#include <iostream>
#include <cmath>
using namespace std;

double r, area;
int DecimalPlaces;

int main()
{
    cout << "Enter a radius where it is greater than or equal to 0: " << endl;
    cin >> r;
    cout << "To how many decimal places do you want the area to be given to you?" << endl;
    cin >> DecimalPlaces;

    area = M_PI * (r*r);

    cout << "The area of the circle with radius " << r << " is ";

    cout.setf (ios_base::fixed, ios_base::floatfield);
    cout.precision (DecimalPlaces);

    cout << area << "." << endl;

    system ("PAUSE");

    return 0;
}

#include<cstdio>
#include<cmath>

double r, area;
main()
{
    printf("Enter a radius greater than or equal to 0\n");
    scanf("%f", &r); // scanf needs a pointer when inputting variables, hence &r instead of r
    area= M_PI * (r*r);
    printf("%.5f\n", area); // the .5 will print to 5 decimal places
}

#include <iostream>
#include <cmath>

class INT
{
        public:
                //does all standard operations like a good integer should
                INT();
                INT(int value) {val=value;}
                INT operator*(INT other) const {return INT(val*other.val);}
                INT operator/(INT other) const {return INT(val/other.val);}
                INT operator+(const INT &other) const {return INT(val+other.val);}
                INT operator-(const INT &other) const {return INT(val-other.val);}
                INT operator^(const INT &other) const {return INT((int)pow((double)val,(double)other.val));}
                INT operator-() const {return INT(val*-1);}
       
                const INT &operator*=(const INT &other){val*=other.val; return *this;}
                const INT &operator/=(const INT &other){val/=other.val; return *this;}
                const INT &operator+=(const INT &other){val+=other.val; return *this;}
                const INT &operator-=(const INT &other){val-=other.val; return *this;}
                const INT &operator=(const INT &other){val=other.val; return *this;}
               
                //some code shamelessly stolen from wtd
                friend std::ostream& operator<<(std::ostream& out, const INT& other);
        private:
                int val;
};

std::ostream& operator<<(std::ostream& out, const INT& other)
{
    return out <<other.val;
}

int main()
{
        INT a=2;
        INT b=3;
        std::cout<<"a="<<a<<std::endl
                    <<"b="<<b<<std::endl
                    <<"a+b="<<(a+b)<<std::endl
                    <<"b-a="<<(b-a)<<std::endl
                    <<"a^b="<<(a^b)<<std::endl;
        return 0;
}