R devel - Jul 2006 - diff, POSIXct, POSIXlt, POSIXt

Dear Listers,

I have encountered a strange problem using diff() and POSIXt:

dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
dts <- strptime(dts, "%d/%m/%Y")
class(dts)

[1] "POSIXt"  "POSIXlt"

diff(dts)

Time differences of  7862400,  7948800, 15811200,  7862400,  7948800, 
15724800,  7862400,  7948800,        0 secs

In this case the result is not the one expected: expressed in seconds 
and not in days, and the difference between the two last dates is not 0.

Now, if one use a vector of 9 dates only (whatever the date removed), 
things come well:

diff(dts[-1])

Time differences of  92, 183,  91,  92, 182,  91,  92, 182 days

Also if one contrains dts to POSIXct

dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
dts <- as.POSIXct(strptime(dts, "%d/%m/%Y"))
diff(dts)

Time differences of  91,  92, 183,  91,  92, 182,  91,  92, 182 days

Any rational in that?

Patrick

Try converting to POSIXct:
> str(dts)
'POSIXlt', format: chr [1:10] "2003-04-15"
"2003-07-15" "2003-10-15"
"2004-04-15" "2004-07-15" "2004-10-15"
"2005-04-15" ...
> dts
 [1] "2003-04-15" "2003-07-15" "2003-10-15"
"2004-04-15" "2004-07-15"
"2004-10-15" "2005-04-15" "2005-07-15"
 [9] "2005-10-15" "2006-04-15"
> dts <- as.POSIXct(dts)
> dts
 [1] "2003-04-15 EDT" "2003-07-15 EDT" "2003-10-15
EDT" "2004-04-15 EDT"
"2004-07-15 EDT" "2004-10-15 EDT"
 [7] "2005-04-15 EDT" "2005-07-15 EDT" "2005-10-15
EDT" "2006-04-15 EDT"
> diff(dts)
Time differences of  91,  92, 183,  91,  92, 182,  91,  92, 182
days
>


On 7/23/06, Patrick Giraudoux <patrick.giraudoux@univ-fcomte.fr>
wrote:
>
> Dear Listers,
>
> I have encountered a strange problem using diff() and POSIXt:
>
>
>
dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
> dts <- strptime(dts, "%d/%m/%Y")
> class(dts)
>
> [1] "POSIXt"  "POSIXlt"
>
> diff(dts)
>
> Time differences of  7862400,  7948800, 15811200,  7862400,  7948800,
> 15724800,  7862400,  7948800,        0 secs
>
> In this case the result is not the one expected: expressed in seconds
> and not in days, and the difference between the two last dates is not 0.
>
> Now, if one use a vector of 9 dates only (whatever the date removed),
> things come well:
>
> diff(dts[-1])
>
> Time differences of  92, 183,  91,  92, 182,  91,  92, 182 days
>
> Also if one contrains dts to POSIXct
>
>
>
dts<-c("15/4/2003","15/7/2003","15/10/2003","15/04/2004","15/07/2004","15/10/2004","15/4/2005","15/07/2005","15/10/2005","15/4/2006")
> dts <- as.POSIXct(strptime(dts, "%d/%m/%Y"))
> diff(dts)
>
> Time differences of  91,  92, 183,  91,  92, 182,  91,  92, 182 days
>
> Any rational in that?
>
> Patrick
>
> ______________________________________________
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

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