Get rows from a matrix by providing a list of rowIndices

Question

I am a beginner in xtensor and I am currently looking to get rows from list of array.

I've the following matrix.

auto matrix = {{  0.,   1.,   0.,   1.,   1.,   1.,   1.},
               {  1.,   2.,   0.,   1.,   3.,   1.,   1.},
               {  2.,   3.,   0.,   2.,   7.,   0.,   0.},
               {  3.,   4.,   0.,   1.,  11.,   0.,   1.},
               {  4.,   0.,   1.,   1.,   0.,   0.,   0.}}

From this matrix, I want to select the following rows.

xt::xarray<int> rowIndices = { 1, 2, 3, 4 }

Now I want to use this rowIndices array to get a sub-matrix with all the rows. How can I achieve this?

I tried the following.

xt::view(matrix, rowIndices, xt::all())

But this doesn't work.

Answer 1

You need to use xt::keep(...) to selects the rows by index.

The full example:

#include <xtensor/xtensor.hpp>
#include <xtensor/xview.hpp>
#include <xtensor/xio.hpp>

int main()
{
  xt::xtensor<double,2> a =
    {{  0.,   1.,   0.,   1.,   1.,   1.,   1.},
     {  1.,   2.,   0.,   1.,   3.,   1.,   1.},
     {  2.,   3.,   0.,   2.,   7.,   0.,   0.},
     {  3.,   4.,   0.,   1.,  11.,   0.,   1.},
     {  4.,   0.,   1.,   1.,   0.,   0.,   0.}};

  xt::xtensor<size_t,1> rowIndices = { 1, 2, 3, 4 };

  auto v = xt::view(a, xt::keep(rowIndices), xt::all());

  std::cout << v << std::endl;

  return 0;
}

which prints:

{{  1.,   2.,   0.,   1.,   3.,   1.,   1.},
 {  2.,   3.,   0.,   2.,   7.,   0.,   0.},
 {  3.,   4.,   0.,   1.,  11.,   0.,   1.},
 {  4.,   0.,   1.,   1.,   0.,   0.,   0.}}

Note that, according to the documentation, in view you can also make use of xt::range(...), xt::all(), xt::newaxis(), xt::keep(...), and xt::drop(...).