[Numpy-discussion] extract elements of an array that are contained in another array?

josef.pktd@gmai... josef.pktd@gmai...
Fri Jun 5 00:56:14 CDT 2009


On Fri, Jun 5, 2009 at 1:48 AM, Robert Cimrman <cimrman3@ntc.zcu.cz> wrote:
> josef.pktd@gmail.com wrote:
>> On Thu, Jun 4, 2009 at 4:30 PM, Gael Varoquaux
>> <gael.varoquaux@normalesup.org> wrote:
>>> On Thu, Jun 04, 2009 at 10:27:11PM +0200, Kim Hansen wrote:
>>>> "in(b)" or "in_iterable(b)" method, such that you could do a.in(b)
>>>> which would return a boolean array of the same shape as a with
>>>> elements true if the equivalent a members were members in the iterable
>>>> b.
>>> That would really by what I would be looking for.
>>>
>>
>> Just using "in" might promise more than it does, eg. it works only for
>> one dimensional arrays, maybe "in1d". With "in", I would expect a
>> generic function as in python that works with many array types and
>> dimensions. (But I haven't checked whether it would work with a 1d
>> structured array or object array.)
>>
>> I found arraysetops because of unique1d, but I didn't figure out what
>> the subpackage really does, because I was reading "arrayse-tops"
>> instead of array-set-ops"
>
> I am bad in choosing names, but note that numpy sub-modules usually do
> not use underscores, so array_set_ops would not fit well.

I would have chosen something like setfun.  Since this is in numpy
that sets refers to arrays should be implied.

>
>> BTW, for the docs, I haven't found a counter example where
>> np.setdiff1d gives the wrong answer for non-unique arrays.
>
> In [4]: np.setmember1d( [1, 1, 2, 4, 2], [3, 2, 4] )
> Out[4]: array([ True, False,  True,  True,  True], dtype=bool)

setdiff1d    diff  not  member
Looking at the source, I think setdiff always works even if for
non-unique arrays.

Josef

>
> r.
>
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