Le ven. 09 mai à 03:44, Dimitris Rizopoulos a écrit :
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
Dimitris,
We basically have the same solution. Actually, I first went exactly
the same path as you, but I didn't think of expand.grid(), so I was
creating the index matrix manually (easy, but ugly).
Now, just out of curiosity, I made a few tests with ~ 100-row matrices
and my solution is slightly faster (to the order of 1 sec. vs 1.2
sec). :-) This is mostly due to slice.index() being faster than
expand.grid(). In fact, if one doesn't mind the ugliness, building the
indexes manually brings your solutions on par with mine in terms of
speed. Doing this is left as an exercise to the reader...
Does this matter? No. It's just that it's a sunny Friday...
Cheers
Vincent
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" <[EMAIL PROTECTED]
>
To: <r-help@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
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