const

Learn to declare immutable variables with the const keyword.

The const keyword

The const keyword tells that a variable's value will not change during the program. After declaration, you cannot assign a new value:

const double PI = 3.14;
PI = 3.15;  // error

Constants are named values that make code readable. Writing PI instead of 3.14 states your intent clearly. The compiler forbids accidental changes.

What is const for?

const serves three main purposes:

Safety: prevents accidental changes
Readability: TAX is more meaningful than 0.18
Documentation: states clearly that the value is fixed

Use const for any value that should not change during the program. This conveys your intent to both the compiler and the reader.

Exercise Compute with a constant

Define const double TAX = 0.18;. Set double price = 100.0; and print price * TAX with printf using %lf.

Show example solution

#include <stdio.h>

int main(void) {
    const double TAX = 0.18;
    double price = 100.0;
    printf("%lf", price * TAX);
    return 0;
}

Test yourself

After const double PI = 3.14;?
- You can assign any value to PI
- PI cannot be reassigned
- PI is undefined
- PI automatically becomes 0
Show answer

PI cannot be reassigned — A const value cannot be reassigned.

const type

const ____ PI = 3.14;

Show answer

const double PI = 3.14;

Match the keywords

const

Constant
int

Integer
double

Decimal
char

Character

Next lesson: Variable scope

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