Thread: [Numpy-discussion] Removing rows and columns of a matrix

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[Numpy-discussion] Removing rows and columns of a matrix

From: Nils W. <nw...@me...> - 2003-07-28 13:39:41

Hi all,

 How can I delete a column/row from a matrix.

In Matlab:

>> a=rand(4,4)

a =

    0.9501    0.8913    0.8214    0.9218
    0.2311    0.7621    0.4447    0.7382
    0.6068    0.4565    0.6154    0.1763
    0.4860    0.0185    0.7919    0.4057

use

>> a(:,2)=[]

a =

    0.9501    0.8214    0.9218
    0.2311    0.4447    0.7382
    0.6068    0.6154    0.1763
    0.4860    0.7919    0.4057

>> a(2,:)=[]

a =

    0.9501    0.8214    0.9218
    0.6068    0.6154    0.1763
    0.4860    0.7919    0.4057

>> 

Is there something similar in scipy, numpy or numarray ?

Any suggestion ?

Nils

Re: [Numpy-discussion] Removing rows and columns of a matrix

From: Konrad H. <hi...@cn...> - 2003-07-28 14:10:07

>  How can I delete a column/row from a matrix.

As an in-place operation, not at all. To get a copy of an array with some=
=20
columns/rows removed, use Numeric.take.

Konrad.
--=20
-------------------------------------------------------------------------=
------
Konrad Hinsen                            | E-Mail: hi...@cn...
Centre de Biophysique Moleculaire (CNRS) | Tel.: +33-2.38.25.56.24
Rue Charles Sadron                       | Fax:  +33-2.38.63.15.17
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Re: [Numpy-discussion] Removing rows and columns of a matrix

From: Nils W. <nw...@me...> - 2003-07-28 14:29:29

Konrad Hinsen schrieb:
> 
> >  How can I delete a column/row from a matrix.
> 
> As an in-place operation, not at all. To get a copy of an array with some
> columns/rows removed, use Numeric.take.
> 
The in-place operation of matlab is a nice feature. Is it thinkable to
have this
in scipy or numarray at a later date ?

Nils

> Konrad.
> --
> -------------------------------------------------------------------------------
> Konrad Hinsen                            | E-Mail: hi...@cn...
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RE: [Numpy-discussion] Removing rows and columns of a matrix

From: Perry G. <pe...@st...> - 2003-07-28 14:34:44

> 
> Konrad Hinsen schrieb:
> > 
> > >  How can I delete a column/row from a matrix.
> > 
> > As an in-place operation, not at all. To get a copy of an array 
> with some
> > columns/rows removed, use Numeric.take.
> > 
> The in-place operation of matlab is a nice feature. Is it thinkable to
> have this
> in scipy or numarray at a later date ?
> 
> Nils
> 
Speaking for numarray, no, not really. But I wonder what is
really done for matlab. Either they have a more complex representation
of arrays, or all they really are doing is making a new copy of
the array and giving you the impression it is being done in place.
After all you are changing the size and structure of the array.

Perry

RE: [Numpy-discussion] Removing rows and columns of a matrix

From: Ralf J. <jue...@in...> - 2003-07-28 15:10:20

On Mon, 2003-07-28 at 07:34, Perry Greenfield wrote:
> Speaking for numarray, no, not really. But I wonder what is
> really done for matlab. Either they have a more complex representation
> of arrays, or all they really are doing is making a new copy of
> the array and giving you the impression it is being done in place.

Right, this is how Matlab does it behind the scenes.

Ralf

Re: [Numpy-discussion] Removing rows and columns of a matrix

From: Konrad H. <hi...@cn...> - 2003-07-28 14:41:24

On Monday 28 July 2003 16:26, Nils Wagner wrote:
> Konrad Hinsen schrieb:
> > >  How can I delete a column/row from a matrix.
> >
> > As an in-place operation, not at all. To get a copy of an array with =
some
> > columns/rows removed, use Numeric.take.
>
> The in-place operation of matlab is a nice feature. Is it thinkable to
> have this
> in scipy or numarray at a later date ?

I'd say that the obvious way to implement it is as

  del a[2:3,:]

That can certainly be done, but someone has to do it.

Personally, I am not convinced that it is very useful. The consequences o=
f an=20
in-place change can be disastrous when many references point to that arra=
y.=20
But then, we have other in-place operations which sometimes are handy, so=
 my=20
usefulness estimation may be wrong.

Konrad.
--=20
-------------------------------------------------------------------------=
------
Konrad Hinsen                            | E-Mail: hi...@cn...
Centre de Biophysique Moleculaire (CNRS) | Tel.: +33-2.38.25.56.24
Rue Charles Sadron                       | Fax:  +33-2.38.63.15.17
45071 Orleans Cedex 2                    | Deutsch/Esperanto/English/
France                                   | Nederlands/Francais
-------------------------------------------------------------------------=
------

Re: [Numpy-discussion] Removing rows and columns of a matrix

From: Nils W. <nw...@me...> - 2003-07-28 15:22:21

Konrad Hinsen schrieb:
> 
> On Monday 28 July 2003 16:26, Nils Wagner wrote:
> > Konrad Hinsen schrieb:
> > > >  How can I delete a column/row from a matrix.
> > >
> > > As an in-place operation, not at all. To get a copy of an array with some
> > > columns/rows removed, use Numeric.take.
> >
> > The in-place operation of matlab is a nice feature. Is it thinkable to
> > have this
> > in scipy or numarray at a later date ?
> 
> I'd say that the obvious way to implement it is as
> 
>   del a[2:3,:]
> 
> That can certainly be done, but someone has to do it.
> 
> Personally, I am not convinced that it is very useful. The consequences of an
> in-place change can be disastrous when many references point to that array.
> But then, we have other in-place operations which sometimes are handy, so my
> usefulness estimation may be wrong.
> 
> Konrad.

In my opinion, this operation is quite useful. For example a finite
element model
where we have to impose several homogeneous boundary conditions. This
can be done 
by deleting columns and associated rows.

Nils

In any case I am interested in a reliable workaround for this feature.

A small example would be appreciated.
Thanks in advance.



> --
> -------------------------------------------------------------------------------
> Konrad Hinsen                            | E-Mail: hi...@cn...
> Centre de Biophysique Moleculaire (CNRS) | Tel.: +33-2.38.25.56.24
> Rue Charles Sadron                       | Fax:  +33-2.38.63.15.17
> 45071 Orleans Cedex 2                    | Deutsch/Esperanto/English/
> France                                   | Nederlands/Francais
> -------------------------------------------------------------------------------
> 
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