Question

I have some data like these

var1   var2
10     NA
101    NA
NA     86
11     NA
NA     11
NA     61

If one variable is NA then the other one is not, and vice-versa.

How can I combine them into a single variable:

var3
10
101
86
11
11
61

I can do it easily with a loop, but it is quite slow, so I would like to find an easier way. I thought about assigning 0 to the values that are NA and then just adding the variables together…is there a better way ?

Answer 1

Various methods exist. Here’s one way:

var3 <- ifelse(!is.na(var1),var1,var2)

Here it is working on your example:

  var1 <- c(10,101,NA,11,NA,NA)
  var2 <- c(NA,NA,86,NA,11,61)

  var3 <- ifelse(!is.na(var1),var1,var2)

 > var3
 [1]  10 101  86  11  11  61

This method is relatively general – it works with non-numeric data for example:

  var1 <- c("AB","WZ",NA,"MN",NA,NA)
  var2 <- c(NA,NA,"QT",NA,"MN","RS")

  var3 <- ifelse(!is.na(var1),var1,var2)

 > var3
 [1] "AB" "WZ" "QT" "MN" "MN" "RS"

The suggestion of replacing NA with 0 and adding wouldn’t work in that case.