this is part of my program...I'm trying to figure out how to test the value of the radius so that it is not equal to zero..if it is...an error message must print and end the program????

void area_circumofcircle()

{

float r1;

float r2;

float Area;

float Circum;

const double pi=3.14159;

cout << endl << endl << "Calculate The Area and Circumference Of A Circle!\n\nEnter A Floating Point For The Radius ";

cout << endl << "Enter radius 1: ";

cin >> r1;

cout << endl << "Enter radius 2: ";

cin >> r2;

Area = pi * r1 * r2;

cout << endl << endl << "The area of circle is : "<< Area;

Circum = 2 * pi * r1;

cout << endl << endl << "The circumference of circle is : "<<Circum;

}