Merge Two Arrays in C: Program to Combine Two Arrays into One

Introduction

In this tutorial, we will learn how to write a C program to merge two arrays into a single array.

Merging two arrays is a common operation in programming. It is used in many real-world applications, such as:

• Combining two lists of data (like merging two class lists)
• Merging sorted arrays (merge sort algorithm)
• Combining results from multiple sources
• Database query result merging

C Program to Merge Two Arrays

#include <stdio.h>

int main() {
    int n1, n2, i;
    
    // ====== FIRST ARRAY ======
    printf("Enter the number of elements in first array: ");
    scanf("%d", &n1);
    int arr1[n1];
    
    printf("Enter %d elements:\n", n1);
    for(i = 0; i < n1; i++) {
        scanf("%d", &arr1[i]);
    }
    
    // ====== SECOND ARRAY ======
    printf("\nEnter the number of elements in second array: ");
    scanf("%d", &n2);
    int arr2[n2];
    
    printf("Enter %d elements:\n", n2);
    for(i = 0; i < n2; i++) {
        scanf("%d", &arr2[i]);
    }
    
    // ====== MERGE ======
    int merged[n1 + n2];
    
    // Copy elements from first array
    for(i = 0; i < n1; i++) {
        merged[i] = arr1[i];
    }
    
    // Copy elements from second array
    for(i = 0; i < n2; i++) {
        merged[n1 + i] = arr2[i];
    }
    
    // ====== DISPLAY ======
    printf("\nFirst array: ");
    for(i = 0; i < n1; i++) {
        printf("%d ", arr1[i]);
    }
    
    printf("\nSecond array: ");
    for(i = 0; i < n2; i++) {
        printf("%d ", arr2[i]);
    }
    
    printf("\nMerged array: ");
    for(i = 0; i < n1 + n2; i++) {
        printf("%d ", merged[i]);
    }
    printf("\n");
    
    return 0;
}

Sample Output

Enter the number of elements in first array: 3
Enter 3 elements:
10 20 30

Enter the number of elements in second array: 4
Enter 4 elements:
40 50 60 70

First array: 10 20 30
Second array: 40 50 60 70
Merged array: 10 20 30 40 50 60 70

Another Example:

Enter the number of elements in first array: 2
Enter 2 elements:
5 15

Enter the number of elements in second array: 3
Enter 3 elements:
25 35 45

First array: 5 15
Second array: 25 35 45
Merged array: 5 15 25 35 45

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 n1, n2; — sizes of the two arrays
   • int i; — loop counter
3. Read First Array: Reads the size and elements of the first array.
4. Read Second Array: Reads the size and elements of the second array.
5. Merge the Arrays:
   • int merged[n1 + n2]; — creates a merged array with total size
   • First loop copies all elements from arr1 into merged
   • Second loop copies all elements from arr2 into merged starting at index n1
6. Display Results: Prints all three arrays.
7. Return: return 0; indicates successful program execution.

Method 2: Merging Sorted Arrays (Merge Sort Style)

When both arrays are already sorted, we can merge them efficiently in O(n1 + n2) time using a two-pointer approach. This is the core of the merge sort algorithm.

#include <stdio.h>

int main() {
    int n1, n2, i, j, k;
    
    // Read first sorted array
    printf("Enter number of elements in first sorted array: ");
    scanf("%d", &n1);
    int arr1[n1];
    printf("Enter %d sorted elements:\n", n1);
    for(i = 0; i < n1; i++) {
        scanf("%d", &arr1[i]);
    }
    
    // Read second sorted array
    printf("\nEnter number of elements in second sorted array: ");
    scanf("%d", &n2);
    int arr2[n2];
    printf("Enter %d sorted elements:\n", n2);
    for(i = 0; i < n2; i++) {
        scanf("%d", &arr2[i]);
    }
    
    // Merge sorted arrays
    int merged[n1 + n2];
    i = 0; j = 0; k = 0;
    
    while(i < n1 && j < n2) {
        if(arr1[i] < arr2[j]) {
            merged[k] = arr1[i];
            i++;
        } else {
            merged[k] = arr2[j];
            j++;
        }
        k++;
    }
    
    // Copy remaining elements from first array
    while(i < n1) {
        merged[k] = arr1[i];
        i++;
        k++;
    }
    
    // Copy remaining elements from second array
    while(j < n2) {
        merged[k] = arr2[j];
        j++;
        k++;
    }
    
    // Display merged array
    printf("\nMerged sorted array: ");
    for(i = 0; i < n1 + n2; i++) {
        printf("%d ", merged[i]);
    }
    printf("\n");
    
    return 0;
}

Sample Output:

Enter number of elements in first sorted array: 4
Enter 4 sorted elements:
10 20 30 40

Enter number of elements in second sorted array: 3
Enter 3 sorted elements:
15 25 35

Merged sorted array: 10 15 20 25 30 35 40

How Sorted Array Merge Works

• Two Pointers: i points to arr1, j points to arr2
• Compare: Compare arr1[i] and arr2[j], add the smaller one to merged array
• Advance: Move the pointer of the array from which the element was taken
• Remaining: After one array is exhausted, copy all remaining elements from the other array
• Time Complexity: O(n1 + n2)

Algorithm to Merge Two Arrays

1. Start
2. Read size and elements of first array (arr1)
3. Read size and elements of second array (arr2)
4. Create merged array of size n1 + n2
5. For i = 0 to n1-1:
   • Set merged[i] = arr1[i]
6. For j = 0 to n2-1:
   • Set merged[n1 + j] = arr2[j]
7. Print the merged array
8. End

#include <stdio.h>

int main() {
    int n1, n2, i, j = 0;
    
    printf("Enter number of elements in first array: ");
    scanf("%d", &n1);
    int arr1[n1];
    printf("Enter %d elements:\n", n1);
    for(i = 0; i < n1; i++) {
        scanf("%d", &arr1[i]);
    }
    
    printf("Enter number of elements in second array: ");
    scanf("%d", &n2);
    int arr2[n2];
    printf("Enter %d elements:\n", n2);
    for(i = 0; i < n2; i++) {
        scanf("%d", &arr2[i]);
    }
    
    // Alternate merge
    int merged[n1 + n2];
    int k = 0;
    int max_size = n1 > n2 ? n1 : n2;
    
    for(i = 0; i < max_size; i++) {
        if(i < n1) {
            merged[k] = arr1[i];
            k++;
        }
        if(i < n2) {
            merged[k] = arr2[i];
            k++;
        }
    }
    
    printf("\nAlternate merged array: ");
    for(i = 0; i < n1 + n2; i++) {
        printf("%d ", merged[i]);
    }
    printf("\n");
    
    return 0;
}

/* Sample Output:
Enter number of elements in first array: 3
Enter 3 elements:
10 20 30
Enter number of elements in second array: 4
Enter 4 elements:
40 50 60 70
Alternate merged array: 10 40 20 50 30 60 70
*/

Frequently Asked Questions

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