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javier garcia - CEBAS

2004-Aug-17 14:25 UTC

[R] Fwd: strptime() problem?

Hi all;
I've already send a similar e-mail to the list and Prof. Brian Ripley 
answered me but my doubts remain unresolved. Thanks for the clarification, 
but perhaps I wasn't clear enough in posting my questions.

I've got a postgres database which I read into R. The first column is
Timestamp with timezone, and my data are already in UTC format. An
'printed'
extract of R character column, resulting from the timestamptz field is:

raincida$ts:

 [2039] "25/03/2000 22:00:00 UTC" "25/03/2000 23:00:00 UTC"
 [2041] "26/03/2000 00:00:00 UTC" "26/03/2000 01:00:00 UTC"
 [2043] "26/03/2000 02:00:00 UTC" "26/03/2000 03:00:00 UTC"
 [2045] "26/03/2000 04:00:00 UTC" "26/03/2000 05:00:00 UTC"

#And I need to convert this character column into POSIXct, for eventual work. 
#As I can see in the documentation, the process is to use strptime(), what 
#creates an object POSIXlt and doesn't allow to specify that the time zone
of
#the data is already UTC; followed by as.POSIXct()
> lluvia.strptime <- strptime(raincida$ts, format="%d/%m/%Y
%H:%M:%S")
> lluvia.strptime.POSIXct <-
as.POSIXct(lluvia.strptime,tz="GMT")
A "printed" extract is:

 [2039] "2000-03-25 22:00:00 GMT" "2000-03-25 23:00:00 GMT"
 [2041] "2000-03-26 00:00:00 GMT" "2000-03-26 01:00:00 GMT"
 [2043] "2000-03-26 03:00:00 GMT" "2000-03-26 03:00:00 GMT"
 [2045] "2000-03-26 04:00:00 GMT" "2000-03-26 05:00:00 GMT"

As we can see, elements [2043] differ. Shouldn't they be similar as the rest
of the other shown elements? I thought this was a bug, but it seems that
I've
got and conceptual error.(?). This happens several times in my data, and 
produces eventual errors.

Please, how could I resolved this?

Thanks all, and best regards,

Javier G.

Gabor Grothendieck

2004-Aug-17 15:57 UTC

head link

[R] Fwd: strptime() problem?

I am in a different time zone, EDT, on Windows XP and can't
replicate this but you might try reading the latest R News
article on dates and times for some ideas, viz. page 32 of:

   http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pd

In particular, try converting them to chron and then doing 
your manipulations in chron or else convert them from chron to 
POSIXct:

   require(chron)
   r.asc <- raincida$ts
   r.chron <- chron(substring(r.asc, 1, 10), 
             substring(r.asc, 12, 19), format = c("d/m/y",
"h:m:s"))

   r.ct <- as.POSIXct(r.chron)
   format(r.ct, tz="GMT")   # display POSIXct in GMT

Date:   	Tue, 17 Aug 2004 16:25:12 +0200
From:   	javier garcia - CEBAS <rn001 at cebas.csic.es>
To:   	Prof Brian Ripley <ripley at stats.ox.ac.uk>, <r-help at
stat.math.ethz.ch>
Subject:   	[R] Fwd: strptime() problem?

Hi all;
I've already send a similar e-mail to the list and Prof. Brian Ripley
answered me but my doubts remain unresolved. Thanks for the clarification,
but perhaps I wasn't clear enough in posting my questions.

