Multidimensional std::valarray and inequalities

Question

The following piece of code does not compile:

#include <valarray>

int main()
{
   std::valarray<std::valarray<int>> a;
   std::valarray<std::valarray<int>> b;
   //std::valarray<std::valarray<bool>> c;
   std::valarray<bool> c;
   c = (a == b);
   return 0;
}

Neither of the declarations of c compile. Is it allowed to use valarray in the above demonstrated manner or are there restrictions on how it should be used.

Answer 1

I believe I have the solution , though I may be wrong.

Because begin and end are non-member overloads , you use begin(a) instead of a.begin() and so on.

My choice in using std::transform is that you want to iterate over a and b , compare them, and store the results in c , which is why the lambda returns a std::valarray<bool> (instead of a bool ).

   std::valarray<std::valarray<int>> a;
   std::valarray<std::valarray<int>> b;
   std::valarray<std::valarray<bool>> c;

   std::transform(begin(a), end(a), begin(b), begin(c),
   [&] (std::valarray<int> a_val, std::valarray<int> b_val) 
       -> std::valarray<bool> {
       return std::valarray<bool>(a_val == b_val);
   });