R help - Aug 2018 - A slightly unorthodox matrix product.

Can anyone think of a sexy way of forming following "product"?

Given matrices A and B, both with m rows, form a 3 dimensional array C 
such that:

     C[i,j,k] = A[i,j]*B[i,k]

I *think* that the following does what I want.  (I keep confusing 
myself, so I'm not sure!)

library(abind)
xxx <- lapply(1:nrow(a),function(i,a,b){a[i,]%o%b[i,]},a=A,b=B)
do.call(abind,c(xxx,list(along=3)))

Is there a cleverer way?

cheers,

Rolf Turner

-- 
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

Hi Rolf,
A few edits because (i) nrow(a) should be nrow(A) and (ii) you have
calculated C[j,k,i] = A[i,j]*B[i,k], (iii) minor style change on lapply.

library(abind)
xxx <- lapply(1:nrow(A),function(i){A[i,]%o%B[i,]})
yyy <- do.call(abind,c(xxx,list(along=3)))
zzz <- aperm(yyy,c(3,1,2))

HTH,
Eric


On Sat, Aug 4, 2018 at 8:52 AM, Rolf Turner <r.turner at auckland.ac.nz>
wrote:
>
> Can anyone think of a sexy way of forming following "product"?
>
> Given matrices A and B, both with m rows, form a 3 dimensional array C
> such that:
>
>     C[i,j,k] = A[i,j]*B[i,k]
>
> I *think* that the following does what I want.  (I keep confusing myself,
> so I'm not sure!)
>
> library(abind)
> xxx <- lapply(1:nrow(a),function(i,a,b){a[i,]%o%b[i,]},a=A,b=B)
> do.call(abind,c(xxx,list(along=3)))
>
> Is there a cleverer way?
>
> cheers,
>
> Rolf Turner
>
> --
> Technical Editor ANZJS
> Department of Statistics
> University of Auckland
> Phone: +64-9-373-7599 ext. 88276
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posti
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>
	[[alternative HTML version deleted]]

> On Aug 4, 2018, at 10:01 AM, Eric Berger <ericjberger at gmail.com>
wrote:
> 
> Hi Rolf,
> A few edits because (i) nrow(a) should be nrow(A) and (ii) you have
> calculated C[j,k,i] = A[i,j]*B[i,k], (iii) minor style change on lapply.
> 
> library(abind)
> xxx <- lapply(1:nrow(A),function(i){A[i,]%o%B[i,]})
> yyy <- do.call(abind,c(xxx,list(along=3)))
Or use the simplify="array" gambit in sapply:

yyy <- sapply(1:nrow(A), function(i) A[i,] %o% B[i,],
simplify="array")
> zzz <- aperm(yyy,c(3,1,2))
> 
HTH, 

Chuck