[Numpy-discussion] How do I make a diagonal matrix? (original) (raw)

David Douard david.douard at logilab.fr
Fri Jun 23 11:08:26 EDT 2006

On Fri, Jun 23, 2006 at 07:55:47AM -0700, Keith Goodman wrote:

On 6/23/06, Sven Schreiber <svetosch at gmx.net> wrote: > Keith Goodman schrieb: > > How do I make a NxN diagonal matrix with a Nx1 column vector x along > > the diagonal? > > > > >>> help(n.diag) > Help on function diag in module numpy.lib.twodimbase: > > diag(v, k=0) > returns the k-th diagonal if v is a array or returns a array > with v as the k-th diagonal if v is a vector.

I tried >> x = rand(3,1) >> diag(x) array([ 0.87113114]) Isn't rand(3,1) a vector?

No: In [13]: rand(3).shape
Out[13]: (3,)

In [14]: rand(3,1).shape Out[14]: (3, 1)

A "vector" is an array with only one dimension. Here, you have a 3x1 "matrix"...

Off list I was given the example: x=rand(3) diag(3)

So you've got the solution!

That works. But my x is a Nx1 matrix. I can't get it to work with matrices.

??? Don't understand what you cannot make work, here.

In [15]: x=rand(3,1)

In [18]: diag(x[:,0]) Out[18]: array([[ 0.2287158 , 0. , 0. ], [ 0. , 0.50571537, 0. ], [ 0. , 0. , 0.72304857]])

What else would you like?

David

Joris: The Numpy Example List looks good. I hadn't come across that before.

David Douard LOGILAB, Paris (France) Formations Python, Zope, Plone, Debian : http://www.logilab.fr/formations Développement logiciel sur mesure : http://www.logilab.fr/services Informatique scientifique : http://www.logilab.fr/science -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 189 bytes Desc: Digital signature URL: <http://mail.python.org/pipermail/numpy-discussion/attachments/20060623/5f3252e4/attachment-0001.sig>

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