Xarray: Add head(), tail() and thin() methods?

[shoyer]

These would be shortcuts for isel/slice syntax:

• ds.head(time=5) -> ds.isel(time=slice(5)): select the first five time values
• ds.tail(time=5) -> ds.isel(time=slice(-5, None)): select the last five time values
• ds.thin(time=5) -> ds.isel(time=slice(None, None, 5)): select every 5th time value

[ebrevdo]

Clojure conventions: .take, .take_last get the first n and last n
pandas/ndarray conventions: .take([3,4,5]) selects rows 3,4,5.

probably want to be consistent with one of these.

On Tue, Feb 10, 2015 at 3:28 PM, Stephan Hoyer [email protected]
wrote:

These would be shortcuts for isel/slice syntax:

• ds.head(time=5) -> ds.isel(time=slice(5)): select the first five
  time values
• ds.tail(time=5) -> ds.isel(time=slice(-5, None)): select the last
  five time values
• ds.subsample(time=5) -> ds.isel(time=slice(None, None, 5)): select
  every 5th time value

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[shoyer]

@ebrevdo yes, I think we probably need to stick to the pandas/numpy convention for the meaning of take.

My inspiration for these names was the head() and tail() methods in pandas, which are quite convenient. But it's not entirely clear how/if these generalize cleanly to N-dimensions. I suppose take_first and take_last could be an improvement over head/tail.

[ebrevdo]

seems like for, e.g., head, you can pass either a single dimension or
multiple ones (e.g., either as **kwargs or a dictionary) and use those as
the start dimension.

that said, about naming conventions, i think for tensors the most common
convention is definitely slice() (which is implemented as isel). head/tail
can be implemented in terms of slice().

e.g.:
ds.slice(dim1=3, dim2=(1,4), dim3=(1,None,5)) -- or --
ds.slice({'dim1': 3, 'dim2': (1,4), 'dim3': (1, None, 5)})

head/tail/whatever are easy calls to this and you can have that in the
documentation. as a result, people won't get confused because they
understand slice.

On Tue, Feb 10, 2015 at 4:14 PM, Stephan Hoyer [email protected]
wrote:

@ebrevdo https://github.com/ebrevdo yes, I think we probably need to
stick to the pandas/numpy convention for the meaning of take.

My inspiration for these names was the head() and tail() methods in
pandas, which are quite convenient. But it's not entirely clear how/if
these generalize cleanly to N-dimensions. I suppose take_first and
take_last could be an improvement over head/tail.

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https://github.com/xray/xray/issues/319#issuecomment-73812481.

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[jhamman]

I think this is still worth doing. I actually prefer @shoyer's original method names and API. I also think this would be a good first issue.

[shoyer]

One virtue of tail() is particular is that we can define array.tail(x=0) to consistently return an array with an x dimension of size 0. This requires special case logic with slicing, since slice(None, -0) is the same as slice(None, 0).

[shoyer]

Anyways so yes, I agree that these would definitely be more readable than the equivalent operations with isel().

[HasanAhmadQ7]

Will work on this issue in the next three weeks if no one else does till then.

[DangoMelon]

Is this being worked on?

If not, I can send a PR today since I have some code ready that might help add this functionality.

[dcherian]

Go for it!