# [Numpy-discussion] How do I make a diagonal matrix?

David Douard david.douard at logilab.fr
Fri Jun 23 10:08:26 CDT 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.twodim_base:
> >
> > 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)
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