std::gslice - cppreference.com (original) (raw)

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 ij and a list of sizes dj, a std::gslice constructed from these values selects the set of indices kj=s+Σj(ijdj).

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.

Contents

• 1 Member functions
• 2 std::gslice::gslice
  • 2.1 Parameters
• 3 std::slice::start, size, stride
  • 3.1 Parameters
  • 3.2 Return value
  • 3.3 Complexity
  • 3.4 Example
  • 3.5 Defect reports
  • 3.6 See also

[edit] Member functions

| | constructs a generic slice (public member function) | | --------------------------------------------------------------- | | | returns the parameters of the slice (public member function) |

std::gslice::gslice

| gslice() | (1) | | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --- | | | gslice( std::size_t start, const std::valarray<std::size_t>& sizes, const std::valarray<std::size_t>& strides ); | (2) | | | gslice( const gslice& other ); | (3) | |

Constructs a new generic slice.

1. Constructs a new slice with parameters start, sizes, strides.

2. Constructs a copy of other.

Parameters

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.

[edit] Example

Demonstrates the use of gslices to address columns of a 3D array:

#include #include   void test_print(std::valarray& 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 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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

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