Re: how to create a permutation matrix

[c.]

On 26 Jan 2015, at 19:57, Jordi Gutiérrez Hermoso <address@hidden> wrote:

> On Mon, 2015-01-26 at 18:58 +0100, c. wrote:
>> Is it expected that :
>> 
>>>> P = eye (4);
>>>> P = P([2 1 3 4], :);
>>>> typeinfo (P)
>> ans = permutation matrix
>> 
>> but 
>> 
>>>> P = eye (4);
>>>> P ([1 2], :) = P([2 1], :);
>>>> typeinfo (P)
>> ans = matrix
> 
> Yes, that's the current implementation. You need to specify the full
> permutation vector in order to get a permutation matrix. It would take
> a bit more magic to realise that P([2 1], :) is being assigned to a
> permuted version of itself and thus create a permutation matrix.

Wouldn't it be better to at least fall back to SparseMatrix rather than Matrix 
in this case?

> - Jordi G. H.

c.