std::gslice

Header: <valarray>

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.

# Declarations

class gslice;

# 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

# Return value

The parameters of the slice – start, sizes and strides respectively.

# Example

#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);
}

# Defect reports

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)