Thomas' solution is better but thought this might be of interest
anyways since it can be written closer to mathematical notation.
That is, the required expression can be written in the
following equivalent way for a suitable matrix A:
X' diag(u) A' A diag(u) X
where diag(u) is a diagonal matrix with u along the diagonal
as in the R diag function, spaces refer to matrix multiplication
and ' means transpose.
Thus we have:
A <- outer(unique(dat$id), dat$id, "==")
crossprod(A %*% diag(dat$u) %*% as.matrix(dat[1:2]))
On 3/8/06, Thomas Lumley <[EMAIL PROTECTED]> wrote:
> On Wed, 8 Mar 2006, ronggui wrote:
>
> > Thank you for all .
> >
> > One more question.How can I calculate these efficiently?
> >
> > set.seed(100)
> > dat<-data.frame(x1=rnorm(20),x2=rnorm(20),u=rnorm(20),id=round(2*runif(20)))
> > # In this example,id's elements are 0,1,2.
> > y<-list()
> > for (i in 0:2){
> > X<-as.matrix(subset(dat,id==i,c("x1","x2")))
> > u<-as.matrix(subset(dat,id==i,c("u")))
> > y[[i+1]]<-t(X)%*%u%*%t(u)%*%X
> > }
> > y[[1]]+y[[2]]+y[[3]]
> >
>
> People have already told you about crossprod, so crossprod(crossprod(X,u))
> would seem an obvious improvement over the matrix multiplications.
>
> There is a better solution, though.
>
> Xu<-dat[,c("x1","x2")]*dat[,"u"]
> crossprod( rowsum(Xu, dat$id))
>
> -thomas
>
>
> > the above code is not elegant.And my second solution to this problem
> > is using by to get a list.
> >
> > matlis<-by(dat, dat$id, function(x){
> > a<-as.matrix(x[,c("x1","x2")])
> > b<-as.matrix(x[, "u"])
> > t(a) %*% b %*% t(b) %*% a
> > })
> >
> > S <- matrix(unlist(matlis), 4, length(matlis))
> > S1 <- matrix(rowSums(S), 2, 2)
> >
> > The code works ,but I want to ask if there is any other more better
> > ways to do it? It seems that this kind of computation is quite common.
> >
> >
> >
> >
> >
> > 2006/2/28, Gabor Grothendieck <[EMAIL PROTECTED]>:
> >> Try:
> >>
> >> crossprod(x)
> >>
> >> or
> >>
> >> t(x) %*% x
> >>
> >> On 2/28/06, ronggui <[EMAIL PROTECTED]> wrote:
> >>> This is the code:
> >>>
> >>> x<-matrix(rnorm(20),5)
> >>> y<-list()
> >>> for (i in seq(nrow(x))) y[[i]]<-t(x[i,,drop=F])%*%x[i,,drop=F]
> >>> y[[1]]+y[[2]]+y[[3]]+y[[4]]+y[[5]]
> >>>
> >>> How can I do it without using for loops?
> >>> Thank you in advance!
> >>> --
> >>> ronggui
> >>> Deparment of Sociology
> >>> Fudan University
> >>>
> >>> ______________________________________________
> >>> R-help@stat.math.ethz.ch mailing list
> >>> https://stat.ethz.ch/mailman/listinfo/r-help
> >>> PLEASE do read the posting guide!
> >>> http://www.R-project.org/posting-guide.html
> >>>
> >>
> >
> >
> > --
> > »ÆÈÙ¹ó
> > Deparment of Sociology
> > Fudan University
> >
> >
>
> Thomas Lumley Assoc. Professor, Biostatistics
> [EMAIL PROTECTED] University of Washington, Seattle
>
> ______________________________________________
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>
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