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  • Manually subtracting two slices of DataArary always produces zero output · 5 ✖
id html_url issue_url node_id user created_at updated_at ▲ author_association body reactions performed_via_github_app issue
378125383 https://github.com/pydata/xarray/issues/2035#issuecomment-378125383 https://api.github.com/repos/pydata/xarray/issues/2035 MDEyOklzc3VlQ29tbWVudDM3ODEyNTM4Mw== shoyer 1217238 2018-04-03T04:33:40Z 2018-04-03T04:33:40Z MEMBER

If you look at the coordinates, the first and last labels get removed in arithmetic (by default we use an inner join).

You might find it more informative to look at the result of doing arithmetic with an outer join, which does result in some NaNs: ``` In [5]: with xr.set_options(arithmetic_join='outer'): ...: print(da[...,1:]-da[...,:-1]) ...: <xarray.DataArray (x: 5, y: 5, z: 5)> array([[[nan, 0., 0., 0., nan], [nan, 0., 0., 0., nan], [nan, 0., 0., 0., nan], [nan, 0., 0., 0., nan], [nan, 0., 0., 0., nan]],

   [[nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan]],

   [[nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan]],

   [[nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan]],

   [[nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan],
    [nan,  0.,  0.,  0., nan]]])

Coordinates: * z (z) int64 0 1 2 3 4 * x (x) int64 0 1 2 3 4 * y (y) int64 0 1 2 3 4 ```

 
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   Manually subtracting two slices of DataArary always produces zero output 310670696
378123622 https://github.com/pydata/xarray/issues/2035#issuecomment-378123622 https://api.github.com/repos/pydata/xarray/issues/2035 MDEyOklzc3VlQ29tbWVudDM3ODEyMzYyMg== max-sixty 5635139 2018-04-03T04:19:37Z 2018-04-03T04:19:37Z MEMBER

Because it's doing 3.0 - 3.0? Or am I misunderstanding something?

Try taking a single x & y slice and looking at the input & the output

 
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   Manually subtracting two slices of DataArary always produces zero output 310670696
378123085 https://github.com/pydata/xarray/issues/2035#issuecomment-378123085 https://api.github.com/repos/pydata/xarray/issues/2035 MDEyOklzc3VlQ29tbWVudDM3ODEyMzA4NQ== tomchor 13205162 2018-04-03T04:15:29Z 2018-04-03T04:15:29Z CONTRIBUTOR

@maxim-lian But my last point remains: should this really return a zero-valued matrix? I feel like this is counter-intuitive

 
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   Manually subtracting two slices of DataArary always produces zero output 310670696
378119865 https://github.com/pydata/xarray/issues/2035#issuecomment-378119865 https://api.github.com/repos/pydata/xarray/issues/2035 MDEyOklzc3VlQ29tbWVudDM3ODExOTg2NQ== max-sixty 5635139 2018-04-03T03:49:41Z 2018-04-03T03:49:41Z MEMBER

Right - it's aligning on the intersection of the overlapping coords - in this case the z dim goes from len=4 to len=3

 
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   Manually subtracting two slices of DataArary always produces zero output 310670696
378114226 https://github.com/pydata/xarray/issues/2035#issuecomment-378114226 https://api.github.com/repos/pydata/xarray/issues/2035 MDEyOklzc3VlQ29tbWVudDM3ODExNDIyNg== tomchor 13205162 2018-04-03T03:08:07Z 2018-04-03T03:08:07Z CONTRIBUTOR

I just realized this is probably the expected behavior (although counter-intuitive for me) since the coordinates of the points I'm operating on are different. I still don't understand why the returned array has zero values though, instead of nans.

 
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   Manually subtracting two slices of DataArary always produces zero output 310670696

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