How to handle missing values in R by replacing it with mean? [closed]

Answer 1

This should do the trick, using your data:

 d <- data.frame(id = rep(c("key1", "key2"), each = 3), x = c(10, 12, NA, 2, 3, NA))  
 library(plyr)
 d2 <- ddply(d, .(id), transform, x=ifelse(is.na(x), mean(x, na.rm=T), x) )

Here, we split the dataframe by your key and transform x depending on whether or not it is NA. The result is

     id    x
 1 key1 10.0
 2 key1 12.0
 3 key1 11.0
 4 key2  2.0
 5 key2  3.0
 6 key2  2.5