std::gslice (3) - Linux Manuals

std::gslice: std::gslice

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 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 + 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

              constructs a generic slice
constructor (public member function)

start returns the parameters of the slice
size (public member function)
stride

 std::gslice::gslice

gslice()
gslice( std::size_t start, const std::valarray<std::size_t>& sizes,
const std::valarray<std::size_t>& strides );
gslice( const gslice& other );

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 rows, int cols, int planes)
  {
      for(int r=0; r<rows; ++r) {
          for(int c=0; c<cols; ++c) {
              for(int z=0; z<planes; ++z)
                  std::cout << v[r*cols*planes + c*planes + z] << ' ';
              std::cout << '\n';
          }
          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 << "After column operations: \n";
      test_print(v, 2, 4, 3);
  }

Output:

  Initial 2x4x3 array:
  111 112 113
  121 122 123
  131 132 133
  141 142 143

  211 212 213
  221 222 223
  231 232 233
  241 242 243

  After column operations:
  1 -1 113
  1 -1 123
  1 -1 133
  1 -1 143

  1 212 213
  1 222 223
  1 232 233
  1 242 243