C Program to Merge the Contents of Two Files into a Third File

In this article, we will write a C program to merge the contents of two files into a third file. The program takes the name of the two files from the user, checks if both files exist or not, reads their contents and writes them into a third file.

If the files corresponding to the file names provided by the user do not exist or there is some error opening the file, the program breaks and prints an error on the output window.

Sample Input:

Enter the name of the first file: first_file.txt
Enter the name of the second file: second_file.txt
Enter the name of the third file: third_file.txt

Sample Output:

The two files merged successfully into file third_file.txt

To merge the contents of the two files into a third file, you have to first open the first file in read mode, read its content character by character, and write it into the third file.

The same steps you have to do for the second file.

Below are the steps you have to follow:

• Open the first file in read mode(“r”) using the fopen() function, read its content character by character using the fgetc() function until the end of the file(EOF) is reached.
• Open the second file in read mode(“r”) using the fopen() function and also read its content until EOF is reached using the fgetc() function.
• Write the content of the first file into the third file character by character using the fputc() function. Follow the same process with the second file.
• Once all the above tasks are performed, close all three files using the fclose() function.

See implementation in the following C program:

// C program to merge the contents
// of two files into a third file

#include <stdio.h>

int main(){
	
	char file1[100], file2[100], file3[100], ch;
	FILE *fptr1, *fptr2, *fptr3;
	
	printf("Enter the name of the first file: ");
	scanf("%s", &file1);
	
	printf("Enter the name of the second file: ");
	scanf("%s", &file2);
	
	printf("Enter the name of the third file: ");
	scanf("%s", &file3);
	
	// Open the files
	fptr1 = fopen(file1, "r");  // Read mode
	fptr2 = fopen(file2, "r");  // Read mode
	fptr3 = fopen(file3, "w");  // Write mode
	
	// Check if there is an error opening the file
	if(fptr1 == NULL || fptr2 == NULL || fptr3 == NULL){
		printf("Error opening the file");
		return 1;
	}
	
	
	// Read the first character of the first file
	ch = fgetc(fptr1);
	
	// Read file1 until EOF(End of file) is reached
	while(ch!=EOF){
		fputc(ch, fptr3);  // write into file3
		ch = fgetc(fptr1);  // Read next character
	}
	
	// Read first character of file 2
	ch = fgetc(fptr2);
	
	// Read file2 until EOF(End of file) is reached
	while(ch!=EOF){
		fputc(ch, fptr3);  // write into file3
		ch = fgetc(fptr2);  // Read next character
	}
	
	printf("The two files merged successfully into file %s", file3);
	
	// Close the files
	fclose(fptr1);
	fclose(fptr2);
	fclose(fptr3);
	
    return 0;
    
}

Output:

Enter the name of the first file: first_file.txt
Enter the name of the second file: second_file.txt
Enter the name of the third file: third_file.txt
The two files merged successfully into file third_file.txt

Enter the name of the first file: first_file.txt
Enter the name of the second file: rando.txt
Enter the name of the third file: temp.txxt
Error opening the file

Below is a sample output image:

Note: Please note that it is important to close all the files once all operations are performed on the files. This is because open files consume system resources such as file handles and memory. Failing to close them can lead to resource exhaustion, especially in long-running programs or when dealing with a large number of files.

That’s all for this article. Thanks for reading.