Impute missing data with mean by group

Question

I have a categorical variable with three levels (A, B, and C).

I also have a continuous variable with some missing values on it.

I would like to replace the NA values with the mean of its group. This is, missing observations from group A has to be replaced with the mean of group A.

I know I can just calculate each group's mean and replace missing values, but I'm sure there's another way to do so more efficiently with loops.

A <- subset(data, group == "A")
mean(A$variable, rm.na = TRUE)
A$variable[which(is.na(A$variable))] <- mean(A$variable, na.rm = TRUE)

Now, I understand I could do the same for group B and C, but perhaps a for loop (with if and else) might do the trick?

Answer 1

require(dplyr)
data %>% group_by(group) %>%
mutate(variable=ifelse(is.na(variable),mean(variable,na.rm=TRUE),variable))

For a faster, base-R version, you can use ave:

data$variable<-ave(data$variable,data$group,FUN=function(x) 
  ifelse(is.na(x), mean(x,na.rm=TRUE), x))