[Numpy-discussion] Re: Newbie indexing question and print order
Lennart Ohlsson
lennart.ohlsson at cs.lth.se
Wed Apr 5 08:12:20 CDT 2006
Hi,
Although I mainly use for 2D takes here is an nd-version of such a function:
def vtake(a, indices):
"""Corresponding to take in numpy but with vector valued indices"""
indexrank = indices.shape[-1]
flattedindex = 0
for i in range(indexrank):
flattedindex = flattedindex*a.shape[i] + indices[...,i]
flattedshape = (-1,) + a.shape[indexrank:]
return a.reshape(flattedshape).take(flattedindex)
- Lennart
On 4/5/06, Pau Gargallo<pau.gargallo at gmail.com> wrote:
hi,
i had the same problem and i defined a function with a similar sintax
to interp2 which i call take2 to solve it:
from numpy import *
def take2( a, x,y ):
return take( ravel(a), x + y*a.shape[0] )
a = array( [[ 0.15, 0.75, 0.2 ],
[ 0.82, 0.5, 0.77],
[ 0.21, 0.91, 0.59]] )
xy = array([ [[1, 1], [1, 1], [2, 1]],
[[2, 2], [0, 0], [1, 0]],
[[1, 1], [0, 0], [2, 1]]] )
print take2( a, xy[...,0], xy[...,1] )
i hope this helps you.
pau
On 4/5/06, amcmorl <amcmorl at gmail.com> wrote:
> Hi all,
>
> I'm having a bit of trouble getting my head around numpy's indexing
> capabilities. A quick summary of the problem is that I want to
> lookup/index in nD from a second array of rank n+1, such that the last
> (or first, I guess) dimension contains the lookup co-ordinates for the
> value to extract from the first array. Here's a 2D (3,3) example:
>
> In [12]:print ar
> [[ 0.15 0.75 0.2 ]
> [ 0.82 0.5 0.77]
> [ 0.21 0.91 0.59]]
>
> In [24]:print inds
> [[[1 1]
> [1 1]
> [2 1]]
>
> [[2 2]
> [0 0]
> [1 0]]
>
> [[1 1]
> [0 0]
> [2 1]]]
>
> then somehow return the array (barring me making any row/column errors):
> In [26]: c = ar.somefancyindexingroutinehere(inds)
>
> In [26]:print c
> [[ 0.5 0.5 0.91]
> [ 0.59 0.15 0.82]
> [ 0.5 0.15 0.91]]
>
> i.e. c[x,y] = a[ inds[x,y,0], inds[x,y,1] ]
>
> Any suggestions? It looks like it should be relatively simple using
> 'put' or 'take' or 'fetch' or 'sit' or something like that, but I'm not
> getting it.
>
> While I'm here, can someone help me understand the rationale behind
> 'print' printing row, column (i.e. a[0,1] = 0.75 in the above example
> rather than x, y (=column, row; in which case 0.75 would be in the first
> column and second row), which seems to me to be more intuitive.
>
> I'm really enjoying getting into numpy - I can see it'll be
> simpler/faster coding than my previous environments, despite me not
> knowing my way at the moment, and that python has better opportunities
> for extensibility. So, many thanks for your great work.
> --
> Angus McMorland
> email a.mcmorland at auckland.ac.nz
> mobile +64-21-155-4906
>
> PhD Student, Neurophysiology / Multiphoton & Confocal Imaging
> Physiology, University of Auckland
> phone +64-9-3737-599 x89707
>
> Armourer, Auckland University Fencing
> Secretary, Fencing North Inc.
>
>
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