[R] speeding up a special product of three arrays

[Dimitris Rizopoulos]

try this:

A <- matrix(rnorm(10*4), 10, 4)
B <- matrix(rnorm(3*4), 3, 4)
C <- matrix(rnorm(5*4), 5, 4)

nrA <- nrow(A); nrB <- nrow(B); nrC <- nrow(C)
ind <- as.matrix(expand.grid(1:nrA, 1:nrB, 1:nrC))
D <- rowSums(A[ind[, 1], ] * B[ind[, 2], ] * C[ind[, 3], ])
dim(D) <- c(nrA, nrB, nrC)
D


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Giuseppe Paleologo" <paleologo at gmail.com>
To: <r-help at r-project.org>
Sent: Thursday, May 08, 2008 11:20 PM
Subject: [R] speeding up a special product of three arrays


>I am struggling with R code optimization, a recurrent topic on this 
>list.
>
> I have three arrays, say A, B and C, all having the same number of 
> columns.
> I need to compute an array D whose generic element is
>
> D[i, j, k] <- sum_n A[i, n]*B[j, n]*C[k, n]
>
> Cycling over the three indices and subsetting the columns won't do. 
> Is there
> any way to implement this efficiently in R or should I resign to do 
> this in
> C?
>
> Thanks,
>
> Giuseppe
>
> [[alternative HTML version deleted]]
>
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