I've got a postgres database which I read into R. The first column is
Timestamp with timezone, and my data are already in UTC format. An
'printed'
extract of R character column, resulting from the timestamptz field is:

raincida$ts:

[2039] "25/03/2000 22:00:00 UTC" "25/03/2000 23:00:00 UTC"
[2041] "26/03/2000 00:00:00 UTC" "26/03/2000 01:00:00 UTC"
[2043] "26/03/2000 02:00:00 UTC" "26/03/2000 03:00:00 UTC"
[2045] "26/03/2000 04:00:00 UTC" "26/03/2000 05:00:00 UTC"

#And I need to convert this character column into POSIXct, for eventual work.
#As I can see in the documentation, the process is to use strptime(), what
#creates an object POSIXlt and doesn't allow to specify that the time zone
of
#the data is already UTC; followed by as.POSIXct()
> lluvia.strptime <- strptime(raincida$ts, format="%d/%m/%Y
%H:%M:%S")
> lluvia.strptime.POSIXct <-
as.POSIXct(lluvia.strptime,tz="GMT")
A "printed" extract is:

[2039] "2000-03-25 22:00:00 GMT" "2000-03-25 23:00:00 GMT"
[2041] "2000-03-26 00:00:00 GMT" "2000-03-26 01:00:00 GMT"
[2043] "2000-03-26 03:00:00 GMT" "2000-03-26 03:00:00 GMT"
[2045] "2000-03-26 04:00:00 GMT" "2000-03-26 05:00:00 GMT"

As we can see, elements [2043] differ. Shouldn't they be similar as the rest
of the other shown elements? I thought this was a bug, but it seems that
I've
got and conceptual error.(?). This happens several times in my data, and
produces eventual errors.

Please, how could I resolved this?

Thanks all, and best regards,

Javier G.

Gabor Grothendieck

2004-Aug-17 15:59 UTC

head link

[R] Fwd: strptime() problem?

javier garcia - CEBAS <rn001 <at> cebas.csic.es> writes:

: 
: Hi all;
: I've already send a similar e-mail to the list and Prof. Brian Ripley 
: answered me but my doubts remain unresolved. Thanks for the clarification, 
: but perhaps I wasn't clear enough in posting my questions.
: 
: I've got a postgres database which I read into R. The first column is
: Timestamp with timezone, and my data are already in UTC format. An
'printed'
: extract of R character column, resulting from the timestamptz field is:
: 
: raincida$ts:
: 
:  [2039] "25/03/2000 22:00:00 UTC" "25/03/2000 23:00:00
UTC"
:  [2041] "26/03/2000 00:00:00 UTC" "26/03/2000 01:00:00
UTC"
:  [2043] "26/03/2000 02:00:00 UTC" "26/03/2000 03:00:00
UTC"
:  [2045] "26/03/2000 04:00:00 UTC" "26/03/2000 05:00:00
UTC"
: 
: #And I need to convert this character column into POSIXct, for eventual 
work. 
: #As I can see in the documentation, the process is to use strptime(), what 
: #creates an object POSIXlt and doesn't allow to specify that the time zone
of 
: #the data is already UTC; followed by as.POSIXct()
: 
: > lluvia.strptime <- strptime(raincida$ts, format="%d/%m/%Y
%H:%M:%S")
: > lluvia.strptime.POSIXct <-
as.POSIXct(lluvia.strptime,tz="GMT")
: 
: A "printed" extract is:
: 
:  [2039] "2000-03-25 22:00:00 GMT" "2000-03-25 23:00:00
GMT"
:  [2041] "2000-03-26 00:00:00 GMT" "2000-03-26 01:00:00
GMT"
:  [2043] "2000-03-26 03:00:00 GMT" "2000-03-26 03:00:00
GMT"
:  [2045] "2000-03-26 04:00:00 GMT" "2000-03-26 05:00:00
GMT"
: 
: As we can see, elements [2043] differ. Shouldn't they be similar as the
rest
: of the other shown elements? I thought this was a bug, but it seems that 
I've 
: got and conceptual error.(?). This happens several times in my data, and 
: produces eventual errors.
: 
: Please, how could I resolved this?

[Sorry if this gets posted twice.  I had a problem posting and
not sure if the first one ever got sent.]

