On Fri, 1 Sep 2017, Andrea Altomani wrote:> I should have formulated my question in a more specific way. > > 1. I suspect this is a floating point precision issue. I am not very > knowledgeable about R internals, can someone else confirm it?Yes. If you represent a series with increment 1/12 it depends on how you do it. As a simple example consider the following two descriptions of the same time point: 2 - 1/12 ## [1] 1.916667 1 + 11/12 ## [1] 1.916667 However, both are not identical: (2 - 1/12) == (1 + 11/12) ## [1] FALSE The difference is just the .Machine$double.eps: (2 - 1/12) - (1 + 11/12) ## [1] 2.220446e-16> 2. Should this be considered a bug or not, because it is "just a > precision issue"? Should I report it?I don't think it is a bug because of the (non-standard) way how you created the time series.> 3. How can it happen? From a quick review of ts.R, it looks like the values > of the time index are never modified, but only possibly removed. In my case: > - x and y have the same index. > - the subtraction operator recognizes this, and create a new ts with one > entry > - the result of the subtraction has an index which is different from the > input. > This is very surprising to me, and I am curious to understand the problem.The object 'x' and hence the object 'y' have the same time index. But in 'z' a new time index is created which is subtly different from that of 'x'. The reason for this is that R doesn't expect an object like 'x' to exist. You should create a "ts" object with ts(), e.g., x <- ts(2017, start = c(2017, 6), freqency = 12) But you created something close to the internal representation...but not close enough: y <- structure(2017, .Tsp = c(2017.416667, 2017.416667, 12), class = "ts") The print functions prints both print(x) and print(y) as Jun 2017 2017 However, aligning the two time indexes in x - y or ts.intersect(x, y) does not work...because they are not the same as.numeric(time(x)) - as.numeric(time(y)) ## [1] -3.333332e-07 The "ts" code tries to avoid these situations by making many time index comparisons only up to a precision of getOption("ts.eps") (1e-5 by default) but this is not used everywhere. See ?options: 'ts.eps': the relative tolerance for certain time series ('ts') computations. Default '1e-05'. Of course, you could ask for this being used in more places, e.g., in stats:::.cbind.ts() where (st > en) is used rather than ((st - en) > getOption("ts.eps")). But it's probably safer to just use ts() rather than structure(). Or if you use the latter make sure that you do at a high enough precision. hth, Z> On Fri, Sep 1, 2017 at 5:53 PM Jeff Newmiller <jdnewmil at dcn.davis.ca.us> > wrote: > >> You already know the answer. Why ask? >> -- >> Sent from my phone. Please excuse my brevity. >> >> On September 1, 2017 7:23:24 AM PDT, Andrea Altomani < >> altomani.andrea at gmail.com> wrote: >>> I have a time series x, and two other series obtained from it: >>> >>> x <- structure(2017, .Tsp = c(2017.41666666667, 2017.41666666667, 12), >>> class = "ts") >>> y <- floor(x) >>> z <- x-y >>> >>> I would expect the three series to have exactly the same index. >>> However I get the following >>> >>>> time(x)-time(y) >>> Jun >>> 2017 0 >>> >>> as expected, but >>> >>>> time(x)-time(z) >>> integer(0) >>> Warning message: >>> In .cbind.ts(list(e1, e2), c(deparse(substitute(e1))[1L], >>> deparse(substitute(e2))[1L]), : >>> non-intersecting series >>> >>> and indeed, comparing the indices gives: >>> >>>> time(x)[1]-time(z)[1] >>> [1] 3.183231e-12 >>> >>> Is this a bug in R, or is it one of the expected precision errors due >>> to the use of limited precision floats? >>> >>> I am using R 3.4.0 (2017-04-21) on Windows (64-bit). >>> >>> Thaks! >>> >>> Andrea Altomani >>> >>> ______________________________________________ >>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see >>> 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. >> > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > 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. >

