#include <stdio.h>
#define PI 3.1415926
#define SQUARE(x)  ( ( x ) * ( x ) )

/* circle area      
   by: A. C. Programmer   
   date started: 12 Sept 97 
   last updated: 2 Oct 97 */

/* @ExplainProgram()@ explains the purpose of the program, displaying
   this on the screen of the computer */

void ExplainProgram( void )
{
    printf( "This program calculates the area of a circle. \n" );
    printf( "Enter the value of the radius and press <Enter> \n" );
    printf( "\n" );
}

/* @GetValue()@ prompts user to enter radius.
   Returns the value as a float. */

float GetValue( void )
{
    float inputValue;				/* value entered by user */
               
    printf( "Value of the radius: " );
    scanf( "%f", &inputValue );
    return inputValue;
}
               

/* @CircleArea()@ calculates the area of a circle.
   Returns the value as a float. */
   
float CircleArea( float radius )
{
    float area;					/* area of the circle */
               
    area = PI * SQUARE( radius );
    return area;
}


/* @DisplayAnswer()@ displays the area of the circle on the screeen. */

void DisplayAnswer( float area )
{
    printf( "\n\n" );				/* two blank lines */
    printf( "The area of the circle is %f square units.\n", area );
}

int main( void )
{
    float radius;
    float area;
               
    ExplainProgram();				/* explains program to user */
    radius = GetValue();			/* get radius from user */
    area = CircleArea( radius );		/* compute the circle area */
    DisplayAnswer( area );			/* display the answer */
}