Question

It always outputs ‘0’ for the area. I can’t figure how to work in the int r to calculate the area.

// Define a class and use it to test out some math stuff
#include <iostream>
#include <cmath>
using namespace std;

class Circle {
        public:
                // function that calculates the area of a circle
                float circle_area(int r) {
                        area = 3.14 * (r*r);
                        return area;
                } // end function circle_area
                void display_msg() {
                        cout << "Area: " << circle_area(r) << endl;
                } // end function display_msg
        private:
                float area;
                int r;
}; // end class Circle

int main(void) {
        int r;

        Circle circle_calc; // create a Circle object named circle_calc

        cout << "Radius of circle: ";
        cin >> r;
        circle_calc.display_msg();
}

Answer 1

The member variable Circle::r is never set, the local variable r in main() is set via cin but never used.

Something like this may be want you want:

class Circle {
public:
    Circle(int a_radius) : r(a_radius) {}
    // function that calculates the area of a circle
    float circle_area(int r) {
            area = 3.14 * (r*r);
            return area;
    } // end function circle_area
    void display_msg() {
            cout << "Area: " << circle_area(r) << endl;
    } // end function display_msg
private:
    float area;
    int r;
}; // end class Circle

int main(void) {
    int r;

    cout << "Radius of circle: ";
    cin >> r;

    Circle circle_calc(r);

    circle_calc.display_msg();
}

The variable int r declared in main() is not related to the member variable r in class Circle, even though they have the same name: they are two different variables.

EDIT:

Further refinement of class Circle:

class Circle {
    public:
        // Constructor.
        Circle(int a_radius) : r(a_radius) {}

        // function that calculates the area of a circle
        float circle_area() { return 3.14 * r * r; }

        void display_msg() {
            cout << "Area: " << circle_area() << endl;
        }
    private:
        int r;
}; // end class Circle

Changes:

• area does not need to be a member variable (as stated by Fred Larson)
• r is not required to be passed to circle_area() member function as r is a member variable, so circle_area() has access to it