Adding a column of means by group to original data

I want to add a column of means based on factor column in `R` `data.frame`. Like this: ``` df1 <- data.frame(X = rep(x = LETTERS[1:2], each = 3), Y = 1:6) df2 <- aggregate(data = df1, Y ~ X, FUN = mean) df3 <- merge(x = df1, y = df2, by = "X", suffixes = c(".Old",".New")) df3 # X Y.Old Y.New # 1 A 1 2 # 2 A 2 2 # 3 A 3 2 # 4 B 4 5 # 5 B 5 5 # 6 B 6 5 ``` To accomplish this problem I've to create two unnecessary `data.frames`. I'd like to know a way to append a column of means by factor column into my original `data.frame` without creating any extra `data.frames`. Thanks for your time and help.