  here is the data:
 y<-c(5,2,3,7,9,0,1,4,5)
id<-c(1,1,6,6,7,8,15,15,19)
t<-c(50,56,50,56,50,50,50,60,50)
table1<-data.frame(y,id,t)//longitudinal data

what  I want to do is to use the linear model for each id ,then get the
estimate value,like:

fit1<-lm(y~t,data=table1,subset=(id==1))

but ,you can see the variable "id" is quite irregular,they are not arranaged
in order and many number missing,if I write a loop by using "for",it will
give me a lot "NA",
and for sure ,I dont want to type id=## for about 500 times,any one know how
to deal with it?

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