Re: [Numpy-discussion] Why does np.repeat build a full array?

[Sebastian Berg]

On Di, 2015-12-15 at 08:56 +0100, Sebastian Berg wrote:
> On Di, 2015-12-15 at 17:49 +1100, Juan Nunez-Iglesias wrote:
> > Hi,
> > 
> > 
> > I've recently been using the following pattern to create arrays of a
> > specific repeating value:
> > 
> > 
> > from numpy.lib.stride_tricks import as_strided
> > 
> > value = np.ones((1,), dtype=float)
> > arr = as_strided(value, shape=input_array.shape, strides=(0,))
> > 
> > 
> > I can then use arr e.g. to count certain pairs of elements using
> > sparse.coo_matrix. It occurred to me that numpy might have a similar
> > function, and found np.repeat. But it seems that repeat actually
> > creates the full, replicated array, rather than using stride tricks to
> > keep it small. Is there any reason for this?
> > 
> 
> Two reasons:
>  1. For most arrays, arrays even the simple repeats cannot be done with
> stride tricks. (yours has a dimension size of 1)
>  2. Stride tricks can be nice, but they can also be
> unexpected/inconsistent when you start writing to the result array, so
> you should not do it (and the array should preferably be read-only IMO,
> as_strided itself does not do that).
> 
> But yes, there might be room for a function or so to make some stride
> tricks more convenient.
> 

Actually, your particular use-case is covered by the new `broadcast_to`
function.


> - Sebastian
> 
> > 
> > Thanks!
> > 
> > 
> > Juan.
> > ___
> > NumPy-Discussion mailing list
> > NumPy-Discussion@scipy.org
> > https://mail.scipy.org/mailman/listinfo/numpy-discussion
> 
> ___
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> https://mail.scipy.org/mailman/listinfo/numpy-discussion



signature.asc
Description: This is a digitally signed message part
___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
https://mail.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Why does np.repeat build a full array?

[Juan Nunez-Iglesias]

On Tue, Dec 15, 2015 at 8:29 PM, Sebastian Berg 
wrote:

> Actually, your particular use-case is covered by the new `broadcast_to`
> function.
>

So it is! Fascinating, thanks for pointing that out! =)
___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
https://mail.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Why does np.repeat build a full array?

[Sebastian Berg]

On Di, 2015-12-15 at 17:49 +1100, Juan Nunez-Iglesias wrote:
> Hi,
> 
> 
> I've recently been using the following pattern to create arrays of a
> specific repeating value:
> 
> 
> from numpy.lib.stride_tricks import as_strided
> 
> value = np.ones((1,), dtype=float)
> arr = as_strided(value, shape=input_array.shape, strides=(0,))
> 
> 
> I can then use arr e.g. to count certain pairs of elements using
> sparse.coo_matrix. It occurred to me that numpy might have a similar
> function, and found np.repeat. But it seems that repeat actually
> creates the full, replicated array, rather than using stride tricks to
> keep it small. Is there any reason for this?
> 

Two reasons:
 1. For most arrays, arrays even the simple repeats cannot be done with
stride tricks. (yours has a dimension size of 1)
 2. Stride tricks can be nice, but they can also be
unexpected/inconsistent when you start writing to the result array, so
you should not do it (and the array should preferably be read-only IMO,
as_strided itself does not do that).

But yes, there might be room for a function or so to make some stride
tricks more convenient.

- Sebastian

> 
> Thanks!
> 
> 
> Juan.
> ___
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> https://mail.scipy.org/mailman/listinfo/numpy-discussion



signature.asc
Description: This is a digitally signed message part
___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
https://mail.scipy.org/mailman/listinfo/numpy-discussion