r - Perform numerical operation in subset of IDs and return the results in the same data frame -


i have large dataset consisting of longitudinal measurements in various subjects (ids) , variables lets say:

test.df <- data.frame(id=c(rep("a", 50),rep("b", 50)), x1=rnorm(100), x2=rnorm(100))

i want perform numerical operation on records of each id , return results in same dataset.

right doing is:

test.df <- data.frame(id=c(rep("a", 50),rep("b", 50)), x1=rnorm(50), x2=rnorm(50))  test.df$mean.of.x1<-na test.df$mean.of.x2<-na  for(i in unique(test.df$id)){   test.df$mean.of.x1[test.df$id==i]<-mean(test.df$x1[test.df$id==i])   test.df$mean.of.x2[test.df$id==i]<-mean(test.df$x2[test.df$id==i]) }

the example simplistic (and perhaps silly), shows need (in original problem there several function run each id not mean). there more efficient way this? can *apply function help?

transform(test.df, mean.of.x1 = ave(x1, id, fun=mean),                     mean.of.x2 = ave(x2, id, fun=mean))

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