std::slice (3) Linux Manual Page

std::slice – std::slice

Synopsis

Defined in header <valarray>

class slice;

std::slice is the selector class that identifies a subset of std::valarray similar to BLAS slice. An object of type std::slice holds three values: the starting index, the stride, and the total number of values in the subset. Objects of type std::slice can be used as indexes with valarray’s operator[].

Member functions

              constructs a slice
constructor (public member function)

start returns the parameters of the slice
size (public member function)
stride

 std::slice::slice

slice()
slice( std::size_t start, std::size_t size, std::size_t stride );
slice( const slice& other );

Constructs a new slice.
1) Default constructor. Equivalent to slice(0, 0, 0). This constructor exists only to allow construction of arrays of slices.
2) Constructs a new slice with parameters start, size, stride. This slice will refer to size number of elements, each with the position:
start + 0*stride
start + 1*stride
…
start + (size-1)*stride
3) Constructs a copy of other.

Parameters

start – the position of the first element
size – the number of elements in the slice
stride – the number of positions between successive elements in the slice
other – another slice to copy

 std::slice::start, size, stride

std::size_t start() const; (1)
std::size_t size() const; (2)
std::size_t stride() const; (3)

Returns the parameters passed to the slice on construction – start, size and stride respectively.

Parameters

(none)

Return value

The parameters of the slice — start, size and stride respectively.

Complexity

Constant.

Example

Barebones valarray-backed Matrix class with a trace calculating function.
// Run this code

#include <iostream>

#include <valarray>

class Matrix
{
    std::valarray<int> data;
    int dim;

  public:
    Matrix(int r, int c)
        : data(r * c), dim(c)
    {
    }
    int &operator()(int r, int c)
    {
        return data[r * dim + c];
    }
    int trace() const
    {
        return data[std::slice(0, dim, dim + 1)].sum();
    }
};

int main()

{
    Matrix m(3, 3);
    int n = 0;
    for (int r = 0; r < 3; ++r)
        for (int c = 0; c < 3; ++c)
            m(r, c) = ++n;
    std::cout << "Trace of the matrix (1,2,3) (4,5,6) (7,8,9) is " << m.trace() << '\n';
}

Output:

  Trace of the matrix (1,2,3) (4,5,6) (7,8,9) is 15

See also

            get/set valarray element, slice, or mask
operator[] (public member function)
            generalized slice of a valarray: starting index, set of lengths, set of strides
gslice (class)
            proxy to a subset of a valarray after applying a slice
slice_array (class template)