Example: Remove Duplicates In-Place (Sorted Array) in C

// Remove Duplicates In-Place (Sorted Array)
#include <stdio.h>
#include <stdlib.h> // For qsort
#include <string.h> // For memcpy (not strictly needed but useful for array manipulation)

// Comparison function for qsort
int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

int main() {
    int arr[] = {10, 20, 20, 30, 40, 40, 50, 60, 10};
    int n = sizeof(arr) / sizeof(arr[0]);

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

    // Step 1: Sort the array
    qsort(arr, n, sizeof(int), compare);

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

    if (n == 0) {
        printf("Array is empty, no duplicates to remove.\n");
        return 0;
    }

    // Step 2: Use two pointers for in-place removal
    int i = 0; // Pointer for the next unique element position
    // j iterates through the array
    for (int j = 1; j < n; j++) {
        // If current element is different from the last unique element
        if (arr[j] != arr[i]) {
            i++; // Move the unique pointer forward
            arr[i] = arr[j]; // Place the unique element
        }
    }

    // The new effective size of the array is i + 1
    int new_n = i + 1;

    printf("Array after removing duplicates (in-place): ");
    for (int k = 0; k < new_n; k++) {
        printf("%d ", arr[k]);
    }
    printf("\n");

    return 0;
}