[R] columnwise multiplication?

[Marc Schwartz (via MN)]

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