Writing Comments

This lesson covers writing comments in C, so your code's intent comes across clearly to others and to your future self. It's for anyone searching "C comment syntax."

A comment is a note for humans with no effect on execution. // starts a single-line comment, and /* */ wraps a multi-line one.

The example shows three patterns: a whole-line comment like // This is a single-line comment, an inline comment at the end of a line of code like int price = 100; // product price (yen), and a multi-line comment wrapped in /* */.

A common early mistake is over-commenting until it becomes harder to read, not easier. Writing down "why this logic is needed" is genuinely useful, but there's no need to explain what's already obvious from the code.

In real projects, comments are also frequently used to temporarily disable a piece of code (commenting it out) — an essential technique in day-to-day work.

📖 Reference code

#include <stdio.h>

int main(void) {
    // This is a single-line comment
    int price = 100; // product price

    /*
       This is a
       multi-line comment
    */
    printf("Price: %d\n", price);
    return 0;
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).