[Numpy-discussion] Strange behavior for argsort() and take()

[Thomas J. Duck]

Hi,

      I have found what I think is some strange behavior for argsort 
() and take().  First, here is an example that works as expected:

      >>> x = numpy.array([1,0,3,2])
      >>> x.argsort()
      array([1, 0, 3, 2])

argsort() returns the original array, which is self-indexing for  
numbers 0 through 3...

      >>> x.take(x.argsort())
      array([0, 1, 2, 3])

...and take() provides the correct sort with ascending order.

Now check out what happens when we swap the middle two numbers:

      >>> x = numpy.array([1,3,0,2])
      >>> x.argsort()
      array([2, 0, 3, 1])

argsort() appears to have indexed the numbers in descending order  
(which was not expected)....

      >>> x.take(x.argsort())
      array([0, 1, 2, 3])

... but the take() operation puts them in ascending order anyway  
(which was really not expected)!

      Can anyone shed some light on what is happening here for me?  I  
have tested this on both my OS X/Fink and Debian/Lenny systems with  
the same result.

Thanks,
Tom

--

Thomas J. Duck <tom.duck@dal.ca>

Associate Professor,
Department of Physics and Atmospheric Science, Dalhousie University,
Halifax, Nova Scotia, Canada, B3H 3J5.
Tel: (902)494-1456 | Fax: (902)494-5191 | Lab: (902)494-3813
Web: http://aolab.phys.dal.ca/