R devel - Feb 2008 - Arithmetic bug? (found when use POSIXct) (PR#10776)

Full_Name: Bo Zhou
Version: 2.6.1 (2007-11-26)
OS: Windows XP
Submission from: (NULL) (207.237.54.242)


Hi,

I found an arithmetic problem when I'm doing something with POSIXct
The code to reproduce it is as follows (This is the recommended way of finding
out time zone difference on R News 2004-1 Page 32 URL
http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pdf)

a=Sys.time()
b<-as.POSIXct(format(a,tz="GMT"))
a-b
unclass(a)
unclass(b)
unclass(a)-unclass(b)
as.numeric(a)
as.numeric(b)
as.numeric(a)-as.numeric(b)

The result on my machine
> a=Sys.time()
> b<-as.POSIXct(format(a,tz="GMT"))
> a-b
Time difference of -4.999969 hours
> unclass(a)
[1] 1203184447
> unclass(b)
[1] 1203202447
attr(,"tzone")
[1] ""
> unclass(a)-unclass(b)
[1] -17999.89
attr(,"tzone")
[1] ""
> as.numeric(a)
[1] 1203184447
> as.numeric(b)
[1] 1203202447
> as.numeric(a)-as.numeric(b)
[1] -17999.89
>

See FAQ 7.31 or explain what you believe to be the problem.

On Feb 16, 2008 1:05 PM,  <gmail at kubism.ku.dk>
wrote:
> Full_Name: Bo Zhou
> Version: 2.6.1 (2007-11-26)
> OS: Windows XP
> Submission from: (NULL) (207.237.54.242)
>
>
> Hi,
>
> I found an arithmetic problem when I'm doing something with POSIXct
> The code to reproduce it is as follows (This is the recommended way of
finding
> out time zone difference on R News 2004-1 Page 32 URL
> http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pdf)
>
> a=Sys.time()
> b<-as.POSIXct(format(a,tz="GMT"))
> a-b
> unclass(a)
> unclass(b)
> unclass(a)-unclass(b)
> as.numeric(a)
> as.numeric(b)
> as.numeric(a)-as.numeric(b)
>
> The result on my machine
>
> > a=Sys.time()
> > b<-as.POSIXct(format(a,tz="GMT"))
> > a-b
> Time difference of -4.999969 hours
> > unclass(a)
> [1] 1203184447
> > unclass(b)
> [1] 1203202447
> attr(,"tzone")
> [1] ""
> > unclass(a)-unclass(b)
> [1] -17999.89
> attr(,"tzone")
> [1] ""
> > as.numeric(a)
> [1] 1203184447
> > as.numeric(b)
> [1] 1203202447
> > as.numeric(a)-as.numeric(b)
> [1] -17999.89

Hi Gabo,

FAQ 7.31 does not apply to this. "numeric" in R is 64bit which is
large enough to handle this.

I
figured out the cause is that Sys.time() gives millisecond value while
b<-as.POSIXct(format(a,tz=3D"GMT")) cuts off that part.=20

Then of course a-b is not 5 hours!

Do it again with "options(digits=3D22)" and you will see what actually
happened there.

The recommended code from R News is not aware the truth that POSIXlt can
represent sub-second time.

Cheers,

B
_________________________________________________________________
Helping your favorite cause is as easy as instant messaging.=A0You IM, we give.

	[[alternative HTML version deleted]]

OK.  Good point.

Note that that article was written 4 years ago
when R was at version 1.9 and POSIXt did not fully support
subseconds.  At that time, POSIXct could represent subseconds
internally but it was not used as POSIXlt did not yet support it
and that and the associated digits.sec option did not come until
R version 2.3, released two years after the article was written.

Try setting the digits.secs option before you run the command
like this:
> options(digits.secs = 3)
> dp <- Sys.time()
> dp-as.POSIXct(format(dp,tz="GMT"))
Time difference of -5 hours

On Feb 17, 2008 3:50 PM,  <bozhou1981 at hotmail.com>
wrote:
>
> Hi Gabo,
>
> FAQ 7.31 does not apply to this. "numeric" in R is 64bit which is
large eno> ugh to handle this.
>
> I
> figured out the cause is that Sys.time() gives millisecond value while
> b<-as.POSIXct(format(a,tz=3D"GMT")) cuts off that part.=20
>
> Then of course a-b is not 5 hours!
>
> Do it again with "options(digits=3D22)" and you will see what
actually happ> ened there.
>
> The recommended code from R News is not aware the truth that POSIXlt can
re> present sub-second time.
>
> Cheers,
>
> B
> _________________________________________________________________
> Helping your favorite cause is as easy as instant messaging.=A0You IM, we
g> ive.
>
>        [[alternative HTML version deleted]]
>
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