On 29.03.2012 23:44, Steven R Corsi wrote:
Thanks for this feedback. I was able to get that example to work, but I
have gotten conflicting results with factors converted via POSIXct using
the format= and tz= options to bring in excel-type dates and times. Here
is what I have tried:
dt.factor<- factor(c("3/17/2011 14:07","3/20/2011 7:18","3/22/2011
14:10","3/29/2011 9:07","7/15/2010 7:49"))
dt.factor
[1] 3/17/2011 14:07 3/20/2011 7:18 3/22/2011 14:10 3/29/2011 9:07 7/15/2010
7:49
Levels: 3/17/2011 14:07 3/20/2011 7:18 3/22/2011 14:10 3/29/2011 9:07 7/15/2010
7:49
as.POSIXct(dt.factor,format="%m/%d/%Y %H:%M",tz="CST6CDT")
Error in as.POSIXlt.character(as.character(x)) :
character string is not in a standard unambiguous format
Works for me in R-release.
Uwe Ligges
as.POSIXct(as.character(dt.factor),format="%m/%d/%Y %H:%M",tz="CST6CDT")
[1] "2011-03-17 14:07:00 CDT" "2011-03-20 07:18:00 CDT" "2011-03-22 14:10:00
CDT"
[4] "2011-03-29 09:07:00 CDT" "2010-07-15 07:49:00 CDT"
So, it works only after converting the factor variable to character, but
it is a simple matter to do that conversion.
Thanks
Steve
On 3/29/2012 3:24 PM, Prof Brian Ripley wrote:
On 29/03/2012 21:05, Steven R Corsi wrote:
That is a good tip. I have tried it and found that it works if I make
sure it was converted to character first. strptime appears to accept
factor variables which is the default when reading in the file
originally.
I would say that POSIXct is the way to go if you know the timezone of
the datetimes (it is essential to do the conversion). But you don't
always do so, and then POSIXlt can be useful (although assuming UTC
can also work).
Several times a month we get help requests about times on DST
transitions which shows that people too often think they know the
timezone and in fact do not (as the times are invalid or ambiguous in
the assumed timezone).
as.POSIXct does work with factors ....
z<- "2012-03-29 21:20:05"
as.POSIXct(z)
[1] "2012-03-29 21:20:05 BST"
as.POSIXct(factor(z))
[1] "2012-03-29 21:20:05 BST"
Thanks
Steve
On 3/29/2012 2:31 PM, MacQueen, Don wrote:
I also find that POSIXct is generally the most useful, and only use
POSIXlt in special cases.
But have you considered as.POSIXct() instead of strptime()? It works
for
me, and I can't remember the last time I had to use strptime() for
converting character to date/time. (But I mostly don't work with
multiple
time zones, except for converting to/from UTC.
-Don
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