Thread: Removing duplicate elements when merging two arrays

The common solution is to search the third array before inserting values from the second array.

Tim S.

I tried using an conditional statements to check if an element in the
second already exists in the first array. The first few test I ran worked fine but the output was backwards... and now when I try to run more test I keep getting at least one duplicate. Is there something wrong with my logic?

Code:

 #include <stdio.h>

void merge_array(int arr1[], int num1, int arr2[], int num2);

int main(void){

    int first_arr[30];
    int second_arr[30];
    int num1, num2;

    printf("Enter the length of array 1: ");
    scanf("%d", &num1);
    printf("Enter the elements of the array: ");
    for(int i = 0; i < num1; i++){
        // scan the first array of elements
        scanf("%d", &first_arr[i]);
    }
    
    printf("Enter the length of the array 2: ");
    scanf("%d", &num2);
    printf("Enter the elements of the array: ");
    for(int i = 0; i < num2; i++){
        // scan the second array of elements
        scanf("%d", &second_arr[i]);
    }
    printf("Output array: ");
    merge_array(first_arr, num1, second_arr, num2);
    return 0;
}

void merge_array(int arr1[], int num1, int arr2[], int num2){

    int arr3[60];
    int i, j, k;
    
    // loop through every element in the array
    // and remove any dupilicated elements that 
    // are found in both arrays.
    for(i = 0, j = 0, k = 0; (i < num1) && (j < num2); ){

        if(arr1[i] < arr2[j]){
            
            arr3[k] = arr1[i];
            i++;
            k++;
        }
        else if(arr1[i] > arr2[j]){
            
            arr3[k] = arr2[j];
            j++;
            k++;
        }
        else{

            arr3[k] = arr1[i];
            i++;
            j++;
            k++;
        }
    }
    if (i < num1){

        for(; i < num1; i++){
            //copy all the elements from the first array into third array
            arr3[k] = arr1[i];
            k++;
        }
    }
    else if(j < num2){
        
        for(; j < num2; j++){
            // copy all the elements from the second array into third array
            arr3[k] = arr2[j];
            k++;
        }
    }
    //output the array
    for(i = 0; i < k; i++){
        printf("%d ", arr3[i]);
    }
}

The output i get is:

What happens if you have something like this.
1 3 5 7 9
10 8 6 4 2 1

Somewhere along the line, you need to sort things.

For me, it's just
- copy both arrays
- sort
- remove duplicates (which will be adjacent after sort).

Well that's not entirely surprising, because your approach is wrong.

If you want a practical demonstration
- write all those numbers on small pieces of paper
- distribute into two piles as shown previously

Now try to extract numbers from each pile one at a time and figure out what steps you actually perform to ensure you have a unique merged list.

I fixed the conditional statements inside the body of my for loop and I got the proper output. I still have some doubt that I might be incorrect even though the output array was correct.

Here's my code:

Code:

#include <stdio.h>
#include <stdlib.h>

void merge_array(int arr1[], int num1, int arr2[], int num2);

int main(void){

    int first_arr[100];
    int second_arr[100];
    int num1, num2;

    printf("Enter the length of array 1: ");
    scanf("%d", &num1);
    printf("Enter the elements of the array: ");
    for(int i = 0; i < num1; i++){
        // scan the first array of elements
        scanf("%d", &first_arr[i]);
    }
    
    printf("Enter the length of the array 2: ");
    scanf("%d", &num2);
    printf("Enter the elements of the array: ");
    for(int i = 0; i < num2; i++){
        // scan the second array of elements
        scanf("%d", &second_arr[i]);
    }
    printf("Output array: ");
    merge_array(first_arr, num1, second_arr, num2);
    return 0;
}

void merge_array(int arr1[], int num1, int arr2[], int num2){

    int arr3[200];
    int i, j, k; // k is the count
    
    // loop through every element in the array
    // and remove any dupilicated elements that 
    // are found in both arrays.
    for(i = 0, j = 0, k = 0; (i < num1) && (j < num2); ){
        // check if any elements in arr1 are in arr2
        if(arr1[i] == arr2[j]){
            
            arr3[k] = arr2[j]; // remove the duplicate here?
            j++;
            k++;
        }
        else{

            arr3[k] = arr1[i]; // copy elements to arr1
            i++;
            k++;
        }   
    }
    //output the array
    for(i = 0; i < k; i++){
        printf("%d ", arr3[i]);
    }

EDIT: It isn't working for all outputs...
Here are some of the test I ran:

It didn't work with my test case.

Code:

$ ./a.out 
Enter the length of array 1: 5
Enter the elements of the array: 1 3 5 7 9
Enter the length of the array 2: 6
Enter the elements of the array: 10 8 6 4 2 1
Output array: 1 3 5 7 9 $

There's a whole bunch of even numbers missing.

Have you tried the exercise of trying this using bits of paper?

You can't write the code until you actually understand the problem.

Just writing code which copies arrays with some "if" thrown in isn't the way to go.

> Do you mind explaining to me what I should do?
I already have.
Read my posts again.

I can not help you with you current code because I do not understand how it could made to work.

This problem has two clear paths to work on solving.
1. sort array 1 and 2; then merge the sorted arrays into the third array.
2. copy array 1 into 2; then scan though array 2 one element at a time; check if element is in array 3. If not, add to array 3.

Tim S.

Changing num1 and num2 to size1 and size2 would make your code more readable.

Tim S.