[Numpy-discussion] strange behavior of numpy.unique

Warren Weckesser warren.weckesser@gmail....
Wed Nov 7 13:18:08 CST 2012


On Wed, Nov 7, 2012 at 11:24 AM, <josef.pktd@gmail.com> wrote:

> On Tue, Nov 6, 2012 at 9:52 PM, Warren Weckesser
> <warren.weckesser@gmail.com> wrote:
> >
> >
> > On Tue, Nov 6, 2012 at 8:27 PM, Phillip Feldman
> > <phillip.m.feldman@gmail.com> wrote:
> >>
> >> numpy.unique behaves as I would expect for small inputs like the
> >> following:
> >>
> >> In [12]: x= [0, 0, 1, 0, 1, 2, 0, 1, 2, 3]
> >>
> >> In [13]: unique(x, return_index=True)
> >> Out[13]: (array([0, 1, 2, 3]), array([0, 2, 5, 9], dtype=int64))
> >>
> >> But, when I give it something larger, the return index values do not
> >> always correspond to the first occurrences in the input. The
> documentation
> >> is silent on the question of how the return index values are chosen
> when a
> >> given element of x appears more than once. Either the documentation
> should
> >> be
> >> clarified, or better yet, the behavior should be changed.
> >
> >
> >
> > In fact, it was changed (in the master branch on github) several months
> ago,
> > but there has not yet been a release with the changes.  The sort method
> that
> > np.unique passes to np.argsort is now 'mergesort', and the docstring
> states
> > that the indices returned are for the first occurrences of the unique
> > elements.  The new docstring is here:
> >
> http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.unique.html#numpy.unique
> >
> > See
> >
> https://github.com/numpy/numpy/commit/dbf235169ed3386b359caaa9217f5280bf1d6749
> > for the commit, and
> > https://github.com/numpy/numpy/blob/master/numpy/lib/arraysetops.py for
> the
> > latest version of the source.
>
> I think it's in 1.6.2 and it broke return_index for structured dtypes,
> IIRC.
>
>

You are correct, Josef, that change is in 1.6.2.  Thanks.

Warren


Josef
>
>
> >
> > Warren
> >
> >
> >>
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> >>
> >
> >
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