[R] numeric derivation

[Gabor Grothendieck]

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 <dwinsemius at comcast.net> 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
>> <oliver at first.in-berlin.de> wrote:
>>>
>>> Zitat von Gabor Grothendieck <ggrothendieck at gmail.com>:
>>>
>>>> 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 at 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.
>
>