[R] as.vector with mode="list" and POSIXct

[Alexandre Sieira]

I was trying to convert a vector of POSIXct into a list of POSIXct, However, I had a problem that I wanted to share with you.

Works fine with, say, numeric:


> v = c(1, 2, 3)
> v
[1] 1 2 3
> str(v)
 num [1:3] 1 2 3
> l = as.vector(v, mode="list")
> l
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

> str(l)
List of 3
 $ : num 1
 $ : num 2
 $ : num 3

If you try it with POSIXct, on the other hand…


> v = c(Sys.time(), Sys.time())
> v
[1] "2013-05-20 18:02:07 BRT" "2013-05-20 18:02:07 BRT"
> str(v)
 POSIXct[1:2], format: "2013-05-20 18:02:07" "2013-05-20 18:02:07"
> l = as.vector(v, mode="list")
> l
[[1]]
[1] 1369083728

[[2]]
[1] 1369083728

> str(l)
List of 2
 $ : num 1.37e+09
 $ : num 1.37e+09

The POSIXct values are coerced to numeric, which is unexpected.

The documentation for as.vector says: "The default method handles 24 input types and 12 values of type: the details of most coercions are undocumented and subject to change." It would appear that treatment for POSIXct is either missing or needs adjustment.

Unlist (for the reverse) is documented to converting to base types, so I can't complain. Just wanted to share that I ended up giving up on vectorization and writing the two following functions:


unlistPOSIXct <- function(x) {
  retval = rep(Sys.time(), length(x))
  for (i in 1:length(x)) retval[i] = x[[i]]
  return(retval)
}

listPOSIXct <- function(x) {
  retval = list()
  for (i in 1:length(x)) retval[[i]] = x[i]
  return(retval)
}

Is there a better way to do this (other than using *apply instead of for above) that better leverages vectorization? Am I missing something here?

Thanks!




-- 
Alexandre Sieira
CISA, CISSP, ISO 27001 Lead Auditor

"The truth is rarely pure and never simple."
Oscar Wilde, The Importance of Being Earnest, 1895, Act I