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 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¶

constructor constructs a generic slice
(public member function)
start returns the parameters of the slice
size (public member function)
stride

std::gslice::gslice

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

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 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 it is usable (as an empty
constructed generic slice is usable subset)

See also¶

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