[Numpy-discussion] Slice nested arrays, How to
William Ray Wing
wrw at mac.com
Mon Jul 24 13:15:15 EDT 2017
> On Jul 24, 2017, at 10:37 AM, Bob <bobmerhebi at gmail.com> wrote:
>
> Hello,
>
> I created the following array by converting it from a nested list:
>
> a = np.array([np.array([ 17.56578416, 16.82712825, 16.57992292,
> 15.83534836]),
> np.array([ 17.9002445 , 17.35024876, 16.69733472, 15.78809856]),
> np.array([ 17.90086839, 17.64315136, 17.40653009, 17.26346787,
> 16.99901931, 16.87787178, 16.68278558, 16.56006419,
> 16.43672445]),
> np.array([ 17.91147242, 17.2770623 , 17.0320501 ,
> 16.73729491, 16.4910479 ])], dtype=object)
>
> I wish to slice the first element of each sub-array so I can perform
> basic statistics (mean, sd, etc...0).
>
Have you considered using Pandas? Assuming I understand what you are trying to do, that nested list could read directly into a Pandas 2D data frame. Extracting the first element of each column (or row) is then fast and efficient.
Bill
> How can I do that for large data without resorting to loops? Here's the
> result I want with a loop:
>
> s = np.zeros(4)
> for i in np.arange(4):
> s[i] = a[i][0]
>
> array([ 17.56578416, 17.9002445 , 17.90086839, 17.91147242])
>
> Thank you
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