Combining two vectors element-by-element

Question

I have 2 vectors, such as these:

A <- c(1,2,NA,NA,NA,NA,7)
B <- c(NA,NA,3,4,NA,NA,7)

I would like to combine them so that the resulting vector is

1,2,3,4,NA,NA,-1

That is

1. when only 1 value (say X) in either vector at position i exists (the other being NA) the new vector should take the value X at position i.

2. when both values are NA at position i, the new vector should take the value NA at position i

3. when both vectors have a value at position i, the new vector should take the value -1 at position i.

I can easily do this with a loop, but it is very slow on a large dataset so can anyone provide a fast way to do this ?

Answer 1

These commands create the vector:

X <- A
X[is.na(A)] <- B[is.na(A)]
X[is.na(B)] <- A[is.na(B)]
X[!is.na(A & B)] <- -1

#[1]  1  2  3  4 NA NA -1