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.
Constructs a new generic slice.
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, strideReturns the parameters passed to the slice on construction - start, sizes and strides respectively.
Parameters(none)
Return valueThe parameters of the slice -- start, sizes and strides respectively.
ComplexityConstant.
[edit] ExampleDemonstrates 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[edit] 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) [edit] See also get/set valarray element, slice, or maskRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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