How to compare C++ slice_array? Why it cannot be compared as valarray?

Question

I wonder why I cannot compare some_valarray[first_slice] < another_valarray[second_slice] as I compare some_valarray < another_valarray and how I can do that in simple way without copying? Of course, I can iterate over them in plain loop but, maybe, there is more elegant and, probably, optimizable way?

Answer 1

To compare a slice_array you currently have to use valarray( const std::slice_array& ).

#include <valarray>

int main() {
  //Initialize valarray
  std::valarray<int> val0{0,1,2,3,4,5,6,7,8,9};
  std::valarray<int> val1{9,8,7,6,5,4,3,2,1,0};

  //Compare valarray
  std::valarray<bool> cmp0 = val0 < val1;

  //Compare slice_array
  std::valarray<bool> cmp1 = 
std::valarray<int>(val0[std::slice(0,5,2)]) < std::valarray<int>(val1[std::slice(1,5,2)]);

  //Compare slice_array since c++17
  std::valarray<bool> cmp2 =
std::valarray(val0[std::slice(0,5,2)]) < std::valarray(val1[std::slice(1,5,2)]);

  return 0;
}