Remove Duplicate Elements from Array in C: Program to Delete Duplicates

Introduction

In this tutorial, we will learn how to write a C program to remove duplicate elements from an array.

Removing duplicates from an array is a common programming task. It is used in many real-world applications, such as:

• Cleaning up data by removing repeated entries
• Creating a unique list of items from a dataset
• Removing duplicate user records from a database
• Preparing data for analysis or reporting

C Program to Remove Duplicates from Unsorted Array

#include <stdio.h>

int main() {
    int n, i, j, k;
    
    // Ask user for number of elements
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    // Declare array
    int arr[n];
    
    // Read elements into the array
    printf("Enter %d elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    // Remove duplicates
    for(i = 0; i < n; i++) {
        for(j = i + 1; j < n; j++) {
            if(arr[i] == arr[j]) {
                // Shift all elements to the left
                for(k = j; k < n - 1; k++) {
                    arr[k] = arr[k + 1];
                }
                n--;  // Decrease array size
                j--;  // Check the new element at position j
            }
        }
    }
    
    // Display array after removing duplicates
    printf("\nArray after removing duplicates: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    
    return 0;
}

Sample Output

Enter the number of elements: 8
Enter 8 elements:
10 20 10 30 20 40 10 50

Array after removing duplicates: 10 20 30 40 50

Another Example:

Enter the number of elements: 6
Enter 6 elements:
5 5 5 5 5 5

Array after removing duplicates: 5

Example with No Duplicates:

Enter the number of elements: 5
Enter 5 elements:
10 20 30 40 50

Array after removing duplicates: 10 20 30 40 50

Program Explanation

Let's break down the code step by step:

1. Include Header File: #include <stdio.h> includes the standard input/output library.
2. Declare Variables:
   • int n; — number of elements
   • int i, j, k; — loop counters
3. Get User Input: Reads the array elements from the user.
4. Remove Duplicates:
   • The outer loop (i) iterates through each element
   • The inner loop (j) checks for duplicates of arr[i] in the remaining array
   • If a duplicate is found (arr[i] == arr[j]):
     • Shift all elements from j+1 to n-1 one position left
     • Decrease n by 1 (array size reduced)
     • Decrease j by 1 to check the new element at position j
5. Display Result: Prints the array after removing duplicates.
6. Return: return 0; indicates successful program execution.

Algorithm to Remove Duplicates (Unsorted Array)

Step-by-step algorithm:

1. Start
2. Read the array elements
3. For i = 0 to n-1:
   • For j = i+1 to n-1:
     • If arr[i] == arr[j]:
       • For k = j to n-2:
         • arr[k] = arr[k+1]
       • n--
       • j--
4. Print the updated array
5. End

Alternative Method: Using Visited Array

This method uses a separate array to track which elements have already been added to the result.

#include <stdio.h>

int main() {
    int n, i, j;
    int count = 0;
    
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    int arr[n];
    int visited[n];  // To mark visited elements
    int unique[n];   // To store unique elements
    
    printf("Enter %d elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
        visited[i] = 0;
    }
    
    // Find unique elements
    for(i = 0; i < n; i++) {
        if(visited[i] == 0) {
            unique[count] = arr[i];
            count++;
            
            // Mark all duplicates as visited
            for(j = i + 1; j < n; j++) {
                if(arr[i] == arr[j]) {
                    visited[j] = 1;
                }
            }
        }
    }
    
    // Display unique elements
    printf("\nArray after removing duplicates: ");
    for(i = 0; i < count; i++) {
        printf("%d ", unique[i]);
    }
    printf("\n");
    
    return 0;
}

Removing Duplicates from a Sorted Array

If the array is already sorted, we can remove duplicates in O(n) time using a two-pointer approach.

#include <stdio.h>

int main() {
    int n, i, j;
    
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    int arr[n];
    
    printf("Enter %d sorted elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    // Remove duplicates from sorted array
    j = 0;
    for(i = 1; i < n; i++) {
        if(arr[i] != arr[j]) {
            j++;
            arr[j] = arr[i];
        }
    }
    
    // j+1 is the new size
    int newSize = j + 1;
    
    printf("\nArray after removing duplicates: ");
    for(i = 0; i < newSize; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    
    return 0;
}

Sample Output:

Enter the number of elements: 8
Enter 8 sorted elements:
10 10 20 20 30 40 40 50

Array after removing duplicates: 10 20 30 40 50

How Sorted Array Approach Works

• Two Pointers: j tracks the position of the last unique element
• Compare: If arr[i] != arr[j], it's a new unique element
• Copy: Copy the new element to position j+1
• Increment: Move j forward
• Result: The first j+1 elements are unique

Time and Space Complexity

#include <stdio.h>

int main() {
    int n, i, j, k;
    int duplicates_removed = 0;
    
    printf("Enter the number of elements: ");
    scanf("%d", &n);
    
    int arr[n];
    
    printf("Enter %d elements:\n", n);
    for(i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
    
    int original_n = n;
    
    // Remove duplicates and count
    for(i = 0; i < n; i++) {
        for(j = i + 1; j < n; j++) {
            if(arr[i] == arr[j]) {
                for(k = j; k < n - 1; k++) {
                    arr[k] = arr[k + 1];
                }
                n--;
                j--;
                duplicates_removed++;
            }
        }
    }
    
    printf("\nArray after removing duplicates: ");
    for(i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    printf("Total duplicates removed: %d\n", duplicates_removed);
    printf("Original size: %d, New size: %d\n", original_n, n);
    
    return 0;
}

Frequently Asked Questions

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