Example: Merge Sort with User Input in C

// Merge Sort with User Input
#include <stdio.h>
#include <stdlib.h> // For malloc and free

// Function to merge two sorted subarrays
void merge(int arr[], int left, int mid, int right) {
    int i, j, k;
    int n1 = mid - left + 1;
    int n2 = right - mid;

    // Create temporary arrays
    int *L = (int *)malloc(n1 * sizeof(int));
    int *R = (int *)malloc(n2 * sizeof(int));

    // Copy data to temp arrays L[] and R[]
    for (i = 0; i < n1; i++)
        L[i] = arr[left + i];
    for (j = 0; j < n2; j++)
        R[j] = arr[mid + 1 + j];

    // Merge the temp arrays back into arr[left..right]
    i = 0; // Initial index of first subarray
    j = 0; // Initial index of second subarray
    k = left; // Initial index of merged subarray

    while (i < n1 && j < n2) {
        if (L[i] <= R[j]) {
            arr[k] = L[i];
            i++;
        } else {
            arr[k] = R[j];
            j++;
        }
        k++;
    }

    // Copy the remaining elements of L[], if any
    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }

    // Copy the remaining elements of R[], if any
    while (j < n2) {
        arr[k] = R[j];
        j++;
        k++;
    }

    // Free the dynamically allocated temporary arrays
    free(L);
    free(R);
}

// Function to implement Merge Sort
void mergeSort(int arr[], int left, int right) {
    if (left < right) {
        // Find the middle point
        int mid = left + (right - left) / 2;

        // Sort first and second halves
        mergeSort(arr, left, mid);
        mergeSort(arr, mid + 1, right);

        // Merge the sorted halves
        merge(arr, left, mid, right);
    }
}

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

int main() {
    // Step 1: Declare variables for array size and pointer
    int size;
    int *arr;

    // Step 2: Get array size from user
    printf("Enter the number of elements: ");
    scanf("%d", &size);

    // Step 3: Dynamically allocate memory for the array
    arr = (int *)malloc(size * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed!\n");
        return 1; // Indicate an error
    }

    // Step 4: Get array elements from user
    printf("Enter %d integers:\n", size);
    for (int i = 0; i < size; i++) {
        printf("Element %d: ", i + 1);
        scanf("%d", &arr[i]);
    }

    // Step 5: Print the original array
    printf("Original array: ");
    printArray(arr, size);

    // Step 6: Call mergeSort to sort the array
    mergeSort(arr, 0, size - 1);

    // Step 7: Print the sorted array
    printf("Sorted array: ");
    printArray(arr, size);

    // Step 8: Free the dynamically allocated memory
    free(arr);

    return 0;
}