Test for NA and select values based on result

Question

My question is rather simple. What I want is if A[i]!=NA, then C[i]=A[i], if A[i]=NA, then C[i]=B[i], however, I always g

Answer 1

use is.na :

DF <- within(DF,
   C <- ifelse(!is.na(A),A,B)
)

with DF being your dataframe.