Constants

In the previous topic, we learned about variables, whose values can change during program execution.

But what if we want to store a value that should not change?

That's where constants come in.

What is a Constant?

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

For example, if we want to store the value of Pi:

const float PI = 3.14159;

Here, PI is a constant. We should not change its value later.

Think of it like a locked box:

       PI
    ┌────────┐
    │ 3.14159│ 🔒
    └────────┘

The value is stored, but it cannot be modified.

Creating Constants Using const

The most common way to create a constant is by using the const keyword.

const int DAYS = 7;

Now if we try:

DAYS = 10;

the compiler will report an error because DAYS is a constant.

Variable vs Constant

The main difference is simple:

For example:

int age = 20;
age = 21;       // Allowed

const int DAYS = 7;
DAYS = 10;      // Not allowed

Constants Using #define

C also allows us to create constants using #define.

#define PI 3.14159

Now we can use PI in our program:

printf("%f", PI);

Unlike const, #define is handled by the preprocessor before the actual compilation of the C program.

For beginners, just remember that both approaches can be used to represent fixed values.

Why Use Constants?

Constants are useful when a value should remain fixed throughout the program.

For example:

const int MAX_MARKS = 100;
const float PI = 3.14159;

This makes the program easier to understand and prevents accidental changes to important values.

In Simple Words

A variable stores a value that can change, while a constant stores a value that should remain fixed.

For example, a student's marks can change, but the maximum marks might always be 100.