std::slice (Valarray slice selector)

05 Oct, 2017
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[].
class slice;

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.Syntax :
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

start + 0*stride
start + 1*stride
....
....
start + (size-1)*stride

Example:
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.
// C++ program to test the functioning of std::slice#include <iostream> // std::cout#include <cstddef> // std::size_t#include <valarray> // std::valarray, std::slice  int main(){    std::valarray<int> sample(12);    // initialising valarray    for (int i = 0; i < 13; ++i)        sample[i] = i;      // using slice from start 1 and size 3 and stride 4    std::valarray<int> bar = sample[std::slice(2, 3, 4)];      // display slice result    std::cout << "slice(2, 3, 4):";    for (std::size_t n = 0; n < bar.size(); n++)        std::cout << ' ' << bar[n];    std::cout << '\n';      return 0;}
Output:
slice(2, 3, 4): 2 6 10

Application : A simple application of slice is finding the trace of a matrix.
// C++ program to find trace of a matrix by using std::slice#include <iostream> // std::cout#include <valarray> // std::valarray, std::slice  using namespace std;  int main(){    // row and column of matrix    int row = 3, col = 3;      // matrix of size row*col in row major form.    std::valarray<int> matrix(row * col);      // initialising matrix    for (int i = 0; i < row * col; ++i)        matrix[i] = i + 1;      // using slice from start 0 with size as col and stride col+1    std::valarray<int> diagonal = matrix[std::slice(0, col, col + 1)];      // finding trace using diagonal we got using slice    int index = 0;    for (int i = 0; i < row; i++) {        for (int j = 0; j < col; j++)            std::cout << matrix[index++] << " "; // same as matrix[i][j]        std::cout << endl;    }      int sum = 0; // initialising sum as 0    // calculating trace of matrix    for (int i = 0; i < diagonal.size(); i++)        sum += diagonal[i];    std::cout << "Trace of matrix is : ";    std::cout << sum << endl; // sum is trace of matrix    return 0;}
Output:
1 2 3 
4 5 6 
7 8 9 
Trace of matrix is : 15


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