[SciPy-User] creating a view of an array

[Zachary Pincus]

Hi,

Indexing like a[:,(0,2)] creates a copy. (The logic isn't smart enough to determine if the index tuple is regular enough that it might be able to represent it as a view...)

import numpy
a = numpy.zeros((1000,1000))
b = a[:,:3:2] # This is what you want
b.base is a # True
c = a[:,(0,2)] # Makes a copy
c.base is a # False

Pretty much any simple slice will just create a view. (Right, experts?)

In addition, you could muck with the numpy.ndarray constructor and pass base, offset, dtype, and strides parameters to make a new view on an old array that does things that normal slicing couldn't. (Like zero-length strides, etc.)

Zach




On Jul 30, 2011, at 5:48 PM, Martin van Leeuwen wrote:

> Hi Mathieu,
> 
> You can index using a tuple too, like:
> 
> a[:,(0,2)]
> 
> That way you can index into any columns.
> 
> Martin
> 
> 2011/7/30 mathieu lacage <mathieu.lacage at alcmeon.com>:
>> hi,
>> 
>> Let's say I have a big large array:
>> 
>> a = numpy.empty((10000*1000,10))
>> 
>> and I want to create a view of that array to be able to process a subset of
>> its data without making a copy.
>> 
>> One column:
>> b = a[:,1]
>> 
>> Two adjacent columns:
>> b = a[:,1:2]
>> 
>> Can I do the same (no memory allocation) for two columns that are not
>> adjacent ? i.e. if I want to create an array that is a view for the 1st and
>> 3rd columns.
>> 
>> 
>> Mathieu
>> 
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>> 
>> 
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