| 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.
#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 (public member function) | |
| generalized slice of a valarray: starting index, set of lengths, set of strides (class) | |
| proxy to a subset of a valarray after applying a slice (class template) |
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