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

[Ranjan Maitra]

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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