[SciPy-User] extracting row from 2D array

josef.pktd@gmai... josef.pktd@gmai...
Wed Sep 29 12:09:48 CDT 2010


On Wed, Sep 29, 2010 at 1:05 PM, Dan Lussier <dtlussier@gmail.com> wrote:
> I want to extract a row from an 2D array given a criteria that is
> based on each entry in that row.
>
> By way of an example:
>
> xyz = np.array([[ 159.278  ,   14.7733 ,   11.6785 ],
>               [ 158.251  ,    1.11666,    4.76546],
>               [  21.1418 ,   84.8988 ,   56.5169 ],
>               [ 154.149  ,    4.9442 ,  131.672  ],
>               [  31.578  ,   11.1078 ,    8.47435]])
>
> magnitude_sq = xyz*xyz.sum(axis=1)
>
> I want to pull out the rows that satisfy a condition per row like
> magnitude_sq < 16. I've found and tried np.extract, np.take and
> np.compress. While np.extract fails to do what I want given that it
> effectively wants to treat the 2D array was a 1D vector, I moved onto
> np.compress and can get what I want via:
>
> np.compress( magnitude_sq < 16 , xyz, axis=0)

with indexing

xyz[magnitude_sq < 16, :]

Josef

>
> What I'm trying to understand is the relationship between these
> different functions (i.e. which one(s) are more fundamental) as the
> documentation doesn't really identify how they might relate to each
> other.
>
> I have also posted the question to
> http://ask.scipy.org/en/topic/50-extracting-entries-from-2d-numpy-array
> to give that system at try.
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