std::slice - cppreference.com (original) (raw)

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[].

Contents

• 1 Member functions
• 2 std::slice::slice
  • 2.1 Parameters
• 3 std::slice::start, size, stride
  • 3.1 Parameters
  • 3.2 Return value
  • 3.3 Complexity
  • 3.4 Non-member functions
• 4 operator==(std::slice)
  • 4.1 Parameters
  • 4.2 Return value
  • 4.3 Example
  • 4.4 Defect reports
  • 4.5 See also

[edit] Member functions

| | constructs a slice (public member function) | | --------------------------------------------------------------- | | | 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

1. Constructs a copy of other.

Parameters

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.

[edit] Non-member functions

operator==(std::slice)

| friend bool 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

Return value

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

[edit] Example

Barebones valarray-backed Matrix class with a trace calculating function.

#include #include   class Matrix { std::valarray 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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

| | get/set valarray element, slice, or mask (public member function) [edit] | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | generalized slice of a valarray: starting index, set of lengths, set of strides (class) [edit] | | | proxy to a subset of a valarray after applying a slice (class template) [edit] | | | a multi-dimensional non-owning array view (class template) [edit] |