I am in a different time zone, EDT, on Windows XP and can't
replicate this but you might try reading the latest R News
article on dates and times for some ideas, viz. page 32 of:

   http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pd

In particular, try converting the datetimes to chron and then doing 
your manipulations in chron or else converting them from chron to 
POSIXct rather than going through POSIXlt:

   require(chron)
   r.asc <- raincida$ts
   r.chron <- chron(substring(r.asc, 1, 10), 
             substring(r.asc, 12, 19), format = c("d/m/y",
"h:m:s"))

   r.ct <- as.POSIXct(r.chron)
   format(r.ct, tz="GMT") # display in GMT

Whit Armstrong

2004-Aug-17 16:14 UTC

head link

[R] Fwd: strptime() problem?

Javier,

I recently had a problem with dates.  This example might shed some light on
your problem.
> x <- ISOdate(rep(2000,2),rep(3,2),rep(26,2),hour=0)
> x
[1] "2000-03-26 GMT" "2000-03-26 GMT"
> unclass(x)
[1] 954028800 954028800
attr(,"tzone")
[1] "GMT"

When one creates a date with ISOdate, the resulting object is of class
POSIXct and is given the attribute "tzone" which is set to
"GMT."

When one prints an object of class POSIXct the function
"print.POSIXct" is
called:> print.POSIXctfunction (x, ...) 
{
    print(format(x, usetz = TRUE, ...), ...)
    invisible(x)
}
<environment: namespace:base>> 
So, that function is just calling "format" which gets dispatched to
"format.POSIXct":
> format.POSIXctfunction (x, format = "", tz = "", usetz = FALSE, ...) 
{
    if (!inherits(x, "POSIXct")) 
        stop("wrong class")
    if (missing(tz) && !is.null(tzone <- attr(x, "tzone")))
        tz <- tzone
    structure(format.POSIXlt(as.POSIXlt(x, tz), format, usetz, 
        ...), names = names(x))
}
<environment: namespace:base>> 
Now, if one looks carefully at this code, you will see that it tests for the
attribute "tzone" on the object that is passed in.  If it finds that
attribute, then it is passed on to "format.POSIXlt" (which is the
function
that ultimately does the printing).  If there is no "tzone" attribute,
then
"" is passed to "format.POSIXlt" as the tzone, which causes
the object to be
printed in your locale specific format.

See:
> attr(x,"tzone") <- ""
> x[1] "2000-03-25 19:00:00 Eastern Standard Time" "2000-03-25
19:00:00 Eastern
Standard Time"> attr(x,"tzone") <- "GMT"
> x[1] "2000-03-26 GMT" "2000-03-26
GMT"> 
Now this is the part that really got me confused:
> x[1] "2000-03-26 GMT" "2000-03-26
GMT"> x[1][1] "2000-03-25 19:00:00 Eastern Standard
Time"> 
What happens in the above case is that the code for "[.POSIXct" looks
like
this:
> get("[.POSIXct")function (x, ..., drop = TRUE) 
{
    cl <- oldClass(x)
    class(x) <- NULL
    val <- NextMethod("[")
    class(val) <- cl
    val
}
<environment: namespace:base>> 
The attribute "tzone" is not preserved!!  when "val" is
created from the
call to NextMethod, its class is restored, but not its "tzone"
attribute.
So any dates of class POSIXct that are printed after they have been
subscripted ("[") will have their "tzone" attribute
stripped, and will print
in the local specific format.

For your specific case, I would convert all my dates to POSIXct, then set
the attribute "tzone" to "GMT."  After that, be very careful
when
subscripting them, or you will find them printing in local specific formats
again.

for you:> y <- strptime("4/3/2000",format="%m/%d/%Y")
> y
[1] "2000-04-03"> y <- as.POSIXct(y,"GMT")
> y
[1] "2000-04-03 GMT"> unclass(y)[1] 954720000
attr(,"tzone")
[1] "GMT"> 
I think that should straighten out your problem.

Hope that helps,
Whit

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