calculating sum and dealing with NAs

Question

I am having a problem with the rowSum function. What's happening is that any rows with NAs are being counted as 0 and I don't want that. Here is my data:

V1 V2 V3   V4
1  0  0    1
0  1  NA   1
NA  NA NA  NA

Here is what is happening:

V1 V2 V3   V4  SUM
1  0  0    1    2
0  1  NA   1    2
NA  NA NA  NA   0

I want this:

V1 V2 V3   V4  SUM
1  0  0    1    2
0  1  NA   1    2
NA  NA NA  NA   NA 

I've looked on several websites and I have tried so many different iterations of code and I keep getting the same thing. This is the most basic piece of code I have used, although I tried using dplyr. Can someone please help me?

df$sum <- rowSums(df, na.rm = T)

Answer 1

We can take advantage of the fact that

NA ^ 0
#[1] 1
NA ^ 1
#[1] NA

Using it in rowSums , we can do:

rowSums(df, na.rm = TRUE) * NA^(rowSums(!is.na(df)) == 0)
#[1]  2  2 NA