Confusion over Initializing variables and changing them in C++ - c++

Question

I'm trying to make a program that will allow the user to enter in a heighth and width of a rectangle, and display the area, however every step of my program is done using functions as part of the assignment.
The issue I'm having is that I have the variables assigned and intialized, though I'm not sure how to overwrite them with user inputted data. If I don't initialize the variables at all the program will not run. I was hoping someone could tell me what I'm doing wrong. My code is:
#include <iostream>
using namespace std;
double getWidth(int x);
double getLength(int y);
double getArea(double x, double y, double a);
double displayData(double a);
int main()
{
int x = 0, y = 0, a = 0;
getWidth(x);
getLength(y);
getArea(x, y, a);
displayData(a);
system("pause");
return 0;
}
double getWidth(int x)
{
cout << "Please enter the width: ";
cin >> x;
return x;
}
double getLength(int y)
{
cout << "Please enter the length: ";
cin >> y;
return y;
}
double getArea(double x, double y, double a)
{
a = x*y;
return a;
}
double displayData(double a)
{
cout << a << endl;
return a;
}

Answer 1

There are two ways of passing variables. Method number one is to pass by value. This is the most common method, and it is the one your program is doing. In this method, a copy of the data in the variable is being made and supplied to the function. Your function only changes the copy and not the original variable.
The second method is to pass by reference. When passing by reference, your function effectively has a pointer to the original variable and can thus change it. To pass by reference, put in an ampersand (&) in front of the variable in the function header. Note in the code below that it is not necessary to pass x and y to getArea by reference because getArea only needs to read these variables not write to them.
This however will introduce a new problem for you. When you pass by value it is possible to change the variable type to a larger type without an explicit cast. This is not possible with passing by reference because the different parts of the program would then be trying to treat the variable as a different type. i.e. main wants to write to/read from a as if it is an integer and getArea wants to write to/read from a as if it is a double. These two data types have different sizes and and different formats so this is not possible. Thus you have to declare a is a double in main.
#include <iostream>
using namespace std;
double getWidth(int &x);
double getLength(int &y);
double getArea(double x, double y, double &a);
double displayData(double a);
int main()
{
int x = 0, y = 0;
double a;
getWidth(x);
getLength(y);
getArea(x, y, a);
displayData(a);
system("pause");
return 0;
}
double getWidth(int &x)
{
cout << "Please enter the width: ";
cin >> x;
return x;
}
double getLength(int &y)
{
cout << "Please enter the length: ";
cin >> y;
return y;
}
double getArea(double x, double y, double &a)
{
a = x*y;
return a;
}
double displayData(double a)
{
cout << a << endl;
return a;
}

Answer 2

You seem to be confusing a few different concepts. You should either be passing references and assigning them within the functions, or passing less values and assigning them to some variable in main. For example, your getWidth should be:
double getWidth() {
double w;
cin >> w;
return w;
}
and in your main you should have:
int main() {
/* ... */
double width = getWidth();
/* ... */
}
And so on for the others as well. You should be looking into references and pointers in C++ as well, that would be the other way that you could do this (and you are seemingly confused about). Finally you should definitely find an introduction to functions in some intro to C++ book, as someone said above.