第四个C++程序:ComputeAreaWithConsoleInput

#include<iostream>
using namespace std;
int main()
{
	double r, s;

	cout << "Please input the radius:" ;
	cin >> r;

	s = r * r * 3.14159;
	cout << "The area is:" << s << endl;

	return 0;
}

《C++程序设计》P28