std::gslice
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 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.
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.
gslice(0, std::valarray<std::size_t>(), std::valarray<std::size_t>()). This constructor exists only to allow construction of arrays of slices.start, sizes, strides.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:
#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 constructed generic slice is usable | it is usable (as an empty subset) |
See also
| get/set valarray element, slice, or mask (public member function) |
|
| BLAS-like slice of a valarray: starting index, length, stride (class) |
|
| proxy to a subset of a valarray after applying a gslice (class template) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/cpp/numeric/valarray/gslice
