I think the problem is that difftime objects have a "units" attribute so when converting to plain numbers you need to supply a unit with respect to which it is to be converted:
d <- Sys.Date() + 1:10 dd <- diff(d) sum(as.double(dd, units = "days")) On Sun, Oct 12, 2008 at 7:46 PM, David Winsemius <[EMAIL PROTECTED]> wrote: > Two follow-up questions: > > A) I get an error message when using Harrell's describe() function on one of > my variable, telling me that sum() is not meaningful for a difftime object. > Why should sum() not be meaningful for a collection of interval lengths? > >> describe(pref900) > Error in Summary.difftime(c(1075, 3429, 2915, 2002, 967, 1759, 532, 589, : > 'sum' not defined for "difftime" objects > > summary() is informative and throws no error, but does not report means. > Even with na.rm=TRUE, sum fails: >> sum(pref900$deatht, na.rm=TRUE) > Error in Summary.difftime(c(NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, : > 'sum' not defined for "difftime" objects > > My interest in the sum of difftime objects comes from my interest in > calculating the number of person-years of observation in various categories. > I have durations created by subtracting times. > > > B) The help pages are not particularly expansive regarding the output of > deltat() but your answer suggests that it should work on non-time objects as > well? Am I correct in assuming you meant that diff(x)/deltat(x) should be > meaningful for any numeric x. > > -- > David Winsemius > R 2.7.1 / Mac OS 10.5.4 / Intel CPUs > > s> > On Oct 12, 2008, at 10:34 AM, Gabor Grothendieck wrote: > >> ?deltat >> >> On Sun, Oct 12, 2008 at 9:45 AM, Oliver Bandel >> <[EMAIL PROTECTED]> wrote: >>> >>> Zitat von Gabor Grothendieck <[EMAIL PROTECTED]>: >>> >>>> If you simply want successive differences use diff: >>>> >>>> x <- seq(4)^2 >>>> diff(x) >>>> >>>> tx <- ts(x) >>>> diff(tx) >>> >>> [...] >>> >>> Oh, cool, thanks. >>> >>> But what about diff / delta_t ? >>> >>> Do I have to calculate it by my own, or is there >>> already a function for making a difference-qoutient? >>> >>> This would be fine to have, because for example >>> coming from space vs. time to velocity vs. time >>> and acceleration vs. time (and further derivatives) >>> are also a time-series. >>> >>> The possibility of using the advantages of the time series class here, >>> would be fine. >>> >>> >>> Ciao, >>> Oliver >>> >> >> ______________________________________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide >> http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
