question about "filter" over rows/cols

[tort-dla-psa]

How to do so? Let's say, I have an xarray and I want to get 1st col is a shape of a col ({-1,1}) and 1st row ({1,-1}). But xt::filter seems to apply bool array to whole xarray, so if we have array{2,2}, array of filter should be 4 bools in size.

Is there any way to write desired functional not like below? It's very inefficient

xt::xarray<int> arr = xt::arange(4);
arr.reshape({2,2});
xt::xarray<bool> f = {true, false, true, false};

xt::xarray<int> f_col = xt::filter(arr, f);
f = {true, true, false, false};
f_col.reshape({-1,1});

auto f_row = xt::filter(arr, f);

std::cout<<arr<<"\n"
	<<f_col<<"\n"
	<<f_row<<"\n";

{{0, 1},
{2, 3}}
{{0},
{2}}
{0, 1}

[JohanMabille]

Maybe views can help here?

auto row = xt::view(arr, 0, xt::all());
auto col = xt::view(arr, xt::all(), 0);

[tort-dla-psa]

in my example yes, but i meant something like this:

>>> import numpy as np
>>> a = np.arange(6).reshape((2,-3))
>>> a
array([[0, 1, 2],
       [3, 4, 5]])
>>> b = np.ones(3, dtype=np.bool)
>>> b[1] = False
>>> b
array([ True, False,  True])
>>> c = a[:, b]
>>> c
array([[0, 2],
       [3, 5]])

in other words, how to apply filter just for one dimension?

[JohanMabille]

There is actually a bug regarding the broadcasting of the condition in the filter function. A PR has been opened about that, however it is not complete.

You can ask explicitly for the broadcasting (it's a bit verbose but it is an acceptable workaround):

xt::xarray<int> a = xt::arange(6).reshape({2, -3});
xt::xarray<bool> filter = { true, false, true };
xt::xarray<int> res = xt::filter(a, xt::broadcast(filter, a.shape()));

Then you can reshape res to the shape you need.

You can also use the drop and keep slices with views:

xt::xarray<int> res = xt::view(a, xt::all(), xt::drop(2));

[tort-dla-psa]

Oh, is there any plans on fixing that? Thanks for the workaround

[JohanMabille]

Well ideally we want to fix everything, it is just a matter of time and ressources ;) We are busy with the core part for now, but index views and filters should be the next big refactoring after the next release.

In the meantime, if that's something urgent for you and/or if you want to contribute, feel free to open a PR, we will be happy to review it.