[Rd] diff(time) vs. difftime?

From: Spencer Graves <spencer.graves_at_structuremonitoring.com>
Date: Mon, 19 Mar 2012 16:07:59 -0700

          I've just uncovered an infelicity in as.difftime:

 > as.difftime(diff(as.POSIXct(c('2012-12-12', '2012-12-13'))), units='hours')
Time difference of 1 days

          Is this a bug in the code or in my understanding?

          Thanks again for all your hard work in making R the great product it is and in answering so many questions.

	  Best Wishes,

 > sessionInfo()
R version 2.14.1 (2011-12-22)
Platform: i386-pc-mingw32/i386 (32-bit)

[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats graphics grDevices utils datasets methods base

other attached packages:
[1] zoo_1.7-7 RODBC_1.3-4 WriteXLS_2.1.0 sos_1.3-1

loaded via a namespace (and not attached):
[1] grid_2.14.1 lattice_0.20-0 tools_2.14.1


       I just encountered another RTFM problem:  With 
diff(as.POSIXct(...), ...) I was unable to control the units of the results. Examples:

> (d.d <- diff(as.POSIXct(c('2012-12-12', '2012-12-13')))) Time difference of 1 days
> (d.h <- diff(as.POSIXct(c('2012-12-12 08:00', '2012-12-12 09:00')))) Time difference of 1 hours
> (d.m <- diff(as.POSIXct(c('2012-12-12 08:00', '2012-12-12 08:01')))) Time difference of 1 mins
> (d.s <- diff(as.POSIXct(c('2012-12-12 08:00:00', '2012-12-12 08:00:01'))))
Time difference of 1 secs
> as.numeric(d.d)
[1] 1

> as.numeric(d.s)
[1] 1

       methods('diff') identified the following:

[1] diff.Date diff.default diff.POSIXt diff.ts diff.zoo*

       In looking at the help pages for each of these functions, I found no mention of "difftime" or any other way to control the units.

[1] -.Date -.POSIXt -.yearmon* -.yearqtr*

        ?"-.Date" didn't seem helpful on this, either. ?"-.POSIXt" contained a link to "difftime", but I didn't see that until after I read the code for "-.POSIXt" and found "difftime".

       I humbly beseech ye to consider adding a difftime example to all these help pages.


p.s. In case there is any doubt, I very much appreciate all the work that the R Core team has invested in making R what it is today.

Spencer Graves, PE, PhD
President and Chief Technology Officer
Structure Inspection and Monitoring, Inc.
751 Emerson Ct.
San Josť, CA 95126
ph:  408-655-4567
web:  www.structuremonitoring.com

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Received on Mon 19 Mar 2012 - 23:10:19 GMT

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