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

[Jeff Newmiller]

Strictly speaking, the answer is yes because you can unroll the loop, but that probably is not what you really want or need to do.

Your example seems about right, but it is not clear how you plan to make 44 conform with 5500.
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Sent from my phone. Please excuse my brevity.

On July 10, 2015 6:44:19 AM PDT, Ranjan Maitra <maitra.mbox.ignored at inbox.com> wrote:
>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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