[R] Summing elements in a list

[Giovanni Petris]

The following seems to give what you want.


tmp <- rep(w, each=nrow1*ncol1+nrow2*ncol2) * unlist(matlist)
tmp <- rowSums(matrix(tmp,nr=nrow1*ncol1+nrow2*ncol2))
mat1 <- tmp[1:(nrow1*ncol1)]; dim(mat1) <- c(nrow1,ncol1)
mat2 <- tmp[nrow1*ncol1+1:(nrow2*ncol2)]; dim(mat2) <- c(nrow2,ncol2)


Giovanni

-- 

 __________________________________________________
[                                                  ]
[ Giovanni Petris                 GPetris at uark.edu ]
[ Department of Mathematical Sciences              ]
[ University of Arkansas - Fayetteville, AR 72701  ]
[ Ph: (479) 575-6324, 575-8630 (fax)               ]
[ http://definetti.uark.edu/~gpetris/              ]
[__________________________________________________]

> Date: Fri, 31 Oct 2003 14:16:31 -0500
> From: Angelo Canty <canty at math.mcmaster.ca>
> Sender: r-help-bounces at stat.math.ethz.ch
> Organization: McMaster University
> Precedence: list
> 
> Hi,
> 
> Suppose that I have a list where each component is a list of two
> matrices.  I also have a vector of weights.  How can I collapse my
> list of lists into a single list of two matrices where each matrix
> in the result is the weighted sum of the corresponding matrices.
> 
> I could use a loop but this is a nested calculation so I was hoping
> there is a more efficient way to do this.  To help clarify, here is
> the code I would use with a for loop
> 
> result <- list(mat1=matrix(0,nrow1,ncol1),
>                mat2=matrix(0,nrow2,ncol2))
> for (i in seq(along=matlist)) {
>    result$mat1 <- result$mat1+w[i]*matlist[[i]]$mat1
>    result$mat2 <- result$mat2+w[i]*matlist[[i]]$mat2
> }
> 
> I apologise if this is a trivial question.  Unfortunately I don't have
> my copy of V&R S Programming to hand.
> 
> Thanks for your help,
> Angelo
> -- 
> ------------------------------------------------------------------
> |   Angelo J. Canty                Email: cantya at mcmaster.ca     |
> |   Mathematics and Statistics     Phone: (905) 525-9140 x 27079 |
> |   McMaster University            Fax  : (905) 522-0935         |
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