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

[Jeff Newmiller]

I don't know what you plan to do with this list, but lists are quite a bit less efficient than fixed-mode vectors, so you are likely losing a lot of computational speed by using this list. I don't hesitate to use simple data frames (lists of vectors), but processing lists is on par with for loops, not vectorized computation. It may still support a simpler model of computation, but that is an analyst comprehension benefit rather than a computational efficiency benefit.
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Sent from my phone. Please excuse my brevity.

Alexandre Sieira <alexandre.sieira at gmail.com> wrote:

>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
>
>------------------------------------------------------------------------
>
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