how to efficiently select multiple slices from an array?
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. Cheers, Andreas.
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. _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
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. _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
_______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
cool, thanks!
participants (2)
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Andreas Hilboll -
Sebastian Berg