[Numpy-discussion] how to efficiently select multiple slices from an array?

Sebastian Berg sebastian@sipsolutions....
Wed Mar 20 10:43:17 CDT 2013


Hey,

On Wed, 2013-03-20 at 16:31 +0100, Andreas Hilboll wrote:
> Cross-posting a question I asked on SO
> (http://stackoverflow.com/q/15527666/152439):
> 
> 
> Given an array
> 
>     d = np.random.randn(100)
> 
> and an index array
> 
>     i = np.random.random_integers(low=3, high=d.size - 5, size=20)
> 
> how can I efficiently create a 2d array r with
> 
>     r.shape = (20, 8)
> 
> such that for all j=0..19,
> 
>     r[j] = d[i[j]-3:i[j]+5]
> 
> In my case, the arrays are quite large (~200000 instead of 100 and 20),
> so something quick would be useful.


You can use stride tricks, its simple to do by hand, but since I got it,
maybe just use this: https://gist.github.com/seberg/3866040

d = np.random.randn(100)
windowed_d = rolling_window(d, 8)
i = np.random_integers(len(windowed_d))
r = d[i,:]

Or use stride_tricks by hand, with:
windowed_d = np.lib.stride_tricks.as_strided(d, (d.shape[0]-7, 8),
(d.strides[0],)*2)

Since the fancy indexing will create a copy, while windowed_d views the
same data as the original array, of course that is not the case for the
end result.

Regards,

Sebastian

> 
> Cheers, Andreas.
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