[R] Corrected: approaches tpmatrix multiplication of each layer of 3-d array with different matrix

[Ranjan Maitra]

Hi,

Sorry to post again, but there is a careless error in my first R code snippet:

--- begin R code ---
 
AA <- array(1:60, dim = c(5500,44,33))
BB <- array(1:60, dim = c(44,44,33))
 
 
arraymatprod <- function(A, B) {
    EE <- array(dim=dim(A));
    for (i in 1:dim(A)[3]) EE[,,i] <- A[,,i] %*% B[,,i]
    EE
}

system.time(arraymatprod(AA, BB))

--- end R code ---

The second snippet is correct:

--- begin R code ---

arraymatrixproduct <- function(A, B) {
	require(abind)
	dA <- dim(A)[1]
	AB <- abind(A, B, along = 1)
	array(apply(X = AB, MARGIN = 3, FUN = (function(mat) ((mat[1:dA, ] %*% mat[(dA+1):(dim(AB)[1]),])))), dim = dim(A))
}

system.time(arraymatrixproduct(AA, BB))

--- end R code ---

However, the second is almost twice as long as the first snippet.

Many thanks,
Ranjan


On Fri, 10 Jul 2015 08:23:49 -0500 Ranjan Maitra <maitra.mbox.ignored at inbox.com> wrote:

> Dear friends,
> 
> I have two 3-d arrays of appropriate dimensions. I want to postmultiply the 2-D matrix layers of the first with the 2-D matrix layers of the second. Can I do this easily in R avoiding loops?
> 
> As an example:
> 
> --- begin R code ---
> 
> AA <- array(1:60, dim = c(5500,44,33))
> BB <- array(1:60, dim = c(44,44,33))
> 
> 
> arraymatprod <- function(A, B) {
>     EE <- array(dim=dim(A));
>     for (i in 1:3) EE[,,i] <- A[,,i] %*% B[,,i]
>     EE
> }
> 
> system.time(arraymatprod(AA, BB))
> 
> --- end R code ---
> 
> So, is there a way to do this without the loop?
> 
> Of course, I could abind the arrays and then use apply with an appropriate function which would be as follows:
> 
> --- begin R code ---
> 
> arraymatrixproduct <- function(A, B) {
> 	require(abind)
> 	dA <- dim(A)[1]
> 	AB <- abind(A, B, along = 1)
> 	array(apply(X = AB, MARGIN = 3, FUN = (function(mat) ((mat[1:dA, ] %*% mat[(dA+1):(dim(AB)[1]),])))), dim = dim(A))
> }
> 
> system.time(arraymatrixproduct(AA, BB))
> 
> --- end R code ---
> 
> However, this turns out to be slower -- perhaps because of the use of abind and filling the array inside the function.
> 
> I just wanted suggestions to get this operation done more efficiently.
> 
> Many thanks and best wishes,
> Ranjan
> 
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