How to implement coalesce efficiently in R

Question

Background

Several SQL languages (I mostly use postgreSQL) have a function called coalesce which returns the first non null column element for each row. This can b

Answer 1

Another apply method, with mapply.

mapply(function(...) {temp <- c(...); temp[!is.na(temp)][1]}, a, b, c)
[1]  1  2 NA  4  6

This selects the first non-NA value if more than one exists. The last non-missing element could be selected using tail.

Maybe a bit more speed could be squeezed out of this alternative using the bare bones .mapply function, which looks a little different.

unlist(.mapply(function(...) {temp <- c(...); temp[!is.na(temp)][1]},
               dots=list(a, b, c), MoreArgs=NULL))
[1]  1  2 NA  4  6

.mapplydiffers in important ways from its non-dotted cousin.

• it returns a list (like Map) and so must be wrapped in some function like unlist or c to return a vector.
• the set of arguments to be fed in parallel to the function in FUN must be given in a list to the dots argument.
• Finally, mapply, the moreArgs argument does not have a default, so must explicitly be fed NULL.