C Program to delete the duplicate elements in an array

In C programming, removing duplicate elements from an array means creating a new array that contains only unique elements. This is a common array manipulation task that helps eliminate redundancy in data.

Syntax

for(i = 0; i < size; i++) {
    for(j = i + 1; j < size; j++) {
        if(arr[i] == arr[j]) {
            // Shift elements left to remove duplicate
            for(k = j; k < size - 1; k++) {
                arr[k] = arr[k + 1];
            }
            size--;  // Reduce array size
            j--;     // Check current position again
        }
    }
}

Method 1: In-Place Removal Using Nested Loops

This approach removes duplicates by shifting elements left whenever a duplicate is found −

#include <stdio.h>

int main() {
    int a[50], i, j, k, number;
    
    printf("Enter size of the array: ");
    scanf("%d", &number);
    
    printf("Enter elements of the array: ");
    for(i = 0; i < number; i++) {
        scanf("%d", &a[i]);
    }
    
    printf("Original array: ");
    for(i = 0; i < number; i++) {
        printf("%d ", a[i]);
    }
    
    /* Remove duplicates using nested loops */
    for(i = 0; i < number; i++) {
        for(j = i + 1; j < number; j++) {
            if(a[i] == a[j]) {
                /* Shift elements left to remove duplicate */
                for(k = j; k < number - 1; k++) {
                    a[k] = a[k + 1];
                }
                number--;  /* Reduce array size */
                j--;       /* Check current position again */
            }
        }
    }
    
    printf("\nAfter removing duplicates: ");
    for(i = 0; i < number; i++) {
        printf("%d ", a[i]);
    }
    printf("<br>");
    
    return 0;
}

Enter size of the array: 8
Enter elements of the array: 1 2 3 2 4 3 5 1
Original array: 1 2 3 2 4 3 5 1 
After removing duplicates: 1 2 3 4 5

Method 2: Using Separate Array for Unique Elements

This approach creates a new array to store only unique elements −

#include <stdio.h>

int main() {
    int arr[50], unique[50], i, j, n, uniqueCount = 0;
    int isUnique;
    
    printf("Enter number of elements: ");
    scanf("%d", &n);
    
    printf("Enter elements: ");
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    printf("Original array: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    
    /* Find unique elements */
    for(i = 0; i < n; i++) {
        isUnique = 1;
        
        /* Check if element already exists in unique array */
        for(j = 0; j < uniqueCount; j++) {
            if(arr[i] == unique[j]) {
                isUnique = 0;
                break;
            }
        }
        
        /* Add to unique array if not found */
        if(isUnique) {
            unique[uniqueCount] = arr[i];
            uniqueCount++;
        }
    }
    
    printf("\nUnique elements: ");
    for(i = 0; i < uniqueCount; i++) {
        printf("%d ", unique[i]);
    }
    printf("<br>");
    
    return 0;
}

Enter number of elements: 6
Enter elements: 5 3 7 3 5 9
Original array: 5 3 7 3 5 9 
Unique elements: 5 3 7 9

Key Points

• Time Complexity: Both methods have O(n²) time complexity due to nested loops.
• Space Complexity: Method 1 uses O(1) extra space; Method 2 uses O(n) extra space.
• Method 1 modifies the original array, while Method 2 preserves it.
• Both methods maintain the order of first occurrence of elements.

Conclusion

Removing duplicates from an array can be achieved using in-place shifting or creating a separate array for unique elements. The choice depends on whether you need to preserve the original array and space constraints.