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

ifelse(test, yes, no) is a handy function to do just that, and it can be used on vectors. Using your last data.frame:

s <- data.frame(ID = c(191, 282, 202, 210),
    Group = c("", "A", "", "B"),
    Group2 = c("D", "G", "G", "D"))

s$Group <- ifelse(test = s$Group != "", yes = s$Group, no = s$Group2)

The first argument is the test. For each value in the vector, if the test is true, then it will take the value in yes, otherwise it will take the value in no.

Answer 2

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.