luc087.c

Metadata

• Author — Amit Dutta
• Last updated — 08 Feb 2026
• License — MIT License (See the LICENSE file for details)

Problem Statement

Problem Statement

Write a program that merges lines alternately from two files and writes the results to a new file. Handle remaining lines if file sizes differ.

Source Code

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

void create_sample_files();

int main()
{
    FILE *fp1, *fp2, *fp3;
    char line1[100], line2[100];
    char *res1, *res2;

    create_sample_files();

    fp1 = fopen("file1.txt", "r");
    fp2 = fopen("file2.txt", "r");
    fp3 = fopen("merge.txt", "w");

    if (fp1 == NULL || fp2 == NULL || fp3 == NULL)
    {
        printf("File error.\n");
        exit(1);
    }

    // Read lines from both files
    res1 = fgets(line1, sizeof(line1), fp1);
    res2 = fgets(line2, sizeof(line2), fp2);

    while (res1 != NULL && res2 != NULL)
    {
        fputs(line1, fp3);
        fputs(line2, fp3);
        
        res1 = fgets(line1, sizeof(line1), fp1);
        res2 = fgets(line2, sizeof(line2), fp2);
    }

    // Append remaining lines from file 1
    while (res1 != NULL)
    {
        fputs(line1, fp3);
        res1 = fgets(line1, sizeof(line1), fp1);
    }

    // Append remaining lines from file 2
    while (res2 != NULL)
    {
        fputs(line2, fp3);
        res2 = fgets(line2, sizeof(line2), fp2);
    }

    printf("Files merged alternately into 'merge.txt'.\n");

    fclose(fp1);
    fclose(fp2);
    fclose(fp3);

    return 0;
}

void create_sample_files()
{
    FILE *f1 = fopen("file1.txt", "w");
    FILE *f2 = fopen("file2.txt", "w");
    
    fprintf(f1, "File 1 - Line A\nFile 1 - Line B\nFile 1 - Line C\n");
    fprintf(f2, "File 2 - Line 1\nFile 2 - Line 2\nFile 2 - Line 3\nFile 2 - Line 4\n");
    
    fclose(f1);
    fclose(f2);
}