R help - Sep 2010 - POSIXct: Extract the hour for a list of elements

Hi,

I have a list/data.frame 'pk' of POSIXct dates, and I'd like to
extract the
hour for each row.  I know that if I have an individual POSIXct object, I
can extract the hour by converting to a new object with:

new.lt <- as.POSIXlt(<single POSIXct object>)
new.lt$hour

But I can't figure out how to apply this for a list of such dates in a
vectorized form.  I can write a loop, I guess and implement this, but I
think I'm missing a way that takes advantage of vectorization.  Here is my
loop to just print the hour extracts:

for (ct in pk) {
    lt <- as.POSIXlt(ct, origin="1970-01-01")
    print(lt$hour)
}

So is there a shorter vectorization idiom that lets me do this?  I can't
figure out how to use 'lapply' to apply the '$' operator...

Thanks,
Matt

	[[alternative HTML version deleted]]

Matthew -
    It's a bit simpler than you think:

       as.POSIXlt(pk)$hour

should return what you want.  (If not, please provide a 
reproducible example.)
 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu




On Fri, 24 Sep 2010, Matthew Pettis wrote:
> Hi,
>
> I have a list/data.frame 'pk' of POSIXct dates, and I'd like to
extract the
> hour for each row.  I know that if I have an individual POSIXct object, I
> can extract the hour by converting to a new object with:
>
> new.lt <- as.POSIXlt(<single POSIXct object>)
> new.lt$hour
>
> But I can't figure out how to apply this for a list of such dates in a
> vectorized form.  I can write a loop, I guess and implement this, but I
> think I'm missing a way that takes advantage of vectorization.  Here is
my
> loop to just print the hour extracts:
>
> for (ct in pk) {
>    lt <- as.POSIXlt(ct, origin="1970-01-01")
>    print(lt$hour)
> }
>
> So is there a shorter vectorization idiom that lets me do this?  I
can't
> figure out how to use 'lapply' to apply the '$' operator...
>
> Thanks,
> Matt
>
> 	[[alternative HTML version deleted]]
>
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