[Numpy-discussion] use index array of len n to select columns of n x m array

Keith Goodman kwgoodman@gmail....
Thu Aug 5 12:31:03 CDT 2010


On Thu, Aug 5, 2010 at 10:26 AM,  <josef.pktd@gmail.com> wrote:
> On Thu, Aug 5, 2010 at 1:12 PM, Martin Spacek <numpy@mspacek.mm.st> wrote:
>> I want to take an n x m array "a" and index into it using an integer index array
>> "i" of length n that will pull out the value at the designated column from each
>> corresponding row of "a".
>>
>>>>> a = np.arange(10)
>>>>> a.shape = 5, 2
>>>>> a
>> array([[0, 1],
>>        [2, 3],
>>        [4, 5],
>>        [6, 7],
>>        [8, 9]])
>>>>> i = np.array([0, 1, 1, 0, 1])
>>
>> I want:
>>
>>>>> b = a.foo(i)
>>>>> b
>> array([0, 3, 5, 6, 9])
>>
>> What's foo? I can't get take() to do what I want. I went and wrote my own little
>> Cython function to do this, but that seems silly (and is also array dtype
>> dependent). I've tried reading through the numpy book, and I'm sure this is
>> somewhere on the list, but I can't find it. I think it has something to do with
>> fancy indexing. I should know how to do this by know...
>
> like this ?

Yes, I like it. Textbook case of fancy indexing.

>
>>>> a= np.array([[0, 1],
>       [2, 3],
>       [4, 5],
>       [6, 7],
>       [8, 9]])
>>>> i = np.array([0, 1, 1, 0, 1])
>>>> a[range(a.shape[0]), i]
> array([0, 3, 5, 6, 9])
>>>> a[np.arange(a.shape[0]), i]
> array([0, 3, 5, 6, 9])
>
> Josef
>
>>
>> Cheers,
>>
>> Martin
>>
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>>
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