Why does unlist() kill dates in R?

Question

When I unlist a list of dates it turns them back into numerics. Is that normal? Any workaround other than re-applying as.Date?

> dd <- as.Date(c("2013-01-01", "2013-02-01", "2013-03-01")) > class(dd) [1] "Date" > unlist(dd) [1] "2013-01-01" "2013-02-01" "2013-03-01" > list(dd) [[1]] [1] "2013-01-01" "2013-02-01" "2013-03-01"  > unlist(list(dd)) [1] 15706 15737 15765 

Is this a bug?

Answer 1

do.call is a handy function to "do something" with a list. In our case, concatenate it using c. It's not uncommon to cbind or rbind data.frames from a list into a single big data.frame.

What we're doing here is actually concatenating elements of the dd list. This would be analogous to c(dd[[1]], dd[[2]]). Note that c can be supplied as a function or as a character.

> dd <- list(dd, dd) > (d <- do.call("c", dd)) [1] "2013-01-01" "2013-02-01" "2013-03-01" "2013-01-01" "2013-02-01" "2013-03-01" > class(d) # proof that class is still Date [1] "Date"