[R] avoiding loops

[Bill.Venables at csiro.au]

If you have lots of memory there is an obvious strategy:

d12 <- prod(dim(A)[1:2])
A <- A * array(rep(B, each = d12), dim = dim(A))

I don't really see much wrong with the obvious for() loop, though:

for(b in 1:length(B)) A[,,b] <- A[,,b] * B[b]


Bill Venables
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-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Ingmar Visser
Sent: Thursday, 27 March 2008 7:58 AM
To: R-help at r-project.org
Subject: [R] avoiding loops

Hi,
I need to compute an array from a matrix and an array:

A <- array(1:20,c(2,2,5))
B <- matrix(1:10,5)

And I would like the result to be an array consisting of the following:

rbind(A[1,,1]*B[1,],
A[2,,1]*B[1,])

rbind(A[1,,2]*B[2,],
A[2,,2]*B[2,])

rbind(A[1,,3]*B[2,],
A[2,,3]*B[2,])

etc.

Hence the result should have the same dimension as A, ie a series of  
2 by 2 matrices.

Short of a for loop over the the last index of A I have struggled  
with versions of apply but
to no avail ...

Any insights much appreciated,

Best, Ingmar

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