How do I transfer NA's from one data frame into another data frame of the same size in R

I want to update my main data frame so that I can copy the NA entries at the exact position as in my reference data frame ref. Here is an example:

main <- data.frame(Q1 = 1:5,
                   Q2 = rev(1:5),
                   Q3 = c(4, 5, 1, 2, 3))

ref <- data.frame(Q1 = c(0.4, NA, 1.6, 1.1, 0.9),
                  Q2 = c(NA, 2.7, 1.8, 1.5, 0.8),
                  Q3 = c(1.3, 0.9, NA, NA, 2.6))

> main
  Q1 Q2 Q3
1  1  5  4
2  2  4  5
3  3  3  1
4  4  2  2
5  5  1  3

> ref
   Q1  Q2  Q3
1 0.4  NA 1.3
2  NA 2.7 0.9
3 1.6 1.8  NA
4 1.1 1.5  NA
5 0.9 0.8 2.6

How do I create a data frame that is the same as main, but contains NAs at the same positions as in ref? The result should look like this:

> res
  Q1 Q2 Q3
1  1 NA  4
2 NA  4  5
3  3  3 NA
4  4  2 NA
5  5  1  3

>Solution :

The simplest way:

main[is.na(ref)] <- NA