[Numpy-discussion] in1d, but preserve shape of ar1

[Brenton R S Recht]

I started an enhancement request in the Github bug tracker at
https://github.com/numpy/numpy/issues/8331 , but Jaime Frio recommended I
bring it to the mailing list.

`in1d` takes two arrays, `ar1` and `ar2`, and returns a 1d array with the
same number of elements as `ar1`. The logical extension would be a function
that does the same thing but returns a (possibly multi-dimensional) array
of the same shape as `ar1`. The code already has a comment suggesting this
could be done (see https://github.com/numpy/numpy
/blob/master/numpy/lib/arraysetops.py#L444 ).

I agree that changing the behavior of the existing function isn't an
option, since it would break backwards compatability. I'm not sure adding
an option keep_shape is good, since the name of the function ("1d")
wouldn't match what it does (returns an array that might not be 1d). I
think a new function is the way to go. This would be it, more or less:

def items_in(ar1, ar2, **kwargs):
  return np.in1d(ar1, ar2, **kwargs).reshape(ar1.shape)

Questions I have are:
* Function name? I was thinking something like `items_in` or `item_in`: the
function returns whether each item in `ar1` is in `ar2`. Is "item" or
"element" the right term here?
* Are there any other changes that need to happen in arraysetops.py? Or
other files? I ask this because although the file says "Set operations for
1D numeric arrays" right at the top, it's growing increasingly not 1D:
`unique` recently changed to operate on multidimensional arrays, and I'm
proposing a multidimensional version of `in1d`. `ediff1d` could probably be
tweaked into a version that operates along an axis the same way unique does
now, fwiw. Mostly I want to know if I should put my code changes in this
file or somewhere else.

Thanks,

-brsr
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