Constants and Literals in C

In the previous lesson, you learned about variables and data types. Variables can change their values during program execution. However, sometimes we need values that should never change. These values are called constants.

You will also learn about literals, which are fixed values written directly in a program.

Understanding constants and literals helps you write safer, cleaner, and more maintainable programs.

🎯 Learning Objectives

After completing this lesson, you will be able to:

• Understand constants and literals.
• Differentiate between variables and constants.
• Use the const keyword.
• Create symbolic constants using #define.
• Learn different types of literals.
• Write programs using constants.

📖 What is a Constant?

A constant is a value that cannot be changed during program execution.

Once assigned, its value remains fixed.

Examples:

• Number of days in a week → 7
• Value of PI → 3.14159
• Speed of light
• GST percentage (if fixed)

Variable vs Constant

Example

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int age = 20;

age = 25;      // Allowed

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const int age = 20;

age = 25;      // Error

Using the const Keyword

Syntax

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const datatype variableName = value;

Example

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#include <stdio.h>

int main()
{
    const float PI = 3.14159;

    printf("%.5f", PI);

    return 0;
}

Output

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3.14159

Attempting to Modify a Constant

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#include <stdio.h>

int main()
{
    const int marks = 100;

    marks = 90;

    return 0;
}

The compiler generates an error because constants cannot be modified.

Symbolic Constants Using #define

The C Preprocessor provides the #define directive.

Syntax

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#define NAME value

Example

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#include <stdio.h>

#define PI 3.14159

int main()
{
    printf("%.5f", PI);

    return 0;
}

Output

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3.14159

Unlike variables, #define simply replaces the symbol before compilation.

const vs #define

What is a Literal?

A literal is a fixed value written directly in the source code.

Examples

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10

25.5

'A'

"KauraX"

These values are called literals because they appear exactly as written.

Types of Literals

C supports several types of literals.

• Integer Literals
• Floating-Point Literals
• Character Literals
• String Literals

Integer Literals

Whole numbers are called integer literals.

Examples

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10

250

-45

1000

Example Program

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#include <stdio.h>

int main()
{
    int students = 250;

    printf("%d", students);

    return 0;
}

Floating-Point Literals

Numbers containing decimal points.

Examples

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2.5

10.75

3.14159

Program

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#include <stdio.h>

int main()
{
    float rating = 4.9;

    printf("%.1f", rating);

    return 0;
}

Output

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4.9

Character Literals

A character literal represents a single character enclosed in single quotes.

Examples

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'A'

'B'

'9'

'#'

Program

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#include <stdio.h>

int main()
{
    char grade = 'A';

    printf("%c", grade);

    return 0;
}

Output

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A

String Literals

A string literal is enclosed in double quotes.

Example

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"KauraX"

"Programming"

"Welcome"

Program

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#include <stdio.h>

int main()
{
    printf("Welcome to KauraX");

    return 0;
}

Output

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Welcome to KauraX

Real-World Example

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#include <stdio.h>

#define PLATFORM "KauraX"

int main()
{
    const int TOTAL_COURSES = 20;

    printf("Platform : %s\n", PLATFORM);

    printf("Courses  : %d", TOTAL_COURSES);

    return 0;
}

Output

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Platform : KauraX
Courses  : 20

Advantages of Constants

• Prevent accidental changes.
• Improve readability.
• Make code easier to maintain.
• Avoid repeating values.
• Increase program reliability.

Common Beginner Mistakes

Modifying a Constant

Wrong

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const int MAX = 100;

MAX = 50;

Correct

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const int MAX = 100;

Using Character Instead of String

Wrong

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char name = "KauraX";

Correct

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char name[] = "KauraX";

Forgetting Quotes

Wrong

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printf(KauraX);

Correct

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printf("KauraX");

Using #define with Semicolon

Wrong

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#define PI 3.14;

Correct

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#define PI 3.14

Best Practices

• Use const whenever possible.
• Use uppercase names for constants.
• Keep constants meaningful.
• Avoid magic numbers in programs.
• Use #define for symbolic values used throughout the program.

🧠 Interview Questions

1. What is a constant?
2. What is a literal?
3. Differentiate between variables and constants.
4. What is the purpose of the const keyword?
5. Explain #define.
6. Differentiate between const and #define.
7. What are string literals?
8. What are integer literals?

✍ Practice Questions

1. Declare three constants using const.
2. Write a program using #define.
3. Display the value of PI using a constant.
4. Print your college name using a string literal.
5. List five examples of literals used in daily programming.

📚 Lesson Summary

In this lesson, you learned:

• What constants are.
• Difference between variables and constants.
• Using the const keyword.
• Creating symbolic constants with #define.
• Different types of literals.
• Advantages of constants.
• Best practices and common mistakes.

Using constants makes your programs more reliable, readable, and easier to maintain.

🚀 What's Next?

In the next lesson, Input and Output in C, you will learn:

• printf()
• scanf()
• Format specifiers
• Reading user input
• Displaying formatted output
• Escape sequences
• Interactive programs

By the end of the next lesson, your programs will be able to interact with users instead of displaying only fixed output.

Happy Coding with KauraX 💙