Files in C: solved exercise to count lines and characters

Files in C: solved exercise

If you are searching for files in C solved exercises, this example covers a very common real-world task: open a text file, scan characters, and compute basic metrics.

Problem statement

Open a text file in read mode and count:

• number of lines,
• total number of characters.

C solution

#include <stdio.h>

int main(void) {
    FILE *f = fopen("data.txt", "r");
    if (f == NULL) {
        perror("Could not open data.txt");
        return 1;
    }

    int c;
    int lines = 0;
    int chars = 0;

    while ((c = fgetc(f)) != EOF) {
        chars++;
        if (c == '\n') {
            lines++;
        }
    }

    fclose(f);

    printf("Lines: %d\n", lines);
    printf("Chars: %d\n", chars);

    return 0;
}

Expected output

Lines: 3
Chars: 74

Exact values depend on the contents of data.txt.

Recommended edge case

Test a file without a trailing newline.
Example content:

one
two
three

If the file does not end with \n, line counting can be off by one unless your logic handles it.

Common mistakes

• Not checking whether fopen returned NULL.
• Forgetting to call fclose.
• Treating EOF as a real file character.
• Assuming every line ends with \n and miscounting the final line.

Time and space complexity

• Time: O(n), scanning each character once.
• Extra space: O(1).

Practical use

This pattern is used in log processing, tracing pipelines, and event-file analysis in modern observability workflows.

Recommended next exercise

• Strings in C: solved exercises with strlen, strcpy, and strcmp
• fread and fwrite in C: solved binary file exercise
• Binary search in C: solved exercise on sorted arrays
• All C exercises

Guided practice and next step

If you want a complete path with progressive difficulty:

• Programming in C in 100 Solved Exercises
• View on Amazon (included in Kindle Unlimited)

FAQ

Is this good to learn file I/O in C?

Yes. It is a direct base before writing files (fprintf) and binary I/O (fread/fwrite).

Where can I continue with guided practice?

At Programming in C in 100 Solved Exercises and C Exercises. Kindle Unlimited: View on Amazon.

How should I practice this exercise type to improve faster?

Start with small inputs, run edge cases (empty, one item, max capacity), then rewrite the solution from scratch without copying.