Replace NAs in one variable with values from another variable

Question

How can I replace the "NAs" of a variable VAR1 with with the values of the second variable VAR2 to create a third variable VAR3 in R? The data looks like this:

 VAR1:    VAR2:
 1        NA
 3        NA
 NA       1 
 NA       3
 2        NA 
 NA       1

Afterwards it should look likes this:

 VAR1:    VAR2:    VAR3:
 1        NA       1   
 3        NA       3
 NA       1        1
 NA       3        3
 2        NA       2
 NA       1        1  

Answer 1

One way is to use ifelse:

DF <- transform(DF, VAR3 = ifelse(!is.na(VAR1), VAR1, VAR2))

where transform was used to avoid typing DF$ over and over, but maybe you will prefer:

DF$VAR3 <- ifelse(!is.na(DF$VAR1), DF$VAR1, DF$VAR2)