std::gslice(3)

NAME
std::gslice - std::gslice

Synopsis
   Defined in header <valarray>
   class gslice;
   std::gslice is the selector class that identifies a subset of std::valarray indices
   defined by a multi-level set of strides and sizes. Objects of type std::gslice can
   be used as indices with valarray's operator[] to select, for example, columns of a
   multidimensional array represented as a valarray.
   Given the starting value s, a list of strides i
   j and a list of sizes d
   j, a std::gslice constructed from these values selects the set of indices k
   j=s+Σ
   j(i
   jd
   j).
   For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3}
   generates the following set of 24=2*4*3 indices:
   3 + 0*19 + 0*4 + 0*1 = 3,
   3 + 0*19 + 0*4 + 1*1 = 4,
   3 + 0*19 + 0*4 + 2*1 = 5,
   3 + 0*19 + 1*4 + 0*1 = 7,
   3 + 0*19 + 1*4 + 1*1 = 8,
   3 + 0*19 + 1*4 + 2*1 = 9,
   3 + 0*19 + 2*4 + 0*1 = 11,
   ...
   3 + 1*19 + 3*4 + 1*1 = 35,
   3 + 1*19 + 3*4 + 2*1 = 36
   It is possible to construct std::gslice objects that select some indices more than
   once: if the above example used the strides {1,1,1}, the indices would have been {3,
   4, 5, 4, 5, 6, ...}. Such gslices may only be used as arguments to the const version
   of std::valarray::operator[], otherwise the behavior is undefined.

Member functions
   constructor constructs a generic slice
                 (public member function)
   start returns the parameters of the slice
   size (public member function)
   stride std::gslice::gslice
   gslice() (1)
   gslice( std::size_t start, const std::valarray<std::size_t>& sizes, (2)
                        const std::valarray<std::size_t>& strides );
   gslice( const gslice& other ); (3)
   Constructs a new generic slice.
   1) Default constructor. Equivalent to gslice(0, std::valarray<std::size_t>(),
   std::valarray<std::size_t>()). This constructor exists only to allow construction of
   arrays of slices.
   2) Constructs a new slice with parameters start, sizes, strides.
   3) Constructs a copy of other.

Parameters
   start - the position of the first element
   sizes - an array that defines the number of elements in each dimension
   strides - an array that defines the number of positions between successive elements
             in each dimension
   other - another slice to copy std::slice::start, size, stride
   std::size_t start() const; (1)
   std::valarray<std::size_t> size() const; (2)
   std::valarray<std::size_t> stride() const; (3)
   Returns the parameters passed to the slice on construction - start, sizes and
   strides respectively.

Parameters
   (none)

Return value
   The parameters of the slice -- start, sizes and strides respectively.

Complexity
   Constant.

Example
   Demonstrates the use of gslices to address columns of a 3D array: // Run this code
 #include <iostream>
 #include <valarray>
 void test_print(std::valarray<int>& v, int planes, int rows, int cols)
 {
     for (int r = 0; r < rows; ++r)
     {
         for (int z = 0; z < planes; ++z)
         {
             for (int c = 0; c < cols; ++c)
                 std::cout << v[z * rows * cols + r * cols + c] << ' ';
             std::cout << " ";
         }
         std::cout << '\n';
     }
 }
 int main()
 {
     std::valarray<int> v = // 3d array: 2 x 4 x 3 elements
         {111,112,113 , 121,122,123 , 131,132,133 , 141,142,143,
          211,212,213 , 221,222,223 , 231,232,233 , 241,242,243};
     // int ar3d[2][4][3]
     std::cout << "Initial 2x4x3 array:\n";
     test_print(v, 2, 4, 3);
     // update every value in the first columns of both planes
     v[std::gslice(0, {2, 4}, {4 * 3, 3})] = 1; // two level one strides of 12 elements
                        // then four level two strides of 3 elements
     // subtract the third column from the second column in the 1st plane
     v[std::gslice(1, {1, 4}, {4 * 3, 3})] -= v[std::gslice(2, {1, 4}, {4 * 3, 3})];
     std::cout << "\n" "After column operations:\n";
     test_print(v, 2, 4, 3);
 }

Output:
 Initial 2x4x3 array:
 111 112 113 211 212 213
 121 122 123 221 222 223
 131 132 133 231 232 233
 141 142 143 241 242 243
 After column operations:
 1 -1 113 1 212 213
 1 -1 123 1 222 223
 1 -1 133 1 232 233
 1 -1 143 1 242 243
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   LWG 543 C++98 it was unclear whether a default it is usable (as an empty
                      constructed generic slice is usable subset)

See also
   operator[] get/set valarray element, slice, or mask
                (public member function)
   slice BLAS-like slice of a valarray: starting index, length, stride
                (class)
   gslice_array proxy to a subset of a valarray after applying a gslice
                (class template)