Sum of two Columns of Data Frame with NA Values

dplyr solution, taken from here:

library(dplyr)
dat %>% 
    rowwise() %>% 
    mutate(e = sum(b, c, na.rm = TRUE))

dat$e <- rowSums(dat[,c("b", "c")], na.rm=TRUE)
dat
#   a  b c d e
# 1 1  2 3 4 5
# 2 5 NA 7 8 7