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## Re: how to create a permutation matrix

**From**: |
c. |

**Subject**: |
Re: how to create a permutation matrix |

**Date**: |
Tue, 27 Jan 2015 07:29:53 +0100 |

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.