Thanks for the very detailed explanation. I did not create the series using structure(), that was the result of dump() on an intermediate object created within tsdisagg::ta(), which is where I found the error in the first place. ta() indeed manipulates .Tsp directly, rather than using ts. I guess this is a bug in tsdisagg then. Thanks! -- Andrea Altomani On Sat, Sep 2, 2017 at 12:31 AM Achim Zeileis <Achim.Zeileis at uibk.ac.at> wrote:> On Fri, 1 Sep 2017, Andrea Altomani wrote: > > > I should have formulated my question in a more specific way. > > > > 1. I suspect this is a floating point precision issue. I am not very > > knowledgeable about R internals, can someone else confirm it? > > Yes. If you represent a series with increment 1/12 it depends on how you > do it. As a simple example consider the following two descriptions of the > same time point: > > 2 - 1/12 > ## [1] 1.916667 > > 1 + 11/12 > ## [1] 1.916667 > > However, both are not identical: > > (2 - 1/12) == (1 + 11/12) > ## [1] FALSE > > The difference is just the .Machine$double.eps: > > (2 - 1/12) - (1 + 11/12) > ## [1] 2.220446e-16 > > > 2. Should this be considered a bug or not, because it is "just a > > precision issue"? Should I report it? > > I don't think it is a bug because of the (non-standard) way how you > created the time series. > > > 3. How can it happen? From a quick review of ts.R, it looks like the > values > > of the time index are never modified, but only possibly removed. In my > case: > > - x and y have the same index. > > - the subtraction operator recognizes this, and create a new ts with > one > > entry > > - the result of the subtraction has an index which is different from > the > > input. > > This is very surprising to me, and I am curious to understand the > problem. > > The object 'x' and hence the object 'y' have the same time index. But in > 'z' a new time index is created which is subtly different from that of > 'x'. The reason for this is that R doesn't expect an object like 'x' to > exist. > > You should create a "ts" object with ts(), e.g., > > x <- ts(2017, start = c(2017, 6), freqency = 12) > > But you created something close to the internal representation...but not > close enough: > > y <- structure(2017, .Tsp = c(2017.416667, 2017.416667, 12), class = "ts") > > The print functions prints both print(x) and print(y) as > > Jun > 2017 2017 > > However, aligning the two time indexes in x - y or ts.intersect(x, y) does > not work...because they are not the same > > as.numeric(time(x)) - as.numeric(time(y)) > ## [1] -3.333332e-07 > > The "ts" code tries to avoid these situations by making many time index > comparisons only up to a precision of getOption("ts.eps") (1e-5 by > default) but this is not used everywhere. See ?options: > > 'ts.eps': the relative tolerance for certain time series ('ts') > computations. Default '1e-05'. > > Of course, you could ask for this being used in more places, e.g., in > stats:::.cbind.ts() where (st > en) is used rather than ((st - en) > > getOption("ts.eps")). But it's probably safer to just use ts() rather than > structure(). Or if you use the latter make sure that you do at a high > enough precision. > > hth, > Z > > > > On Fri, Sep 1, 2017 at 5:53 PM Jeff Newmiller <jdnewmil at dcn.davis.ca.us> > > wrote: > > > >> You already know the answer. Why ask? > >> -- > >> Sent from my phone. Please excuse my brevity. > >> > >> On September 1, 2017 7:23:24 AM PDT, Andrea Altomani < > >> altomani.andrea at gmail.com> wrote: > >>> I have a time series x, and two other series obtained from it: > >>> > >>> x <- structure(2017, .Tsp = c(2017.41666666667, 2017.41666666667, 12), > >>> class = "ts") > >>> y <- floor(x) > >>> z <- x-y > >>> > >>> I would expect the three series to have exactly the same index. > >>> However I get the following > >>> > >>>> time(x)-time(y) > >>> Jun > >>> 2017 0 > >>> > >>> as expected, but > >>> > >>>> time(x)-time(z) > >>> integer(0) > >>> Warning message: > >>> In .cbind.ts(list(e1, e2), c(deparse(substitute(e1))[1L], > >>> deparse(substitute(e2))[1L]), : > >>> non-intersecting series > >>> > >>> and indeed, comparing the indices gives: > >>> > >>>> time(x)[1]-time(z)[1] > >>> [1] 3.183231e-12 > >>> > >>> Is this a bug in R, or is it one of the expected precision errors due > >>> to the use of limited precision floats? > >>> > >>> I am using R 3.4.0 (2017-04-21) on Windows (64-bit). > >>> > >>> Thaks! > >>> > >>> Andrea Altomani > >>> > >>> ______________________________________________ > >>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > >>> 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. > >> > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > > 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. > > >[[alternative HTML version deleted]]