std:: strided_slice

An instance of every specialization of strided_slice is a slice specifier used in std::submdspan to select a subset of elements using a set of regularly spaced indices at a specified dimension in std::mdspan .

Each strided_slice object s is characterized by three data members: the offset index s. offset , the extent s. extent , and the stride s. stride .

Given that s. stride is greater than zero, the number of selected indices denoted by N is determined by 1 + ( s. extent - 1 ) / s. stride if s. extent is nonzero, otherwise ​ 0 ​ . The half-open interval from which indices will be selected is given by [ s. offset , s. offset + s. extent ) . The sequence of selected indices is produced as follows: s. offset , ..., s. offset + ( N - 1 ) * s. stride .

This class template has no base classes or declared members other than those shown below.

Template parameters

The program is ill-formed if the type requirement is not satisfied.

Member types

Data members

All these members are declared with [[ no_unique_address ]] attribute and have default member initializers where each data member is value initialized.

Notes

Every specialization of strided_slice is an aggregate class which allows aggregate initialization (including designated initialization) of data members (for instance, std :: strided_slice { . offset = 2 , . extent = 10 , . stride = 3 } ).

The slice specification of strided_slice takes advantage of the data member extent , as opposed to other slice specifications that use end to indicate the upper bound value. This is because it can directly generate a static extent for subview of std::mdspan if both extent and stride are of types that satisfy integral-constant-like . This enables efficient extraction of subviews with static extents by mixing compile time values with a runtime value of offset .

Example

#include <mdspan>
#include <print>
 
template <typename View, typename O = int, typename E = int, typename S = int>
    requires (View::extents_type::rank() == 1)
void print_sliced_view(View v, std::strided_slice<O, E, S> s)
{
    using index_type = View::index_type;
    auto subview = std::submdspan(v, s);
    const auto& submap = subview.mapping();
 
    std::print("[");
    bool uses_comma = false;
    for (index_type i = 0; i != subview.extent(0); ++i)
    {
        if (uses_comma)
            std::print(", ");
        std::print("{}", subview[i]);
        uses_comma = true;
    }
    uses_comma = false;
    std::print("] extracted from indices [");
    for (index_type i = 0; i != subview.extent(0); ++i)
    {
        if (uses_comma)
            std::print(", ");
        std::print("{}", submap(i) + s.offset);
        uses_comma = true;
    }
    std::println("]");
}
 
int main()
{
    static constexpr char letters[]
    {
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
        'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
        'U', 'V', 'W', 'X', 'Y', 'Z'
    };
    constexpr std::mdspan md(letters, 26);
    print_sliced_view(md, {.offset = 0, .extent = 10, .stride = 1});
    print_sliced_view(md, {.offset = 2, .extent = 10, .stride = 1});
    print_sliced_view(md, {.offset = 0, .extent = 5,  .stride = 1});
    print_sliced_view(md, {.offset = 2, .extent = 5,  .stride = 1});
    print_sliced_view(md, {.offset = 0, .extent = 10, .stride = 2});
    print_sliced_view(md, {.offset = 2, .extent = 10, .stride = 3});
    print_sliced_view(md, {.offset = 0, .extent = 15, .stride = 5});
    print_sliced_view(md, {.offset = 6, .extent = 15, .stride = 5});
}

[A, B, C, D, E, F, G, H, I, J] extracted from indices [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[C, D, E, F, G, H, I, J, K, L] extracted from indices [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
[A, B, C, D, E] extracted from indices [0, 1, 2, 3, 4]
[C, D, E, F, G] extracted from indices [2, 3, 4, 5, 6]
[A, C, E, G, I] extracted from indices [0, 2, 4, 6, 8]
[C, F, I, L] extracted from indices [2, 5, 8, 11]
[A, F, K] extracted from indices [0, 5, 10]
[G, L, Q] extracted from indices [6, 11, 16]

See also