#include <iostream>
const double pi = 3.14159265358979323846;
#define SQUARE(x)  ( ( x ) * ( x ) )

/* circle area      
   by: A. C. Programmer   
   date started: 12 Sept 2004 
   last updated: 2 Oct 2004 */

// Note: this program is horribly complicated for such a simple purpose.
// I do not recommend that you write a simple program like this.
// Simplicity is beauty.

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

void explain_program( void )
{
    std::cout << "This program calculates the area of a circle.\n";
    std::cout << "Enter the value of the radius and press <Enter>\n";
    std::cout << '\n';
}

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

float get_value( void )
{
    float inputValue;				/* value entered by user */
               
    std::cout << "Value of the radius: ";
    std::cin >> inputValue;
    return inputValue;
}
               

/* circle_area() calculates the area of a circle.
   Returns the value as a float. */
   
float circle_area( float radius )
{
    float area;					/* area of the circle */
               
    area = pi * SQUARE( radius );
    return area;
}

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

void display_answer( float area )
{
    std::cout << "\n\n";				/* two blank lines */
    std::cout << "The area of the circle is " << area << " square units.\n";
}

int main( void )
{
    explain_program();				/* explains program to user */
    float radius = get_value();			/* get radius from user */
    float area = circle_area( radius );		/* compute the circle area */
    display_answer( area );			/* display the answer */
}