C++11 std::multiplies on vector containing 0's

Question

I have a vector, containing the following values

0 0 1 1

And my aim is to multiply this vector and return a double, for this, I am using the std::accumulate and std::multiplies but I have noticed a problem, because there is 0's contained the returning value is always 0. For example (0 * 0 * 1 * 1) = 0

Is it possible to do use std::multiplies to ignore all values that are 0's? Technically, in this part, I am just after the result of: (1 * 1) for example.

I am using:

std::accumulate(diag1.begin(), diag1.end(), 1, std::multiplies<double>());

Where diag1 contains the values inside this example.

Answer 1

You can use C++11 and lambda expressions as suggested by Piotr Skotnicki in the comments, or you can utilize the pre-11 approach (verbosity intended):

struct F 
{ 
   template<class T>
   T operator()(const T &a, const T &b) const
   {
      if (a != 0 && b != 0)
         return a*b;
      else 
      {
          if (a != 0) return a;
          else if (b != 0) return b;
          else return 1;
       }
    }
};
double res = std::accumulate(diag1.begin(), diag1.end(), 1.0d, F());

But be careful with the comparison against 0. The 0 represented as a double may not be the mathematical 0. Furthermore, the multiplication may not be necessarily correct because of the way the floating point numbers are represented in binary, by computers.