[Numpy-discussion] numpy release

Stéfan van der Walt stefan@sun.ac...
Fri Apr 25 09:54:24 CDT 2008


2008/4/25 Alan G Isaac <aisaac@american.edu>:
> > 2008/4/25 Alan G Isaac :
>
> >>  So, if X is 2 by 2, then X[0] will be a row vector.
>  >>  But if X is 1 by 2, then X[0] will be a scalar?
>  >>  Ouch!
>  >>  Bye bye generic code.
>
>
> On Fri, 25 Apr 2008, Stefan van der Walt apparently wrote:
>  > Yup.  That's the current state of things.
>
>  I do not understand.
>  The released "state of things" is that for matrix ``x``
>  we have that ``x[0]`` is a **matrix**.
>
>  I'm not working with SVN NumPy, but my understanding
>  was that as of revision r5072 ``x[0]`` always
>  returns a 1d array.  The core requirement was that
>  ``x[0][0]`` produce the first element of the matrix.
>
>  I do not have time to look at the revision right now,
>  but if a matrix ``x`` we have that ``x[0]``
>  can return a scalar, that is very undesirable.

In current SVN:

In [3]: x = np.matrix(np.arange(9).reshape((3,3)))

In [5]: x
Out[5]:
matrix([[0, 1, 2],
        [3, 4, 5],
        [6, 7, 8]])

In [6]: x[0]
Out[6]: matrix([[0, 1, 2]])

In [7]: x[0,:]
Out[7]: matrix([[0, 1, 2]])

In [8]: x[0][0]
Out[8]: 0

In [9]: x[0,:][0]
Out[9]: 0

In [10]: x[:,0]
Out[10]:
matrix([[0],
        [3],
        [6]])

In [11]: x[:,0][0]
Out[11]: 0

In [12]: x[:2,:2][0]
Out[12]: matrix([[0, 1]])


Regards
Stéfan


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