[Numpy-discussion] matrix indexing question

Bill Baxter wbaxter@gmail....
Sun Mar 25 15:44:45 CDT 2007


On 3/26/07, Alan G Isaac <aisaac@american.edu> wrote:
> > Em Dom, 2007-03-25 às 13:07 -0400, Alan G Isaac escreveu:
> >>     >>> x[1]
> >>     matrix([[1, 0]])
> >> feels wrong.  (Similarly when iterating across rows.)
>
>
> On Sun, 25 Mar 2007, Paulo Jose da Silva e Silva apparently wrote:
> > I think the point here is that if you are using matrices,
> > then all you "should" want are matrices, just like in
> > MATLAB:
> >         >> b = A(1, :)
> >         b =
> >              1     2
>
>
> Yes, that is the idea behind this, which I am also
> accustomed to from GAUSS.  But note again that the Matlab
> equivalent ::
>
>     >>> x=N.mat('1 2;3 4')
>     >>> x[0,:]
>     matrix([[1, 2]])
>
> does provide this behavior.  The question I am raising
> is a design question and is I think really not addressed
> by the rule of thumb you offer.  Specifically, that rule
> of thumb if it is indeed the justification of  ::
>
>     >>> x[1]
>     matrix([[3, 4]])
>
> finds itself in basic conflict with the idea that I ought to
> be able to iterate over the objects in an iterable container.
>
> I mean really, does this not "feel" wrong? ::
>
>     >>> for item in x: print item.__repr__()
>     ...
>     matrix([[1, 2]])
>     matrix([[3, 4]])

This may sound silly, but I really think seeing all those brackets is
what makes it feel wrong.   Matlab's  output doesn't put it in your
face that your 4 is really a matrix([[4]]), even though that's what it
is to Matlab.  But I don't see a good way to change that behavior.

The other thing I find problematic about matrices is the inability to
go higher than 2d.  To me that means that it's impossible to go "pure
matrix" in my code because I'll have to switch back to arrays any time
I want more than 2d (or use a mixed solution like a list of matrices).
 Matlab allows allows >2D.

--bb


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