CODEWITHSUNDEEP
pythonnumpy
drrossum
drrossum

Reputation: 614

How to store data into a view in numpy?

How do I store data into a numpy view without changing the view into a copy? This code snippet examplifies my question:

>>> import numpy as np

>>> #-- init arrays and view
>>> a = np.ones([4])
>>> z = np.zeros([2,4])
>>> z0 = z[0,:]  #-- view
>>> z0.flags.owndata
False

>>> #-- This works!
>>> #-- modify view in-place
>>> np.add(a,z0,z0)
>>> z0.flags.owndata
False
>>> z
array([[ 1.,  1.,  1.,  1.],
       [ 0.,  0.,  0.,  0.]])


>>> #-- reinit arrays and view
>>> z = np.zeros([2,4])
>>> z0 = z[0,:]  #-- view

>>> #-- This does NOT work!
>>> #-- store data into view
>>> z0 = a 
>>> z0.flags.owndata
True

I know about in-place modifications using += -= *= /= and numpy functions that take an out parameter, so you can do things like np.abs(x, x) to take the absolute value of x in-place.

But how to just store data into a view without modification?

Abusing the add function (to add zero and store) works but doesn't feel 'right':

np.add(a,0,z0)

Upvotes: 2

Views: 103

Answers (1)

seberg
seberg

Reputation: 8975

When you do z0 = a, then z0 is the same object as a by python logic. What you want to do is this:

z0[...] = a

using the slicing syntax. Which uses the in-place __setitem__ python logic. On numpy 1.7. or later you could use np.copyto as well, which is probably a little faster, but I like the slicing syntax personally.

Upvotes: 2

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