std::slice (Valarray slice selector)
Valarray slice selector : This class represents a valarray slice selector. It does not contain nor refers to any element – it only describes a selection of elements to be used as an index in valarray::operator.
std::slice is the selector class that identifies a subset of std::valarray. 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.
In simple terms it is used to slice based on an index. A valarray slice is defined by a starting index, a size, and a stride.
slice( std::size_t start, std::size_t size, std::size_t stride ); size_t star : is the index of the first element in the selection size_t size : is the number of elements in the selection stride : is the span that separates the elements selected.
- In given Syntax, by default constructor isequivalent to slice(0, 0, 0). This constructor exists only to allow construction of arrays of slices.
- It 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
slice(1, 5, 4) Input : 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Output : 1 5 9 13 17 Explanation : starting from index 1 then next index 1 + 1 * 4 = 5, next index 1 + 2 * 4 = 9, next index 1 + 3 * 4 = 13, next index 1 + 4 * 4 = 17.
Therefore, a slice with a stride higher than 1 does not select contiguous elements in the valarray. For example, slice(3, 4, 5) selects the elements 3, 8, 13 and 18.
slice(2, 3, 4): 2 6 10
Application : A simple application of slice is finding the trace of a matrix.
1 2 3 4 5 6 7 8 9 Trace of matrix is : 15
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