[Numpy-discussion] indexing question

Robert Kern robert.kern@gmail....
Mon Jun 21 18:10:46 CDT 2010


On Mon, Jun 21, 2010 at 17:42, Neal Becker <ndbecker2@gmail.com> wrote:
> Robert Kern wrote:
>
>> On Mon, Jun 21, 2010 at 14:01, Neal Becker <ndbecker2@gmail.com> wrote:
>>> Can I find an efficient way to do this?
>>>
>>> I have a 2d array, A, 80 rows by 880 columns.
>>>
>>> I have a vector, B, of length 80, with scalar indexes.
>>
>> I assume you mean 880.
>>
>>> I want a vector output C where
>>> C[i] = A[b[i],i] (i=0,879)
>>
>> C = A[b, np.arange(880)]
>>
>
> Thanks!  Just what I needed.
>
> I wouldn't have guessed this.  Do we have a wiki to save useful examples
> like this?

This kind of indexing is documented here:

  http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#integer

Various examples are on the wiki here:

  http://www.scipy.org/Cookbook/Indexing

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco


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