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Difference between revisions of "cpp/numeric/valarray/gslice array"

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< cpp‎ | numeric‎ | valarray
m (link to ja)
(⌇) Example + (not fancy, rather thecnical)
Line 27: Line 27:
 
{{example
 
{{example
 
  | code=
 
  | code=
 +
#include <cassert>
 +
#include <cstddef>
 +
#include <iomanip>
 +
#include <iostream>
 +
#include <numeric>
 +
#include <valarray>
 +
 +
int main()
 +
{
 +
    std::valarray<int> data(32);
 +
    std::iota(std::begin(data), std::end(data), 0);
 +
 +
    const std::size_t offset = 1, z = 2, y = 3, x = 4;
 +
    const std::valarray<std::size_t> sizes{z, y, x};
 +
    const std::valarray<std::size_t> strides{15, 5, 1};
 +
    const std::gslice gslice = std::gslice(offset, sizes, strides);
 +
    // indices are generated according to the formula:
 +
    // index[k] = offset + [0,1,2)*15 + [0,1,2,3)*5 + [0,1,2,3,4)*1
 +
    //          = offset + inner_product(sizes[k], strides);
 +
    // where sizes[k] = {[0,z), [0,y), [0,x)}, while the rightmost index (x)
 +
    // runs fastest. As a result we have following set of indices:
 +
    //  index[0]  = 1 + 0*15 + 0*5 + 0*1 = 1
 +
    //  index[1]  = 1 + 0*15 + 0*5 + 1*1 = 2
 +
    //  index[2]  = 1 + 0*15 + 0*5 + 2*1 = 3
 +
    //  index[3]  = 1 + 0*15 + 0*5 + 3*1 = 4
 +
    //  index[4]  = 1 + 0*15 + 1*5 + 0*1 = 6
 +
    //  index[5]  = 1 + 0*15 + 1*5 + 1*1 = 7
 +
    //  index[6]  = 1 + 0*15 + 1*5 + 2*1 = 8
 +
    //  index[7]  = 1 + 0*15 + 1*5 + 3*1 = 9
 +
    //  ...
 +
    //  index[22] = 1 + 1*15 + 2*5 + 2*1 = 28
 +
    //  index[23] = 1 + 1*15 + 2*5 + 3*1 = 29
 +
 +
    const std::valarray<int> indices = data[gslice];
 +
        // Used: std::valarray<T> operator[]( const std::gslice& gslicearr ) const;
 +
 +
    for (unsigned i=0; i != indices.size(); ++i) {
 +
        std::cout << std::setfill('0') << std::setw(2) << indices[i] << ' ';
 +
    }
 +
    std::cout << "\nTotal indices: " << indices.size() << '\n';
 +
    assert(indices.size() == x*y*z);
 +
 +
    data = 0;
 +
    std::gslice_array<int> gslice_array = data[gslice];
 +
        // Used: std::gslice_array<T> operator[]( const std::gslice& gslicearr );
 +
 +
    gslice_array = 1;
 +
    // Cells that correspond to generated indices = '1', skipped cells = '0'.
 +
    for (auto i : data) { std::cout << i << ' '; }
 +
    std::cout << "\nSum of ones = " << data.sum() << '\n';
 +
}
 
  | output=
 
  | output=
 +
01 02 03 04 06 07 08 09 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29
 +
Total indices: 24
 +
0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 0
 +
Sum of ones = 24
 
}}
 
}}
  

Revision as of 14:03, 20 July 2020

 
C++
 
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Generic numeric operations
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std::valarray
Member functions
Non-member functions
Helper classes
Deduction guides (C++17)
 
std::gslice_array
 
Defined in header <valarray>
template< class T > class gslice_array;

std::gslice_array is a helper template used by std::gslice subscript operator. It has reference semantics to a subset of the array specified by the std::gslice object.

Contents

Member types

Type Definition
value_type T

Member functions

 constructs a gslice_array
(public member function) [edit]
destroys a gslice_array
(public member function) [edit]
assigns contents
(public member function) [edit]
performs arithmetic operation on the array referred by generic slice.
(public member function) [edit]

Example

Run this code
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <valarray>
 
int main()
{
    std::valarray<int> data(32);
    std::iota(std::begin(data), std::end(data), 0);
 
    const std::size_t offset = 1, z = 2, y = 3, x = 4;
    const std::valarray<std::size_t> sizes{z, y, x};
    const std::valarray<std::size_t> strides{15, 5, 1};
    const std::gslice gslice = std::gslice(offset, sizes, strides);
    // indices are generated according to the formula:
    // index[k] = offset + [0,1,2)*15 + [0,1,2,3)*5 + [0,1,2,3,4)*1
    //          = offset + inner_product(sizes[k], strides);
    // where sizes[k] = {[0,z), [0,y), [0,x)}, while the rightmost index (x)
    // runs fastest. As a result we have following set of indices:
    //  index[0]  = 1 + 0*15 + 0*5 + 0*1 = 1
    //  index[1]  = 1 + 0*15 + 0*5 + 1*1 = 2
    //  index[2]  = 1 + 0*15 + 0*5 + 2*1 = 3
    //  index[3]  = 1 + 0*15 + 0*5 + 3*1 = 4
    //  index[4]  = 1 + 0*15 + 1*5 + 0*1 = 6
    //  index[5]  = 1 + 0*15 + 1*5 + 1*1 = 7
    //  index[6]  = 1 + 0*15 + 1*5 + 2*1 = 8
    //  index[7]  = 1 + 0*15 + 1*5 + 3*1 = 9
    //  ...
    //  index[22] = 1 + 1*15 + 2*5 + 2*1 = 28
    //  index[23] = 1 + 1*15 + 2*5 + 3*1 = 29
 
    const std::valarray<int> indices = data[gslice];
        // Used: std::valarray<T> operator[]( const std::gslice& gslicearr ) const;
 
    for (unsigned i=0; i != indices.size(); ++i) {
        std::cout << std::setfill('0') << std::setw(2) << indices[i] << ' ';
    }
    std::cout << "\nTotal indices: " << indices.size() << '\n';
    assert(indices.size() == x*y*z);
 
    data = 0;
    std::gslice_array<int> gslice_array = data[gslice];
        // Used: std::gslice_array<T> operator[]( const std::gslice& gslicearr );
 
    gslice_array = 1;
    // Cells that correspond to generated indices = '1', skipped cells = '0'.
    for (auto i : data) { std::cout << i << ' '; }
    std::cout << "\nSum of ones = " << data.sum() << '\n';
}

Output: 

01 02 03 04 06 07 08 09 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 
Total indices: 24
0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0 0 
Sum of ones = 24

See also

 proxy to a subset of a valarray after applying a slice
(class template) [edit]
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