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

[Alexandre Sieira]

That's a very interesting point, David. The part I actually didn't know is that the class of an object is an attribute, and that by "removing all attributes" the function would in effect unclass it.

Thank you.

-- 
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
On 22 de maio de 2013 at 17:52:05, David Winsemius (dwinsemius at comcast.net) wrote:

On May 20, 2013, at 2:09 PM, Alexandre Sieira 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.  

You may not have expected it, but that result is what is described: From the help page for `as.vector`:  

"For as.vector, a vector (atomic or of type list). All attributes are removed from the result if it is of an atomic mode, ..."  

And since POSIXct vectors _are_ of atomic mode, all of their attributes are removed.  

> is.atomic(as.POSIXct("2001-01-01") )  
[1] TRUE  

(I realize that this is a belated response, and that you and Jeff Newmiller are continuing a correspondence on what I think appears to be a separate concern) , but I needed to go back to the first posting to figure out what the questions were.  

>  
> 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.  

As in the case of factor-classed vectors, POSIXct-vectors are atomic mode. Yours was a selective reading. (And you found a way around this with `as.list`.)  

> is.atomic(factor("2001-01-01") )  
[1] TRUE  
--  
David.  
>  
> 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!  
>  
>  
--  

David Winsemius  
Alameda, CA, USA