Overview

In this tutorial, we will learn about C constants. Constants are read-only variables. The value of the constant cannot be changed in the program. C allows two ways to define the constants in the program

• const keyword
• #define directive

const keyword

The general syntax to define a constant using the const keyword is as follows:

const <data_type> <constant_name> = <read_only_Value>;

<data_type> const <constant_name> = <read_only_Value>;

Example

const int ROWS=10;

int const ROWS=10;

We can use the const keyword before or after the datatype.

#define directive

The general syntax to define using the #define directive is as follows:

#define <constant_name>  <readOnly_Value>

Example

#define COLUMNS 10

This #define directive instructs the C compiler to replace the COLUMNS with value 10. To increase code readability constant names are defined in UPPERCASE.

 

C Constant Action	Yes/ No
Initialize the constant with an initial value	✅
Read the constant value in the program	✅
Modify/Change the constant value in the program	❌

Example

In this example, we will define a floating-point value as constant using the #define preprocessor directive. In the function, we will use the value to calculate the area of the circle.

#define PI 3.14159265

C Program

/**
*************************************************
 * Program Description:
 * C Constants Demo
 * Filename: constants.c
 * C Tutorials - www.TestingDocs.com
 ************************************************
 */
#include<stdio.h>
#define PI 3.14159265

/*Function declaration*/
void area(const float radius);

int main()
{
 /*local variable */
 float radius=0.0;
 printf("Enter the circle radius (cm):= \n");
 scanf("%f",&radius);
 area(radius); // Function Call
 return 0;
}

// Function definition
void area(const float radius)
{
 float circleArea=0.0;
 circleArea = PI*radius*radius; // use the constant value PI
 printf("Area of Circle = %f cm^2\n",circleArea);
}

Notice that we have used the constant value on the left side of the assignment operator( = ).
The compiler replaces the PI name with the value in the #define directive.

circleArea = PI*radius*radius;

So this statement becomes:

circleArea = 3.14159265*radius*radius;

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C Tutorials

C Tutorials on this website can be found at:

https://www.testingdocs.com/c-language-tutorial/