program to calculate circumference and area not outputting values from void functions - c++

Question

I wrote this program for my course, but when I run it the output within the OutputData() function just spits out zero.
we have been studying void functions and referencing. I do everything that I think I am supposed to be doing. And he gives an outline on what we should have a set up. Sadly, it does not work and I'm sure I'm missing something.
What makes it output zero when run?
#include "stdafx.h"
#include <iostream>
#include <cmath>
using namespace std;
// Constants
const double pi = 3.14;
// Global Variables
double radius, circumference, area;
// Prototypes
void Banner();
void ComputeArea(double, double);
int ComputeCircumference(double);
void GetValue(double);
bool GoAgain();
void OutputData(double, double, double);
int main()
{
Banner();
do {
GetValue(radius);
circumference = ComputeCircumference(radius);
ComputeArea(radius, area);
OutputData(radius, area, circumference);
} while (GoAgain());
return 0;
} // Function main()
// ==================
// =================
void Banner() {
cout << "Welcome to the program!\n";
cout << "Input a radius of a circle,\n";
cout << "I will compute the area and\n";
cout << "circumference of that radius.\n";
cout << "Let's begin!\n";
} // Function Banner()
// =========================
// =====================
void ComputeArea(double, double) {
area = pow((pi * radius), 2);
} // Function ComputeArea()
// ==============================
// =====================================
int ComputeCircumference(double
circumference) {
circumference = 2 * pi * radius;
return circumference;
} // Function ComputeCircumference()
// ======================================
// ==========================
void GetValue(double) {
double radius;
cout << "Please enter your circles radius: " << endl;
cin >> radius;
} // Function GetValue()
// ===========================
// =========================
bool GoAgain() {
bool validAnswer;
char answer;
do {
cout << "Enter another radius?\n";
cout << "[y/Y] to go again. [n/N] to exit: ";
cin >> answer;
if ((answer == 'y') || (answer == 'y') ||
(answer == 'n') || (answer == 'N'))
validAnswer = true;
else {
validAnswer = false;
cout << "Error. Enter a valid character: ";
}
} while (!validAnswer);
if ((answer == 'y') || (answer == 'Y'))
return true;
else if ((answer == 'n') || (answer == 'N'))
return false;
} // Function GoAgain()
// ===========================
// ===========================
void OutputData(double, double, double) {
cout << "Here are the results: ";
cout << "You entered: " << radius << " for the radius." << endl;
cout << "Area: " << area << endl;
cout << "Circumference: " << circumference << endl;
} // Function OutputData()
// ============================

Answer 1

If you want to pass results from functions, there are three methods:
Return the result
Pass the result variable by reference (or pointer)
Use a global variable
Your functions don't have variable names in their declarations or definitions.
Note: when passing values to functions without reference, the values are passed by copy. Modifying of a non-reference parameter modifies the copy not the original variable passed to the function.
Returning the Result
//! Note the return type
double Compute_Area(double radius) // Note the parameter name
{
// Note the "return" statement used to return a value.
return 2.0 * pi * radius;
}
Return by Reference
void Compute_Area(double radius,
double& area) // Note the '&' to designate reference
{
// The "area" variable is the parameter variable,
// which is the caller's variable.
area = 2.0 * pi * radius;
}
Global Variable
double area;
void Compute_Area(double radius)
{
area = 2.0 * pi * radius;
}
You should make a habit of providing parameter names in your function's declarations and definitions.
When using more than one parameter of the same type, parameter names help distinguish the parameters (such as height and width for the area of a rectangle). Otherwise Users of the functions (including yourself) will have a difficult time figuring out the purpose of each parameter.