[Numpy-discussion] matrix indexing question

[Bill Spotz]

On Mar 29, 2007, at 6:48 PM, Alan G Isaac wrote:

> On Thu, 29 Mar 2007, Bill Spotz apparently wrote:
>> What I envisioned was that M[i,:] would return
>> a row_vector and M[:,j] would return a column_vector,
>> because this would be symmetric behavior.  M[i], by
>> convention, would behave the same as M[i,:].
>
> Can you please be explicit about the value added by that
> convention, as you see it?

I assume by "convention" you are referring to the convention that M 
[i] is M[i,:] is a row_vector.  I see it as a design decision:

Does M[i] mean something?
   No:
     Then raise an exception.  If users want to iterate over M,
     this forces them to use the double index notation to specify
     what they are iterating over.  I would find this acceptable.
   Yes:
     What should M[i] represent/return?
        Array representing row vector:
        Array representing column vector:
          I would argue against linear algebra objects returning raw
          arrays because their meaning/interface can be ambiguous, as
          this thread demonstrates
        row_vector:
          Reasonable choice, analogous to conventional mathematical
          notation
        column_vector:
          Reasonable choice, but less analogous to conventional
          mathematical notation

Of all the choices, raising an exception and returning a row_vector  
are (in my opinion, anyway) the best.  Unless someone has a better  
argument.

> Thank you,
> Alan Isaac
>
> PS I assume your "vector" class would always accept a single index,
> for both row and column vectors?  What is the return type of v[i]?

My best guess at a best design would be that it would return a scalar.

And yes, vectors would always accept a single index.  But because  
they are matrices, they would accept two indexes as well.

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** Bill Spotz                                              **
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