numpy-discussion

[Numpy-discussion] Sorting eigenvalues and vectors together

[jeremito <jer...@gm...>]

I am using a = numpy.linalg.eig(A) to get the eigenvalues and
eigenvectors.  I am interested only in the largest eigenvalue so I
would like to sort the first element of a.  But if I do that, I won't
know what is the associated eigenvector.  Is there a function that will
sort the values and vectors together or do I need to write it myself.
Thanks,
Jeremy

Re: [Numpy-discussion] Sorting eigenvalues and vectors together

[A. M. A. <per...@gm...>]

On 27/10/06, jeremito <jer...@gm...> wrote:
> I am using a = numpy.linalg.eig(A) to get the eigenvalues and
> eigenvectors.  I am interested only in the largest eigenvalue so I
> would like to sort the first element of a.  But if I do that, I won't
> know what is the associated eigenvector.  Is there a function that will
> sort the values and vectors together or do I need to write it myself.

I think that they are sorted, but argsort() will do what you want (or
argmax(), even).

A. M. Archibald

Re: [Numpy-discussion] Sorting eigenvalues and vectors together

[Keith G. <kwg...@gm...>]

On 10/27/06, jeremito <jer...@gm...> wrote:
> I am using a = numpy.linalg.eig(A) to get the eigenvalues and
> eigenvectors.  I am interested only in the largest eigenvalue so I
> would like to sort the first element of a.  But if I do that, I won't
> know what is the associated eigenvector.  Is there a function that will
> sort the values and vectors together or do I need to write it myself.

You can get the ranking of the eigenvalues with idx =
a[0].argsort().argsort() or idx = (-a[0]).argsort().argsort(). And
then use that to sort both the eigenvalues and eigenvectors.

Re: [Numpy-discussion] Sorting eigenvalues and vectors together

[Robert K. <rob...@gm...>]

jeremito wrote:
> I am using a = numpy.linalg.eig(A) to get the eigenvalues and
> eigenvectors.  I am interested only in the largest eigenvalue so I
> would like to sort the first element of a.  But if I do that, I won't
> know what is the associated eigenvector.  Is there a function that will
> sort the values and vectors together or do I need to write it myself.

Use argsort() on the eigenvalues and fancy indexing on the eigenvectors.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

Re: [Numpy-discussion] Sorting eigenvalues and vectors together

[jeremito <jer...@gm...>]

argsort() will do the trick.  Thanks once again.  
Jeremy

Re: [Numpy-discussion] Sorting eigenvalues and vectors together

[Sven S. <sve...@gm...>]

jeremito schrieb:
> argsort() will do the trick.  Thanks once again.  
> Jeremy
> 

I was a bit confused by your question, maybe you can clarify what you
did in the end.

IIRC, if the eigenvalues returned by numpy are real numbers (due to the
type of the underlying matrix and algorithm), then they are
automatically returned  ascending (again, IIRC). That would mean you
don't have to do anything. But if they're complex, it's not obvious how
to sort them. Probably by modulus. So what exactly was your problem?

ciao,
Sven

Re: [Numpy-discussion] Sorting eigenvalues and vectors together

[Robert K. <rob...@gm...>]

Sven Schreiber wrote:
> IIRC, if the eigenvalues returned by numpy are real numbers (due to the
> type of the underlying matrix and algorithm), then they are
> automatically returned  ascending (again, IIRC).

This should be considered an implementation detail that may change in the 
future, so don't rely on it.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco