[Numpy-discussion] bug in lexsort with two different dtypes?

Charles R Harris charlesr.harris@gmail....
Sat Jun 30 10:35:06 CDT 2007


On 6/26/07, Charles R Harris <charlesr.harris@gmail.com> wrote:
>
>
>
> On 6/26/07, Charles R Harris <charlesr.harris@gmail.com> wrote:
> >
> >
> >
> > On 6/26/07, Tom Denniston < tom.denniston@alum.dartmouth.org> wrote:
> > >
> > > In [1]: intArr1 = numpy.array([ 0, 1, 2,-2,-1, 5,-5,-5])
> > > In [2]: intArr2 = numpy.array([1,1,1,2,2,2,3,4])
> > > In [3]: charArr = numpy.array(['a','a','a','b','b','b','c','d'])
> > >
> > > Here I sort two int arrays.  As expected intArr2 dominates intArr1 but
> > > the items with the same intArr2 values are sorted forwards according
> > > to intArr1
> > > In [6]: numpy.lexsort((intArr1, intArr2))
> > > Out[6]: array([0, 1, 2, 3, 4, 5, 6, 7])
> > >
> > > This,  however, looks like a bug to me.  Here I sort an int array and
> > > a str array.  As expected charArray dominates intArr1 but the items
> > > with the same charArray values are sorted *backwards* according to
> > > intArr1
> > > In [5]: numpy.lexsort((intArr1, charArr))
> > > Out[5]: array([2, 1, 0, 5, 4, 3, 6, 7])
> > >
> > > Is this a bug or am I missing something?
> >
> >
It was a bug. It is fixed in svn.

Chuck

Looks like a bug.
> >
> > In [12]: numpy.argsort([charArr], kind='m')
> > Out[12]: array([[2, 1, 0, 5, 4, 3, 6, 7]])
> >
> > In [13]: numpy.argsort([intArr2], kind='m')
> > Out[13]: array([[0, 1, 2, 3, 4, 5, 6, 7]])
> >
> > Both of these are stable sorts, and since the elements are in order
> > should return [[0, 1, 2, 3, 4, 5, 6, 7]]. Actually, I think they should
> > return [0, 1, 2, 3, 4, 5, 6, 7], I'm not sure why the returned array is 2D
> > and I suspect that is a bug also. As to why the string array sorts
> > incorrectly, I am not sure. It could be that the sort isn't stable, there
> > could be a stride error, or the comparison is returning wrong values. My bet
> > is on the first being the case.
> >
>
> Nevermind the 2D thingee, that was pilot error in changing lexsort to
> argsort, charArr should not be in a list:
>
> In [25]: numpy.argsort(charArr, kind='m', axis=0)
> Out[25]: array([2, 1, 0, 5, 4, 3, 6, 7])
>
> Works just fine.
>
> Chuck
>
>
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