`luc087.c`¶

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.

Metadata¶

Property	Detail
Author	Amit Dutta amitdutta4255@gmail.com
Date	08 Feb 2026
License	MIT License (See the LICENSE file for details)
Difficulty	Beginner (index: 2 / 10)

Concepts¶

Beta Feature

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Pointers
Recursion
Array
Iteration

Actions¶

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Source Code¶

#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);
}

Explanation¶

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Copy the prompt below and paste it into any AI assistant.

    You are explaining a C programming code to a beginner.

    STRICT RULES:

    - Only use the given code. Do NOT assume anything not present.

    - Do NOT add extra examples.

    - Keep explanation clear and short.

    - If something is unclear, say "Not clear from code".

    - Follow the exact format below. Do NOT change headings.

    FORMAT:

    [START]

    ## What it does

    (Explain the overall purpose in 1-2 sentences)

    ## Step-by-step

    (Explain how the code works in steps, simple language)

    ## Key Concepts

    (List concepts like loop, condition, function, etc.)

    ## Notes

    (Mention any limitations, errors, or assumptions)

    [END]

    CODE (luc087.c):

    #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);
    }

luc087.c¶