[SciPy-user] retrieve indexes of 2D array element

David Warde-Farley dwf@cs.toronto....
Mon Jul 13 06:26:35 CDT 2009


It doesn't require 3 inputs. Two of them are optional. See the  
examples, or this:

In [20]: A = array([[1,4,9],[3,5,6],[9,9,3]])

In [21]: rows, cols = where(A == 9)

In [22]: rows
Out[22]: array([0, 2, 2])

In [23]: cols
Out[23]: array([2, 0, 1])

A == 9 will produce a boolean array equal to True where the element is  
9 and False otherwise. Use this as the first argument to where.

David

On 13-Jul-09, at 4:03 AM, Robert Robert wrote:

> Hi,
> I've seen this function where, but it requires three inputs and I  
> have no clue how to use it. Is it possible to give me an example.  
> E.g I have an array as
>
> a = [ [1,2,3],
>         [4,5,6],
>         [7,8,9] ]
>
> and I would like to retrieve the indexes of values 9 which are 2, 2.
> How do I do that with scipy.where ?
> Thanks,
> robert
>
> --- On Mon, 7/13/09, David Warde-Farley <dwf@cs.toronto.edu> wrote:
>
> From: David Warde-Farley <dwf@cs.toronto.edu>
> Subject: Re: [SciPy-user] retrieve indexes of 2D array element
> To: "SciPy Users List" <scipy-user@scipy.org>
> Date: Monday, July 13, 2009, 7:11 AM
>
> On 13-Jul-09, at 2:56 AM, Robert Robert wrote:
>
> > Hi all,
> > I have a 2D scipy array of integer. I can not figure out how to
> > retrieve the indexes i, j of a specific element in the array. I can
> > loop through the array in python and search for it, but I can not
> > believe that scipy does not have a highly optimized algorithm.
> > Thanks,
> > robert
>
>
> By a specific element, do you mean a certain value? If so use a
> boolean condition and where().
>
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.where.html
>
> David
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