numpy-discussion

[Numpy-discussion] matrix multiplication (newbie question)

[izak m. <iza...@ya...>]

Hi

Sorry if this is an obvious question, but what is the easiest way to multiply matrices in numpy? Suppose I want to do A=B*C*D. The ' * ' operator apparently does element wise multiplication, as does the 'multiply' ufunc. All I could find was the numeric function 'matrix_multiply, but this only takes two arguments.

Thanks in advance!
Izak

 
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Re: [Numpy-discussion] matrix multiplication (newbie question)

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

On 11/8/06, izak marais <iza...@ya...> wrote:

> Sorry if this is an obvious question, but what is the easiest way to
> multiply matrices in numpy? Suppose I want to do A=B*C*D. The ' * ' operator
> apparently does element wise multiplication, as does the 'multiply' ufunc.
> All I could find was the numeric function 'matrix_multiply, but this only
> takes two arguments.

If B and C and D are matrices, then '*' is matrix multiplication.

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Charles R H. <cha...@gm...>]

On 11/8/06, Keith Goodman <kwg...@gm...> wrote:
>
> On 11/8/06, izak marais <iza...@ya...> wrote:
>
> > Sorry if this is an obvious question, but what is the easiest way to
> > multiply matrices in numpy? Suppose I want to do A=B*C*D. The ' * '
> operator
> > apparently does element wise multiplication, as does the 'multiply'
> ufunc.
> > All I could find was the numeric function 'matrix_multiply, but this
> only
> > takes two arguments.


Same with the operator *, it takes two arguments but is in infix order, i.e.,
left side and right side.

If B and C and D are matrices, then '*' is matrix multiplication.


And if they are arrays:

A = dot(B,dot(C,D))

Python has a dearth of recognized operators which makes this necessary once
'*' is used for elementwise multiplication, it's a long standing complaint.
You can use matrices in numpy, in which case '*' is used for matrix
multiplication like in matlab, but I think it would be better to get used to
using arrays as they are the numpy core.

Chuck

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Stefan v. d. W. <st...@su...>]

On Wed, Nov 08, 2006 at 05:54:17AM -0800, izak marais wrote:
> Hi
>=20
> Sorry if this is an obvious question, but what is the easiest way to mu=
ltiply
> matrices in numpy? Suppose I want to do A=3DB*C*D. The ' * ' operator a=
pparently
> does element wise multiplication, as does the 'multiply' ufunc. All I c=
ould
> find was the numeric function 'matrix_multiply, but this only takes two
> arguments.

You can also do N.dot(B, N.dot(C,D)).

Cheers
St=E9fan

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Roberto De A. <ro...@de...>]

On 11/8/06, Keith Goodman <kwg...@gm...> wrote:
> > Sorry if this is an obvious question, but what is the easiest way to
> > multiply matrices in numpy? Suppose I want to do A=B*C*D.
> If B and C and D are matrices, then '*' is matrix multiplication.

I think the difference between arrays and matrices is not clear for
him (he's clearly multiplying arrays).

Izak, you should first convert you arrays to matrices using ``numpy.matrix``.

--Rob

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Albert S. <as...@di...>]

> Izak, you should first convert you arrays to matrices using ``numpy.matrix``.

or numpy.asmatrix()
> 
> --Rob
> 
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Re: [Numpy-discussion] matrix multiplication (newbie question)

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

izak marais schrieb:
> Hi
> 
> Sorry if this is an obvious question, but what is the easiest way to
> multiply matrices in numpy? Suppose I want to do A=B*C*D. The ' * '
> operator apparently does element wise multiplication, as does the
> 'multiply' ufunc. All I could find was the numeric function
> 'matrix_multiply, but this only takes two arguments.
> 
> Thanks in advance!
> Izak

There are (at least) two ways:

You can use 'dot', possibly nested.

or you can convert your arrays into the matrix subclass, for which '*'
is matrix multiplication. I.e. mat(B)*mat(C)*mat(D) does what you want.

If you "only" deal with algebra-style matrices (2d-arrays), consider
using the matrix subclass as much as you can. E.g. use the functions in
numpy.matlib to build your inputs.
-sven

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Joris De R. <jo...@st...>]

   [im]: Sorry if this is an obvious question, but what is the easiest way to multiply matrices in numpy? Suppose I want to do A=B*C*D. The ' * ' operator apparently does element wise multiplication, as does the 'multiply' ufunc. 
   [im] All I could find was the numeric function 'matrix_multiply, but this only takes two arguments.

Have a look at the examples "dot()" and "mat()" in the Numpy Example List.
http://www.scipy.org/Numpy_Example_List

J.

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Bill B. <wb...@gm...>]

Also have a look at the section on Arrays vs Matrices in the Numpy for
Matlab users page.  That particular section has nothing to do with
Matlab, really.
http://www.scipy.org/NumPy_for_Matlab_Users
Most of the suggestions and comments made here are already on that page.

--bb

On 11/8/06, Joris De Ridder <jo...@st...> wrote:
>
>    [im]: Sorry if this is an obvious question, but what is the easiest way to multiply matrices in numpy? Suppose I want to do A=B*C*D. The ' * ' operator apparently does element wise multiplication, as does the 'multiply' ufunc.
>    [im] All I could find was the numeric function 'matrix_multiply, but this only takes two arguments.
>
> Have a look at the examples "dot()" and "mat()" in the Numpy Example List.
> http://www.scipy.org/Numpy_Example_List
>
> J.
>
> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
>
>
> -------------------------------------------------------------------------
> Using Tomcat but need to do more? Need to support web services, security?
> Get stuff done quickly with pre-integrated technology to make your job easier
> Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo
> http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642
> _______________________________________________
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> Num...@li...
> https://lists.sourceforge.net/lists/listinfo/numpy-discussion
>

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Johannes L. <a.u...@gm...>]

Hi,

in extension to the previous answers, I'd like to say that it is strongly 
preferable to use dot(A,dot(B,C)) or dot(dot(A,B),C) instead of A*B*C.

The reason is that with dot(), you can control of which operation is performed 
first, which can *massively* influence the time needed, depending on the 
involved matrices. A*B*C will always be evaluated left-to-right (if I 
remember correctly).

Johannes

Re: [Numpy-discussion] matrix multiplication (newbie question)

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

Johannes Loehnert schrieb:
> Hi,
> 
> in extension to the previous answers, I'd like to say that it is strongly 
> preferable to use dot(A,dot(B,C)) or dot(dot(A,B),C) instead of A*B*C.
> 
> The reason is that with dot(), you can control of which operation is performed 
> first, which can *massively* influence the time needed, depending on the 
> involved matrices. A*B*C will always be evaluated left-to-right (if I 
> remember correctly).
> 

Well what about A*(B*C)?

Re: [Numpy-discussion] matrix multiplication (newbie question)

[Johannes L. <a.u...@gm...>]

> Well what about A*(B*C)?

Point taken...

Johannes