[Numpy-discussion] matrix indexing question

Colin J. Williams cjw@sympatico...
Sun Mar 25 18:34:38 CDT 2007


Bill Baxter wrote:
> 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
"pure matrix" seems to me an area of exploration, does it have any 
application in numerical computation at this time?

A list of matrices seems to be a logical structure.  PyMatrix deals with 
lists in building a larger matrix from sub-matrices.

Suppose that we have matrices A (3, 4), B (3, 6), C (4, 2) and D (4, 8).

Then E= M([[A, B], [C, D]]) gives E (7, 10).

Colin W.



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