R devel - May 2003 - Extending %*%

I have lists of matrices stored in various ways.  I'd like to extend
%*% to work on these.  Is this possible, or should I create my own new
operator?

A simplified example would be as follows:

A <- list( A1, A2, A3)
B <- list( B1, B2, B3)

where A1,...,B3 are all matrices, and I'd like A %*% B to return 

list( A1 %*% B1, A2 %*% B2, A3 %*% B3)

In the real case A and B are sometimes arrays, sometimes something
else; lists haven't come up so far, but the principle should be the
same.

Duncan Murdoch

Duncan Murdoch wrote:
> I have lists of matrices stored in various ways.  I'd like to extend
> %*% to work on these.  Is this possible, or should I create my own new
> operator?
> 
> A simplified example would be as follows:
> 
> A <- list( A1, A2, A3)
> B <- list( B1, B2, B3)
> 
> where A1,...,B3 are all matrices, and I'd like A %*% B to return 
> 
> list( A1 %*% B1, A2 %*% B2, A3 %*% B3)
> 
> In the real case A and B are sometimes arrays, sometimes something
> else; lists haven't come up so far, but the principle should be the
> same.
> 
> Duncan Murdoch
The straightforward way without extending %*% (I feel that extending it 
is dangerous, in a way) is
   mapply("%*%", A, B, SIMPLIFY = FALSE)

Uwe Ligges

On Mon, 19 May 2003 21:56:24 -0400, you wrote in message
<te2jcvkqn8uq9it881386oqi1n4j8ncakh@4ax.com>:
>I have lists of matrices stored in various ways.  I'd like to extend
>%*% to work on these.  Is this possible, or should I create my own new
>operator?
In answer to my own question:  yes, it's possible, using the S4
classes and methods.  It would look something like this:


setClass("special", representation(m = "list"))

setMethod("%*%", 
                signature(x = "special", y="special"),
                function(x,y) 
		new("special", 
			m = mapply("%*%", x@m, y@m, SIMPLIFY=FALSE))
)


A1 <- A2 <- A3 <- B1 <- B2 <- B3 <- diag(c(2,3))

A <- new("special", m = list(A1, A2, A3))

B <- new("special", m = list(A1, A2, A3))

A %*% B

(Thanks Uwe for the compact expression to implement the method!)

Duncan Murdoch