[SciPy-user] Returning the positions of a sorted array

Steve Schmerler elcorto@gmx....
Wed Sep 19 06:33:34 CDT 2007

Perez Kawumi wrote:
> Hullo,
> Im having problems returning the positions of a sorted array. 
> Say I create an array 
> a = ([3,5,1])
>  then if I use the sort command I get
> sort(a) = [1,3,5] 
> But what if i want the actual original positions of the sorted array to be returned instead. Is there a command in python that can do this?
> I'm looking for an answer of the form [2,0,1] which returns the original positions in a of the sorted array instead.

In [7]: numpy.*sort*?

In [8]: numpy.argsort?
Type:           function
Base Class:     <type 'function'>
String Form:    <function argsort at 0xa762b6f4>
Namespace:      Interactive
File:           /usr/lib/python2.4/site-packages/numpy/core/fromnumeric.py
Definition:     numpy.argsort(a, axis=-1, kind='quicksort', order=None)
     Returns array of indices that index 'a' in sorted order.

     Keyword arguments:

     axis  -- axis to be indirectly sorted (default -1)
              Can be None to indicate return indices into the
              flattened array.
     kind  -- sorting algorithm (default 'quicksort')
              Possible values: 'quicksort', 'mergesort', or 'heapsort'
     order -- For an array with fields defined, this argument allows
              specification of which fields to compare first, second,
              etc.  Not all fields need be specified.

     Returns: array of indices that sort 'a' along the specified axis.

     This method executes an indirect sort along the given axis using the
     algorithm specified by the kind keyword. It returns an array of indices of
     the same shape as 'a' that index data along the given axis in sorted order.

     The various sorts are characterized by average speed, worst case
     performance, need for work space, and whether they are stable. A stable
     sort keeps items with the same key in the same relative order. The three
     available algorithms have the following properties:

     |    kind   | speed |  worst case | work space | stable|
     |'quicksort'|   1   | O(n^2)      |     0      |   no  |
     |'mergesort'|   2   | O(n*log(n)) |    ~n/2    |   yes |
     |'heapsort' |   3   | O(n*log(n)) |     0      |   no  |

     All the sort algorithms make temporary copies of the data when the sort is not
     along the last axis. Consequently, sorts along the last axis are faster 
and use
     less space than sorts along other axis.

In [10]: a = numpy.array([3,5,1])

In [11]: numpy.argsort(a)
Out[11]: array([2, 0, 1])

In [12]: a.*sort*?

In [13]: a.argsort()
Out[13]: array([2, 0, 1])


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