creation of a permutation matrix (WAS: Re: remove for-loop from following code)

[c.]

On 6 Nov 2011, at 09:52, Stefaan Himpe wrote:

> Hello,
> 
> Thanks for your reply.
> After scanning the octave documentation once more I finally arrived at the 
> following solution (which uses some special provisions to create permutation 
> matrices):
> 
> function [Ytr] = ytransformed(K,Y)
>  Ytr = eye(K)(Y',:);
> endfunctio

I am a bit confused, 
according to the documentation your code should produce a permutation matrix,
but it seems the result is actually a full matix:

------------------------------
>> K = 5;
>> Y = [1; 3; 2];
>> Ytr = eye(K)(Y',:)
Ytr =

   1   0   0   0   0
   0   0   1   0   0
   0   1   0   0   0

whos Ytr
Variables in the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  ===== 
        Ytr         3x5                        120  double

Total is 15 elements using 120 bytes
------------------------------

so it stores all the zeros, consuming unnecessary memory.

I think this might be a bug and I'll look further into it when I have time, 
in the meantime if you want to work around the issue you can create a sparse
matrix of logicals instead:

------------------------------
>> Ytr2 = sparse (1:numel(Y), Y, true (numel (1), 1), numel(Y), K)
>> whos Ytr2
Variables in the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  ===== 
        Ytr2        3x5                         39  logical
------------------------------

 c.