## Introduction

There are a couple of ways in which you can define constant value.

The first is to define Pi as a symbol that's to be replaced in the program by its value during compilation.

In this case, Pi isn't a variable at all, but more a sort of alias for the value it represents.

## Demo

#include <stdio.h>
#define PI 3.14159f // Definition of the symbol PI
int main(void)
{
float radius = 0.0f;
float diameter = 0.0f;
float circumference = 0.0f;
float area = 0.0f;
printf("Input the diameter of a table:");
scanf("%f", &diameter);
radius = diameter/2.0f;/* w w w . ja v a2s . co m*/
circumference = 2.0f*PI*radius;
area = PI*radius*radius;
printf("\nThe circumference is %.2f. ", circumference);
printf("\nThe area is %.2f.\n", area);
return 0;
}

## Result

## how It Works

The following preprocessing directive creates a symbol.

#define PI 3.14159f // Definition of the symbol PI

This defines PI as a symbol that is to be replaced by the string 3.14159f.

The second option is to define Pi as a constant variable by prefixing the type name with the keyword const. For example:

const float Pi = 3.14159f; // Defines the value of Pi as fixed

The code above defines it as a constant numerical value with a specified type.

## Demo

#include <stdio.h>
int main(void)
{
float diameter = 0.0f; // The diameter of a table
float radius = 0.0f; // The radius of a table
const float Pi = 3.14159f; // Defines the value of Pi as fixed
printf("Input the diameter of the table:");
scanf("%f", &diameter);
radius = diameter/2.0f;/*from w ww. j a v a2 s . c om*/
printf("\nThe circumference is %.2f.", 2.0f*Pi*radius);
printf("\nThe area is %.2f.\n", Pi*radius*radius);
return 0;
}

## Result