std::slice (3) - Linux Manuals

std::slice: std::slice

NAME

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