n<-1000
Id<-sort(sample(1:50,size=n,rep=T))
table(Id)
X<-rnorm(n)

CX<-unlist(tapply(X,Id,cumsum))
Xdf<-data.frame(Id,X,CX)
Xdf[1:40,]

Xm<-matrix(rep(1:10,each=2),ncol=2,byrow=T)
xv<-1:10

apply(Xm,2,function(x){mapply(xv,x,FUN="*")})