std::gslice – cppreference.com

Original:

std::gslice is the selector class that identifies a subset of std::valarray indices defined by a multi-level set of strides and sizes. Objects of type std::gslice can be used as indices with valarray's operator[] to select, for example, columns of a multidimensional array represented as a valarray.

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Original:

Given the starting value s, a list of strides i
j and a list of sizes d
j, a std::gslice constructed from these values selects the set of indices k
j=s+Σ
j(i
jd
j).

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Original:

For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3} generates the following set of indices:

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Original:

It is possible to construct std::gslice objects that select some indices more than once: if the above example used the strides {1,1,1} , the indices would have been {3, 4, 5, 4, 5, 6, ...} . Such gslices may only be used as arguments to the const version of std::valarray::operator[], otherwise the behavior is undefined.

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zeigt die Verwendung von gslices zum Spalten eines 3D-Feld anzusprechen

#include <iostream>
#include <valarray>
void test_print(std::valarray<int>& v, int rows, int cols, int planes)
{
    for(int r=0; r<rows; ++r) {
        for(int c=0; c<cols; ++c) {
            for(int z=0; z<planes; ++z)
                std::cout << v[r*cols*planes + c*planes + z] << ' ';
            std::cout << '\n';
        }
        std::cout << '\n';
    }
}
int main()
{
    std::valarray<int> v = // 3d array: 2 x 4 x 3 elements
    { 111,112,113 , 121,122,123 , 131,132,133 , 141,142,143,
      211,212,213 , 221,222,223 , 231,232,233 , 241,242,243};
    // int ar3d[2][4][3]
    std::cout << "Initial 2x4x3 array:\n";
    test_print(v, 2, 4, 3);

    // update every value in the first columns of both planes
    v[std::gslice(0, {2, 4}, {4*3, 3})] = 1; // two level one strides of 12 elements
                                             // then four level two strides of 3 elements

    // subtract the third column from the second column in the 1st plane
    v[std::gslice(1, {1, 4}, {4*3, 3})] -= v[std::gslice(2, {1, 4}, {4*3, 3})];

    std::cout << "After column operations: \n";
    test_print(v, 2, 4, 3);
}

Initial 2x4x3 array:
111 112 113
121 122 123
131 132 133
141 142 143

211 212 213
221 222 223
231 232 233
241 242 243

After column operations:
1 -1 113
1 -1 123
1 -1 133
1 -1 143

1 212 213
1 222 223
1 232 233
1 242 243