C program count total duplicate elements in array

Write a C program to input elements in array from user and count duplicate elements in array. C program to find all duplicate elements in an unsorted array. How to count duplicate elements in array using loop in C programming.

Input array elements: 1, 10, 20, 1, 25, 1, 10, 30, 25, 1

Total number of duplicate elements = 5

Required knowledge

Basic Input Output, If else, For loop, Nested loop, Array

Logic to count duplicate elements in array

Step by step descriptive logic to count duplicate elements in array.

1. Input size and elements in array from user. Store it in some variable say size and arr.
2. Initialize another variable count with 0 to store duplicate count.
3. To count total duplicate elements in given array we need two loops. Run an outer loop loop from 0 to size. Loop structure must look like for(i=0; i<size; i++). This loop is used to select each element of array and check next subsequent elements for duplicates elements using another nested loop.
4. Run another inner loop to find first duplicate of current array element. Run an inner loop from i + 1 to size, the loop structure should look like for(j=i+1; j<size; j++). Now, why run a loop from i + 1. Because we need to search for duplicate elements in next subsequent elements, from current element.
5. Inside inner loop check for duplicate element. If duplicate element is found then increment duplicate count. Which is if(arr[i] == arr[j]) then, count++. Also terminate inner loop if duplicate element is found.

Learn more about program to print all unique elements in array.

Program to count duplicate elements in array

/**
 * C program to count total number of duplicate elements in an array
 */

#include <stdio.h>

#define MAX_SIZE 100  // Maximum array size

int main()
{
    int arr[MAX_SIZE];
    int i, j, size, count = 0;

    /* Input size of array */
    printf("Enter size of the array : ");
    scanf("%d", &size);

    /* Input elements in array */
    printf("Enter elements in array : ");
    for(i=0; i<size; i++)
    {
        scanf("%d", &arr[i]);
    }

    /*
     * Find all duplicate elements in array
     */
    for(i=0; i<size; i++)
    {
        for(j=i+1; j<size; j++)
        {
            /* If duplicate found then increment count by 1 */
            if(arr[i] == arr[j])
            {
                count++;
                break;
            }
        }
    }

    printf("\nTotal number of duplicate elements found in array = %d", count);

    return 0;
}

Enter size of the array : 10
Enter elements in array : 1 10 20 1 25 1 10 30 25 1

Total number of duplicate elements found in array = 5

Happy coding 😉