[R] avoiding loops

[Ingmar Visser]

Thanks for this.
I was afraid someone was going to say this ...
Does this mean the only way of getting this to run faster is by  
moving to C code?
The cases I'm thinking of applying this in have dimensions of A that  
are much larger than
the example, eg n by n by T where n has a max of 10 or so but T could  
be hundreds
or even thousands.
Best, Ingmar

On Mar 27, 2008, at 12:11 AM, <Bill.Venables at csiro.au>  
<Bill.Venables at csiro.au> wrote:

> 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
> CSIRO Laboratories
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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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