C PROGRAMMING PROGRAM • FILE HANDLING

Append Text to an Existing File

Learn how to append text to an existing file using a clear C program.

IntermediateAppend modefopen()fgets()

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

First line
Appended line

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

int main(void)
{
    FILE *file = fopen("notes.txt", "w");
    if (file == NULL) return 1;
    fputs("First line\n", file);
    fclose(file);
    file = fopen("notes.txt", "a");
    if (file == NULL) return 1;
    fputs("Appended line\n", file);
    fclose(file);
    file = fopen("notes.txt", "r");
    if (file == NULL) return 1;
    char line[80];
    while (fgets(line, sizeof line, file) != NULL) fputs(line, stdout);
    fclose(file);
    return 0;
}

CURRENT STEP

Select Start to walk through the important lines.

SELECTED LINE

No line selected

EXPECTED OUTPUT FOR THE SAMPLE

First line
Appended line

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Create the file, reopen it with append mode, add content and read the complete result.
Display the computed result.

Create the file, reopen it with append mode, add content and read the complete result.

EFFICIENCY

Time and space complexity

Time complexity

O(n)

Auxiliary space

O(1)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

Check this

Initialize variables before using their values.

Check this

Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.