Remove Duplicate Elements from an Array in C

Algorithm

1. Read n and the n integers into arr.
2. Start with an output size of 0.
3. For each element of arr, scan the output array; if the value is already there, skip it.
4. Otherwise append the value to the output array and increase its size.
5. Print the output array, which holds each distinct value once in original order.

C program

#include <stdio.h>

#define MAX 100

int main(void) {
    int arr[MAX], unique[MAX];
    int n, i, j, count = 0, found;

    printf("Enter the number of elements (1-%d):\n", MAX);
    if (scanf("%d", &n) != 1 || n < 1 || n > MAX) {
        printf("Error: invalid size\n");
        return 1;
    }

    printf("Enter %d integers:\n", n);
    for (i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }

    for (i = 0; i < n; i++) {
        found = 0;
        for (j = 0; j < count; j++) {
            if (unique[j] == arr[i]) {
                found = 1;
                break;
            }
        }
        if (!found) {
            unique[count] = arr[i];
            count++;
        }
    }

    printf("Array without duplicates:\n");
    for (i = 0; i < count; i++) {
        printf("%d ", unique[i]);
    }
    printf("\n");
    return 0;
}

Sample input and output

7
1 2 2 3 1 4 3

Enter the number of elements (1-100):
Enter 7 integers:
Array without duplicates:
1 2 3 4

4
5 5 5 5

Enter the number of elements (1-100):
Enter 4 integers:
Array without duplicates:
5

How the program works

The output array is built as you go, so the check is always against values already kept. Scanning the output rather than the input means each value is compared only with the distinct values seen so far.

The loop is quadratic in the worst case, which is acceptable for a lab-sized array. Sorting first would be faster but would change the original order of the values.

Viva questions

Common mistakes

• Searching the input array instead of the output array, so every element is found in itself and nothing is kept.
• Starting the count at 1 or forgetting to increase it, which drops or repeats the last value.