std::slice

 

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swap(std::valarray) (C++11) begin(std::valarray) (C++11)   end(std::valarray) (C++11) abs exp log log10 pow sqrt sin cos tan asin acos atan atan2 sinh cosh tanh	operator*operator/operator%operator+operator-operator^operator&operator|operator<<operator>>operator&&operator|| operator==operator!=operator<operator>operator<=operator>=
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slice_array gslice_array indirect_array	slice gslice mask_array
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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 indices 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()	(1)
slice(std::size_t start, std::size_t size, std::size_t stride );	(2)
slice(const slice& other );	(3)

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.

Non-member functions

operator==(std::slice) (C++20)

checks if two slices are equal (function)

operator==(std::slice)

friendbool operator==(const slice& lhs, const slice& rhs );		(since C++20)

Checks if the parameters of lhs and rhs - start, size and stride are equal respectively.

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::slice is an associated class of the arguments.

The != operator is synthesized from operator==.

Parameters

lhs, rhs

-

slices to compare

Return value

lhs.start()== rhs.start()&& lhs.size()== rhs.size()&& lhs.stride()== rhs.stride()

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

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 slice is usable	it is usable (as an empty subset)

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)
mdspan (C++23)	a multi-dimensional non-owning array view (class template)