std::gslice (3) Linux Manual Page
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
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:
See also
operator[] (public member function)
slice (class)
gslice_array (class template)