Basic Program (Knowledge Needed)

#include <iostream>
#include <string>
#include <cmath>

using namespace std;

const double PI = 3.14159; //Const double means that 3.14159 is const double PI. So just type PI or (const)PI in any function to make it work.

int getPositiveInt(string message)
{
    int num = 0;

    do
    {
    cout << message << endl;
    cin >> num;
    }while(num <= 0);

    return num;
}

double areaOfCircle(int r)
{
       return PI * powf(r, 2);             //powf() is a function and for it to work you need to add the #include <cmath>
}                                          //But pow() could work if you change add #include <math.h>

double volOfSphere(int r)
{
       return 4/3.0 * PI * powf(r, 3);
}

int main()
{
       int radius = getPositiveInt("Enter a positive integer for the radius of a circle/sphere: ");

       double aCircle = areaOfCircle(radius);

       double vSphere = volOfSphere(radius);

       cout << "The area of a circle that has a radius of " <<
       radius << " is " << aCircle << "." << endl;

       cout << "The volume of a sphere that has a radius of " <<
       radius << " is " << vSphere << "." << endl;

       system("pause");

       return 0;
}