std::gslice_array(3)

NAME
std::gslice_array - std::gslice_array

Synopsis
   Defined in header <valarray>
   template< class T > class gslice_array;
   std::gslice_array is a helper template used by the valarray subscript operator with
   std::gslice argument. It has reference semantics to a subset of the array specified
   by the std::gslice object.

Member types
   Type Definition
   value_type T

Member functions
   constructor constructs a gslice_array
                 (public member function)
   destructor destroys a gslice_array
                 (public member function)
   operator= assigns contents
                 (public member function)
   operator+=
   operator-=
   operator*=
   operator/=
   operator%= performs arithmetic operation on the array referred by generic slice.
   operator&= (public member function)
   operator|=
   operator^=
   operator<<=
   operator>>=

Example
// Run this code
 #include <cassert>
 #include <cstddef>
 #include <iomanip>
 #include <iostream>
 #include <numeric>
 #include <valarray>
 int main()
 {
     std::valarray<int> data(32);
     std::iota(std::begin(data), std::end(data), 0);
     const std::size_t offset = 1, z = 2, y = 3, x = 4;
     const std::valarray<std::size_t> sizes{z, y, x};
     const std::valarray<std::size_t> strides{15, 5, 1};
     const std::gslice gslice = std::gslice(offset, sizes, strides);
     // indices are generated according to the formula:
     // index[k] = offset + [0,1,2)*15 + [0,1,2,3)*5 + [0,1,2,3,4)*1
     // = offset + inner_product(sizes[k], strides);
     // where sizes[k] = {[0,z), [0,y), [0,x)}, while the rightmost index (x)
     // runs fastest. As a result we have following set of indices:
     // index[0] = 1 + 0*15 + 0*5 + 0*1 = 1
     // index[1] = 1 + 0*15 + 0*5 + 1*1 = 2
     // index[2] = 1 + 0*15 + 0*5 + 2*1 = 3
     // index[3] = 1 + 0*15 + 0*5 + 3*1 = 4
     // index[4] = 1 + 0*15 + 1*5 + 0*1 = 6
     // index[5] = 1 + 0*15 + 1*5 + 1*1 = 7
     // index[6] = 1 + 0*15 + 1*5 + 2*1 = 8
     // index[7] = 1 + 0*15 + 1*5 + 3*1 = 9
     // ...
     // index[22] = 1 + 1*15 + 2*5 + 2*1 = 28
     // index[23] = 1 + 1*15 + 2*5 + 3*1 = 29
     const std::valarray<int> indices = data[gslice];
     for (unsigned i=0; i != indices.size(); ++i) {
         std::cout << std::setfill('0') << std::setw(2) << indices[i] << ' ';
     }
     std::cout << "\nTotal indices: " << indices.size() << '\n';
     assert(indices.size() == x*y*z);
     data = 0;
     std::gslice_array<int> gslice_array = data[gslice];
     gslice_array = 1;
     // Cells that correspond to generated indices = '1', skipped cells = '0'.
     for (auto i : data) { std::cout << i << ' '; }
     std::cout << "\nSum of ones = " << data.sum() << '\n';
 }

Output:
 01 02 03 04 06 07 08 09 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29
 Total indices: 24
 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 0
 Sum of ones = 24

See also
   slice_array proxy to a subset of a valarray after applying a slice
               (class template)