[R] efficiently multiply different matrices in 3-d array with different vectors?

[Ranjan Maitra]

On Thu, 27 Dec 2012 22:03:19 -0800 arun <smartpink111 at yahoo.com> wrote:

> HI,
> 
> Not sure if this is what you wanted:
> set.seed(14)
> Z<-array(sample(1:100,80,replace=TRUE),dim=c(5,2,8))
> set.seed(21)
> Y<-matrix(sample(1:40,40,replace=TRUE),nrow=8)
> do.call(cbind,lapply(seq_len(dim(Z)[1]),function(i) Z[i,,]%*%Y[,i]))
> #     [,1] [,2]  [,3]  [,4]  [,5]
> #[1,] 5065 6070 12328 11536 13678
> #[2,] 6152 9878 11161 13777  7991

Yes, this does it, many thanks!

Ranjan


> 
> 
> 
> 
> ----- Original Message -----
> From: Ranjan Maitra <maitra.mbox.ignored at inbox.com>
> To: r-help at r-project.org
> Cc: 
> Sent: Friday, December 28, 2012 12:33 AM
> Subject: [R] efficiently multiply different matrices in 3-d array with different vectors?
> 
> Hello,
> 
> I have been wondering of an efficient way to do this:
> 
> I have an n x m x p array Z and a p x n matrix Y.
> 
> I want to multiply each of the n matrices with the corresponding column
> vector of Y.
> 
> In other words, I am wanting to matrix multiply: 
> 
> Z[i, ,] %*% Y[, i]
> 
> which will give me a (two-dimensional) array or matrix of dimension n x
> p with the i'th row storing the above.
> 
> Any pointers on an efficient way of doing this? I considered using
> apply, but was not successful. 
> 
> Many thanks again and best wishes,
> Ranjan
> 
> 
> 
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