R help - Jun 2006 - columnwise multiplication?

Hi all,

I'd like to do a multiplication between 2 matrices buy only want resulsts of
cloumn 1 * column 1, column 2 * column 2 and so on.

Now I do

C <- diag(t(A) %*% B)

Is there a bulit in way to do this?

Thank you.

	[[alternative HTML version deleted]]

On Fri, 23 Jun 2006, Mu Tian wrote:
> Hi all,
>
> I'd like to do a multiplication between 2 matrices buy only want
resulsts of
> cloumn 1 * column 1, column 2 * column 2 and so on.
>
> Now I do
>
> C <- diag(t(A) %*% B)
>
> Is there a bulit in way to do this?
If * here means vector inner product, colSums(A*B) which is a lot more 
efficient.
>
> Thank you.
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
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>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

On Fri, 2006-06-23 at 11:53 -0400, Mu Tian wrote:
> Hi all,
> 
> I'd like to do a multiplication between 2 matrices buy only want
resulsts of
> cloumn 1 * column 1, column 2 * column 2 and so on.
> 
> Now I do
> 
> C <- diag(t(A) %*% B)
> 
> Is there a bulit in way to do this?
> 
> Thank you.

You just want:

  A * B

for the initial multiplication. This technically gives you element by
element multiplication. Since matrices are vectors with a dim attribute
and elements stored by default in column order (top to bottom, then left
to right), the result matrix will yield what you require on a column by
column basis.

For example:
> A <- matrix(1:12, ncol = 3)
> B <- matrix(1:12, ncol = 3)
> A
     [,1] [,2] [,3]
[1,]    1    5    9
[2,]    2    6   10
[3,]    3    7   11
[4,]    4    8   12
> B
     [,1] [,2] [,3]
[1,]    1    5    9
[2,]    2    6   10
[3,]    3    7   11
[4,]    4    8   12
> A * B
     [,1] [,2] [,3]
[1,]    1   25   81
[2,]    4   36  100
[3,]    9   49  121
[4,]   16   64  144


To then get cumulative column totals, you can do:
> colSums(A * B)
[1]  30 174 446


HTH,

Marc Schwartz