[Numpy-discussion] Can not update a submatrix

Timothy Hochberg tim.hochberg@ieee....
Wed Jan 30 14:35:50 CST 2008


On Jan 30, 2008 12:43 PM, Anne Archibald <peridot.faceted@gmail.com> wrote:

> On 30/01/2008, Francesc Altet <faltet@carabos.com> wrote:
> > A Wednesday 30 January 2008, Nadav Horesh escrigué:
> > > In the following piece of code:
> > > >>> import numpy as N
> > > >>> R = N.arange(9).reshape(3,3)
> > > >>> ax = [1,2]
> > > >>> R
> > >
> > > array([[0, 1, 2],
> > >        [3, 4, 5],
> > >        [6, 7, 8]])
> > >
> > > >>> R[ax,:][:,ax] = 100
> > > >>> R
> > >
> > > array([[0, 1, 2],
> > >        [3, 4, 5],
> > >        [6, 7, 8]])
> > >
> > > Why R is not updated?
> >
> > Because R[ax] is not a view of R, but another copy of the original
> > object (fancy indexing does return references to different objects).
> > In order to get views, you must specify only a slice of the original
> > array.  For example:
>
> This is not exactly correct.
>

[...a fine explanation by Anne...]

Another way to look at this is note that:

R[ax,:][:,ax] = 100

is translated to:

R.__getitem__((ax, :)).__setitem__((:,ax), 100)

'__setitem__' is capable of setting values using fancy indexing, but as has
been noted, '__getitem__' makes a copy of R when using fancy indexing, and
therefore the original does not get modified.

[Technically, ':' gets translated to slice(None,None,None), but I didn't
want to write that out].

-- 
.  __
.   |-\
.
.  tim.hochberg@ieee.org
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