C++, my double or int value is always 0

Question

I\'m fairly new to C++ and I\'m experiencing some strange behaviour from a percentage increase method I am writing for some image editing software.

What I want to do is

Answer 1

Do this:

// calculate return value
double returnVal = (static_cast(currPixel) / modifier) * newValue;

Or this:

double returnVal = (currPixel / static_cast(modifier)) * newValue;

As you know that operator / will be performed first, and then the operator *. I have typecasted one of the operands of / to double, and hence division will be performed double. Now, left operand of * would be double (since / produced double), and the multiplication would be performed double also. For clarity and correctness, you may write:

double returnVal = (static_cast(currPixel) / static_cast(modifier)) * static_cast(newValue);

Or simply:

double returnVal = (double(currPixel) / (double)modifier) * (double)newValue;

But, following is WRONG:

double returnVal = (double)(currPixel / modifier) * /*(double)*/ newValue;

Since the division would be performed int only! It is like:

double x = 10/3;

Where you need (either):

double x = 10.0/3;
double x = 10/3.0;
double x = (double)10/3;