How to replace value in a variable based on matching values in another variable in R?

Question

Suppose I have NA values under a variable, Font in my df. How can replace the NA values based on matching values under the variable, Group? For instance, I want all rows where Group = 1 to have Arial as the Font and all rows where Group = 2 to have Helvetica as the Font. I am aware that I can do the following: df$Font[df$Group==1]<-"Arial" or use recode() from the 'car' package. However, suppose that there are thousands of different groups and fonts, and I don't want to keep typing each of them out in my code.

Example data:
   Group  Font
   1      Arial
   1      NA
   2      NA
   2      Helvetica

Answer 1

Use ave to grab a non-missing value and fill:

dat <- read.table(text="   Group  Font
   1      Arial
   1      NA
   2      NA
   2      Helvetica",header=TRUE)

dat$Font <- with(dat, ave(Font, Group, FUN=function(x) replace(x, TRUE, na.omit(x)[1L])))

#  Group      Font
#1     1     Arial
#2     1     Arial
#3     2 Helvetica
#4     2 Helvetica