r - Replace missing values with a value from another column

Question

If I have:

s <- data.frame(ID=c(191, 282, 202, 210), Group=c("", "A", "", "B"), stringsAsFactors=FALSE)
s
   ID Group
1 191      
2 282     A
3 202      
4 210     B

I can replace the empty cells with N like this:

ds$Group[ds$Group==""]<-"N"

s
   ID Group
1 191     N 
2 282     A
3 202     N
4 210     B

But I would need to replace the empty cells with a value from another column. How can I accomplish this?:

s
   ID Group Group2
1 191     D      D
2 282     A      G
3 202     G      G
4 210     B      D

Answer 1

We can use data.table to assign the values in "Group2" to "Group" where the "Group" is "" specified in the "i" condition.

library(data.table)
setDT(s)[Group=="", Group:= Group2]

As the assignment happens in place, it is considered to be efficient.