Comments in C

Comments are explanatory notes added to source code. They help programmers understand what a particular part of the program does.

// This program displays a message

Comments are useful because they can:

• Explain difficult code.
• Describe the purpose of a program.
• Make code easier to maintain.
• Help beginners understand programs.
• Document important logic.

C commonly uses two types of comments:

1. Single-line comments
2. Multi-line comments

A single-line comment begins with two forward slashes: //

// This is a single-line comment

The comment continues until the end of that line.

#include <stdio.h>

int main()
{
    // Display Hello World
    printf("Hello World");

    return 0;
}

A single-line comment can also be written after a statement.

int age = 20; // Store age

The part after // is treated as a comment.

A multi-line comment starts with /* and ends with */.

/*
   This is a
   multi-line comment
*/

#include <stdio.h>

int main()
{
    /*
       Display a welcome message
       to the user
    */

    printf("Welcome to C");

    return 0;
}

Comments are not treated as executable instructions. They are removed or otherwise ignored during the preprocessing stages before the compiler processes the resulting source.

// This does not print anything

A comment can explain the purpose of a variable.

int marks = 85; // Student marks

This makes the purpose of marks clear to another programmer.

int age = 20;       // Student age
float fee = 5000.0; // Course fee
char grade = 'A';   // Student grade

Each comment explains the corresponding variable.

Comments can be placed before a block of code to explain its purpose.

// Calculate the total
int total = price + tax;

#include <stdio.h>

int main()
{
    // Start of program
    printf("Hello");

    // End the program
    return 0;
}

Comments can explain what a function is designed to do.

// Displays a welcome message
void welcome()
{
    printf("Welcome");
}

// Adds two numbers
int add(int a, int b)
{
    return a + b;
}

The comment gives a quick description of the function's purpose.

int length = 10;
int width = 5;

// Calculate rectangle area
int area = length * width;

Comments can explain the purpose of a condition.

if (age >= 18)
{
    // Person is eligible
    printf("Eligible");
}

// Print numbers from 1 to 5
for (int i = 1; i <= 5; i++)
{
    printf("%d\n", i);
}

Comments can be useful when temporarily disabling a line of code during testing.

printf("Hello");

// printf("This line is temporarily disabled");

The commented statement will not execute.

Several lines can be commented using a multi-line comment.

/*
printf("Line 1");
printf("Line 2");
printf("Line 3");
*/

#include <stdio.h>

int main()
{
    // This comment does not appear in output
    printf("Hello World");

    return 0;
}

Output:

Hello World

The comment itself is not displayed.

Good comments should be:

• Short and clear.
• Relevant to the code.
• Easy to understand.
• Updated when the related code changes.

// Calculate total price

Comments should add useful information. Avoid comments that simply repeat obvious code.

int age = 20; // Declare age variable

A more useful comment should explain something that is not already obvious, when such an explanation is needed.

A program can begin with a comment describing the purpose of the file.

/*
   Program: Student Marks
   Purpose: Calculate total marks
*/

#include <stdio.h>

/*
   Program to calculate two numbers
*/

#include <stdio.h>

int main()
{
    int a = 10;
    int b = 20;

    // Calculate the sum
    int sum = a + b;

    printf("Sum = %d", sum);

    return 0;
}

C block comments should not be treated as nested block comments. Avoid placing one /* ... */ comment inside another.

/*
   Outer comment

   /* Inner comment */

   Avoid this style
*/

Use separate comments or single-line comments when necessary.

Large programs can contain hundreds or thousands of lines of code. Well-written comments can help developers quickly understand important parts of the program.

// Calculate final bill after applying discount
finalBill = bill - discount;

Add useful comments to the following program:

#include <stdio.h>

int main()
{
    int a = 10;
    int b = 20;
    int sum = a + b;

    printf("Sum = %d", sum);

    return 0;
}

Try explaining the purpose of the variables and calculation using comments.

/*
   Program to calculate the sum
   of two numbers
*/

#include <stdio.h>

int main()
{
    // Declare two numbers
    int a = 10;
    int b = 20;

    // Calculate the sum
    int sum = a + b;

    // Display the result
    printf("Sum = %d", sum);

    return 0;
}

This example shows how comments can document different sections of a simple C program.