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Re: creation of a permutation matrix (WAS: Re: remove for-loop from foll


From: Carlo de Falco
Subject: Re: creation of a permutation matrix (WAS: Re: remove for-loop from following code)
Date: Sun, 6 Nov 2011 11:05:50 +0100

On 6 Nov 2011, at 10:33, c. wrote:

> ------------------------------
>>> 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, 

OK, looking at the definition of idx_vector::is_permutation in idx-vector.cc 
it seems Y will be considered a permutation vector only if it has exactly K 
elements,
so only square permutation matrices can be created.

c.




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