std::slice

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

(constructor)	constructs a slice (public member function)
startsizestride	returns the parameters of the slice (public member function)

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

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