rowSums but keeping NA values

Question

I am pretty sure this is quite simple, but seem to have got stuck...I have two xts vectors that have been merged together, which contain numeric values and NAs.

I wo

Answer 1

mm$m <- "is.na<-"(rowSums(mm, na.rm = TRUE), !rowSums(!is.na(mm)))

> tail(mm)
#                  dd1        dd2        m
# 2013-08-06        NA  1.3325804 1.332580
# 2013-08-07        NA  0.7726740 0.772674
# 2013-08-08 0.8158402 -0.8158402 0.000000
# 2013-08-09        NA  1.2292919 1.229292
# 2013-08-10        NA         NA       NA
# 2013-08-11        NA  0.9334900 0.933490