Using `:=` in data.table to sum the values of two columns in R, ignoring NAs

Question

I have what I think is a very simple question related to the use of data.table and the := function. I don\'t think I quite understand the behaviour of :=<

Answer 1

This is standard R behaviour, nothing really to do with data.table

Adding anything to NA will return NA

NA + 1
## NA

sum will return a single number

If you want 1 + NA to return 1

then you will have to run something like

mat[,col3 := col1 + col2]
mat[is.na(col1), col3 := col2]
mat[is.na(col2), col3 := col1]

To deal with when col1 or col2 are NA

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EDIT - an easier solution

You could also use rowSums, which has a na.rm argument

mat[ , col3 :=rowSums(.SD, na.rm = TRUE), .SDcols = c("col1", "col2")]

rowSums is what you want (by definition, the rowSums of a matrix containing col1 and col2, removing NA values

(@JoshuaUlrich suggested this as a comment )