std::slice
From cppreference.com
Defined in header <valarray>
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class slice; |
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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() |
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slice( std::size_t start, std::size_t size, std::size_t stride ); |
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slice( const slice& other ); |
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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
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) |