xt::where usage

[YannickLecroart]

Hi,

Here is a sample code in Python to sort data in an ndarray.

a = [1,2,3,4]
b = [5,6,7,8]
c = [9,10,11,12]
d = [13,14,20,22]

z = np.stack([a, b, c, d], axis=1)

output:

[[ 1  5  9 13]
 [ 2  6 10 14]
 [ 3  7 11 20]
 [ 4  8 12 22]]

Now I use the following logical statement to sort data.

result = z[np.where(z[:,3] > 14)]

output:

[[ 3  7 11 20]
 [ 4  8 12 22]]

How can I do something similar with xtensor?

Thanks in advance.

Regards,

[YannickLecroart]

Here is what I tried so far:

xt::xarray<int> a = { 1,2,3,4 };
xt::xarray<int> b = { 5,6,7,8 };
xt::xarray<int> c = { 9,10,11,12 };
xt::xarray<int> d = { 13,14,20,22 };

auto  z = xt::stack(xtuple(a, b, c, d), 1); // axis=1 

auto result = xt::view(z, xt::all(), xt::where((xt::view(z, xt::all(), 3)) > 14));

But I get the following error message:

Error C2440 'static_cast': cannot convert from 'const S' to 'size_t' xtensor_demo c:\users\antho\documents\c++\xtensor_demo\xtensor_demo\packages\xtensor.0.15.10\build\native\include\xtensor\xslice.hpp 456

[YannickLecroart]

I think I have found the solution by looking at the following question on Stackoverflow:

https://stackoverflow.com/questions/51206255/filtering-multidimensional-views-in-xtensor

Here is what I do now via xt::filter that seems to do the trick:

xt::xarray<int> a = { 1,2,3,4 };
xt::xarray<int> b = { 5,6,7,8 };
xt::xarray<int> c = { 9,10,11,12 };
xt::xarray<int> d = { 13,14,20,22 };

auto  z = xt::stack(xtuple(a, b, c, d), 1); // axis=1

int tresh = 14;


auto filtered = xt::filter(z,
		xt::broadcast(xt::view(z, xt::all(), 3, xt::newaxis()),
			z.shape()) > tresh);
	

for (auto val : filtered)
	cout << val << endl;

output:

3
7
11
20
4
8
12
22