Re: Any easy/build-in construct to extract submatrices from larger matrix?

["Nasser M. Abbasi" <nma@12000.org>]

On 6/24/2012 3:24 AM, Dmitry A. Kazakov wrote:
> On Sun, 24 Jun 2012 03:05:37 -0500, Nasser M. Abbasi wrote:
>
>> fyi, modern Fortran has a nice facility to do this, called
>> pack(). It works like this: make up a MASK matrix of same size
>> as A. For those elements we want removed, put in a logical
>> .false at that location.
>
> This looks like a low-level hack with unpredictable results, because it
> does not specify the dimensions and the order of the elements for the
> result matrix.
>

Pack() returns a vector, not a matrix. That is why reshape() is used.
The order is well defined. Fortran uses Fortran order? (column major).
So I am not sure I understand your "unpredictable results" ?

> If I implemented indicator functions for matrices, with the thingy is, I
> would define an abstract type with multiple implementations including a set
> of indices. A set would be far more effective for sparse matrices than
> Boolean matrix.
>

Well. Sounds too advanced for me. I was just looking for a little more
simple solution.

btw, if you think the Fortran solution was short, in Matlab,
this is done like this

EDU>> A=[1 2 3;4 5 6;7 8 9];
EDU>> A(1,:)=[];   % removes row 1
EDU>> A(:,1)=[]    % removes column 1

A =
      5     6
      8     9

And this would also work if A was sparse in Matlab.

regards,
--Nasser