Issue with `int` vs. `double` - why isn't my program working? - c++

Question

I have just started teaching myself C++ after 2+ years of working with MATLAB and wanting something a little more robust. I am currently using code::blocks with MinGW I am trying to get the hang of using int and double, but for some reason my little test program won't work.
The program was a way for me to practice using functions. The program takes in numbers from the user and stops when a negative number is inserted. Then, it spits out the sum of the input numbers, product of input numbers, and a count of the numbers processed.
When I use only int, the program works as intended, but can't handle any "decimal" numbers thrown at it - it freaks out.
#include <cstdio>
#include <iostream>
#include <cstdlib>
using namespace std;
int X;
int sum;
int product;
int SUM()
{
int A = sum+X;
sum = A;
}
int PRODUCT()
{
int B=product*X;
product = B;
}
int main()
{
cout<<"Welcome to my counting program! "<<endl;
sum = 0;
product = 1;
int counter = 0;
for (;;)
{
cout<<"Give me a number: ";
cin>>X;
if (X<0)
{
break;
}
sum = SUM();
product = PRODUCT();
counter = counter+1;
}
cout<<"SUM: ";
cout<< sum <<endl;
cout<<"PRODUCT: ";
cout<< product <<endl;
cout<<"NUMBERS INPUT: ";
cout<< counter <<endl;
system ("PAUSE");
}
However, when I use double where applicable, the program only gives me zeroes for the product and sum.
#include <cstdio>
#include <iostream>
#include <cstdlib>
using namespace std;
double X;
double sum;
double product;
int SUM()
{
double A = sum+X;
sum = A;
}
int PRODUCT()
{
double B=product*X;
product = B;
}
int main()
{
cout<<"Welcome to my counting program! "<<endl;
sum = 0.0;
product = 1.0;
double counter = 0.0;
for (;;)
{
cout<<"Give me a number: ";
cin>>X;
if (X<0)
{
break;
}
sum = SUM();
product = PRODUCT();
counter = counter+1;
}
cout<<"SUM: ";
cout<< sum <<endl;
cout<<"PRODUCT: ";
cout<< product <<endl;
cout<<"NUMBERS INPUT: ";
cout<< counter <<endl;
system ("PAUSE");
}
What am I doing wrong? what do I need to change to make it work?
Also, I think I am calling my functions incorrectly and trying to pass values between them incorrectly - what is the correct way to do so?

Answer 1

You're still returning int from your functions. Also, since you're assigning the results, you need to return an appropriate value (sum = SUM();).
Try the following changes:
double /*int*/ SUM()
{
double A = sum+X;
sum = A;
return sum; // Return a value!
}
double /*int*/ PRODUCT()
{
double B=product*X;
product = B;
return product; // Return a value!
}
I would also recommend avoiding the globals for sum/product/X, and instead declare them local to your functions, and pass them as arguments to your methods.