Treat NA as zero only when adding a number

Question

When calculating the sum of two data tables, NA+n=NA.

> dt1 <- data.table(Name=c(\"Joe\",\"Ann\"), \"1\"=c(0,NA), \"2\"=c(3,NA))
> dt1
         


        

Answer 1

dtsum  <- rbind(dt1, dt2)[, lapply(.SD, function(x) ifelse(all(is.na(x)), as.numeric(NA), sum(x, na.rm=T))), by=Name]

(includes @Arun's suggestion) na.rm=TRUE is very useful to remember