# C - Data Type Constants

## 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 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;//from   w ww.  j  a va 2s  . co  m

printf("\nThe circumference is %.2f. ", circumference);
printf("\nThe area is %.2f.\n", area);
return 0;
}
```

## 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
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  w  w. j a va 2  s  .  co m