Understanding C Constants: Unchangeable and Read-Only Variables

Learn about constants in C, which are unchangeable and read-only variables. Discover how to declare a constant using the const keyword, ensuring that once set, the value cannot be modified, thus enhancing the stability and reliability of your code.

C Constants

What Are Constants?

In C, constants are variables that are unchangeable and read-only. To declare a constant, use the const keyword. Once a variable is declared as constant, its value cannot be modified.

Declaring a Constant

Use the const keyword to declare a variable as constant. For example, the following variable myNum will always have the value of 15:

const int myNum = 15;  // myNum will always be 15
myNum = 10;  // error: assignment of read-only variable 'myNum'

error: assignment of read-only variable 'myNum'

You should declare variables as constants when the values are not meant to change. For example:

const int minutesPerHour = 60;
const float PI = 3.14;

Notes on Constants

• When you declare a constant variable, it must be assigned a value immediately. You cannot assign a value later.

Correct Example

const int minutesPerHour = 60;

Incorrect Example

const int minutesPerHour;
minutesPerHour = 60;  // error

error: uninitialized 'const' variable

Good Practice

It is considered a good practice to declare constant variables in uppercase letters for better code readability. This is a common convention among C programmers:

const int BIRTHYEAR = 1980;