[SciPy-user] Mathematica Element-wise Multiplication

Stefan van der Walt stefan@sun.ac...
Mon Dec 17 13:04:56 CST 2007


On Sun, Dec 16, 2007 at 11:49:38PM -0800, Johann Cohen-Tanugi wrote:
> Matthieu Brucher wrote:
> >
> >
> > 2007/12/17, Johann Cohen-Tanugi <cohen@slac.stanford.edu 
> > <mailto:cohen@slac.stanford.edu>>:
> >
> >     thanks for these precisions, David. Reading it, I still come to think
> >     that it is a potential source of confusion to let a "row array" have a
> >     transpose or T method, that essentially does nothing.
> >
> >
> >
> > In object oriented code, this can happen often, but it is not a 
> > problem. It does what you want : inverse the axis, even if there is 
> > only one axis.
> hmmm...... okay... What I wanted was to transpose a 1D array into a 
> vector, or vice-versa, with the linear algebra behavior in mind. I 
> understand that numpy does not follow this, but I cannot believe that 
> this behavior *is* what everybody wants! Tom's initial email was 
> symptomatic, and Stefan's response, with the proposal to use the T 
> method even more so!

I'm not sure what my response was symptomatic of, but if you don't
like the behaviour of ndarrays, you may consider using 'matlib':

In [1]: from numpy import matlib

In [2]: x = matlib.matrix([1,2,3])

In [3]: x
Out[3]:
matrix([[1, 2, 3]])

In [4]: x.T
Out[4]:
matrix([[1],
        [2],
        [3]])

In [5]: x * x.T
Out[5]:
matrix([[14]])

> Assuming that this natural linear algebra could be retrieved when, and 
> *only* when, the array is 1D, I do not see how such an implementation 
> could break codes that depend on it, because I don't see why someone 
> would call 'a.T' just to have 'a' again.... But it is probably my lack 
> of imagination.

When I call "transpose", I don't expect an array to change dimensions.
Like Matthieu wrote, it swaps the axes, and if you have only one axis
there is nothing to swap.  Start with a two-dimensional array, and the
transpose will do what you expect.

Regards
Stéfan


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