std::gslice

 

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