As one of the approximately 30 names on the 1985 IEEE 754 standard, I should be first to comment about representations. However, a quite large fraction of the computers I've owned or used were decimal beasts. This doesn't remove all the issues, of course, but some of these input-output conversions would be avoided. I rather doubt we'll ever see an R version for decimal arithmetic, since there'd be a lot of awkwardness with comparing with the usual version, and a lot of the test comparisons would likely fail. On the other hand, it might serve as a reminder that IEEE arithmetic, while a great step in computation, is not the only possibility and does not eliminate all the difficulties with finite precision arithmetic.
Best, JN On 2019-01-17 11:52 a.m., peter dalgaard wrote: > > >> On 17 Jan 2019, at 15:56 , POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS >> FOUNDATION TRUST) via R-help <r-help@r-project.org> wrote: >> >> Well I get the issue with finite precision. As in SQRT(2) * SQRT(2) is not 2. > > As Jeff indicates, you also need to get that just like 3rds and 7ths cannot > be represented exactly in base 10, 5ths and 10ths cannot be represented > exactly in base 2 (only powers of 1/2 and their multiples can). > > Specifically, 1.4 decimal is 1.0110011001100.... binary > > -pd > >> >> What surprised me was that seq(1.4, 2.1, by=0.001) starts at >> 1.3999999999999999 and not 1.4! >> >> >> -----Original Message----- >> From: PIKAL Petr [mailto:petr.pi...@precheza.cz] >> Sent: 17 January 2019 14:30 >> To: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION TRUST); Ben >> Tupper >> Cc: r-help@r-project.org >> Subject: RE: [R] I can't get seq to behave how I think it should >> >> Hi >> >> It is not seq problem, it is floating point numbers representation in finit >> precision problem. Ben pointed to it and you could learn about it from FAQ >> 7.31. >> >> Cheers >> Petr >> >>> -----Original Message----- >>> From: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS FOUNDATION >>> TRUST) <calum.polw...@nhs.net> >>> Sent: Thursday, January 17, 2019 2:56 PM >>> To: PIKAL Petr <petr.pi...@precheza.cz>; Ben Tupper >>> <btup...@bigelow.org> >>> Cc: r-help@r-project.org >>> Subject: RE: [R] I can't get seq to behave how I think it should >>> >>> Thanks guys. >>> >>> I've used Petr's method and its working for me. >>> >>> If the data had been from a calculation I'd have rounded it... just >>> didn't expect seq to break it! >>> >>> C >>> >>> -----Original Message----- >>> From: PIKAL Petr [mailto:petr.pi...@precheza.cz] >>> Sent: 17 January 2019 13:53 >>> To: Ben Tupper; POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS >>> FOUNDATION TRUST) >>> Cc: r-help@r-project.org >>> Subject: RE: [R] I can't get seq to behave how I think it should >>> >>> Hi >>> >>> Or you could use rounding. >>> which(round(lut, 3)==1.8) >>> [1] 401 >>> >>> Cheers >>> Petr >>> >>>> -----Original Message----- >>>> From: R-help <r-help-boun...@r-project.org> On Behalf Of Ben Tupper >>>> Sent: Thursday, January 17, 2019 2:43 PM >>>> To: POLWART, Calum (COUNTY DURHAM AND DARLINGTON NHS >>> FOUNDATION TRUST) >>>> <calum.polw...@nhs.net> >>>> Cc: r-help@r-project.org >>>> Subject: Re: [R] I can't get seq to behave how I think it should >>>> >>>> Hi, >>>> >>>> This looks like a floating point reality bump - see >>>> >>>> https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-thin >>>> k- these-numbers-are-equal_003f >>>> <https://cran.r-project.org/doc/FAQ/R- >>>> FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f> >>>> >>>> You can use other methods to finding your row - I would opt for >>>> findInterval() >>>> >>>>> lut = seq(1.4, 2.1, by=0.001) >>>>> findInterval(1.8, lut) >>>> [1] 401 >>>> >>>> findInterval() uses a rapid search to find the index in the look up >>>> table (lut) that is just less than or equal to the search value (in >>>> your example >>> 1.8). >>>> >>>> Cheers, >>>> Ben >>>> >>>>> On Jan 17, 2019, at 8:33 AM, POLWART, Calum (COUNTY DURHAM AND >>>> DARLINGTON NHS FOUNDATION TRUST) via R-help <r-help@r-project.org> >>>> wrote: >>>>> >>>>> I am using seq with the expression seq(1.4, 2.1, by=0.001) to >>>>> create a sequence of references from 1.4 to 2.1 in 0.001 >>>>> increments. They appear to be created correctly. They have a >>>>> related pair of data which for the purposes of this we will call >>>>> val. I'm interested in the content on the row with seq = 1.8. But >>>>> I can't seem to get it returned. I can get other values but not >>>>> 1.8! yet looking at row >>>>> 401 there is nothing to indicate an issue >>>>> >>>>>> a = 1.4 >>>>>> b = 2.1 >>>>>> seq = seq(a, b, by=0.001) >>>>>> val = ceiling(seq * 50) >>>>>> s=data.frame(seq, val) >>>>>> s$val[seq==1.799] >>>>> [1] 90 >>>>>> s$val[s$seq==1.8] >>>>> numeric(0) >>>>>> s$val[seq==1.8] >>>>> numeric(0) >>>>>> s$val[s$seq==1.800] >>>>> numeric(0) >>>>>> s$val[s$seq==1.801] >>>>> [1] 91 >>>>>> head(s[s$seq>1.798,]) >>>>> seq val >>>>> 400 1.799 90 >>>>> 401 1.800 90 >>>>> 402 1.801 91 >>>>> 403 1.802 91 >>>>> 404 1.803 91 >>>>> 405 1.804 91 >>>>> >>>>> >>>>> Can anyone explain what's going on here and how I would correctly >>>>> find the >>>> content of row 401 by using an expression to equal the seq column? >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>> >>> ******************************************************************* >>>> *** >>>>> ********************************************** >>>>> >>>>> This message may contain confidential information. If >>>>> ...{{dropped:25}} >>>> >>>> ______________________________________________ >>>> R-help@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. >>> Osobní údaje: Informace o zpracování a ochraně osobních údajů >>> obchodních partnerů PRECHEZA a.s. jsou zveřejněny na: >>> https://www.precheza.cz/zasady- ochrany-osobnich-udaju/ | Information >>> about processing and protection of business partner’s personal data are >>> available on website: >>> https://www.precheza.cz/en/personal-data-protection-principles/ >>> Důvěrnost: Tento e-mail a jakékoliv k němu připojené dokumenty jsou >>> důvěrné a podléhají tomuto právně závaznému prohláąení o vyloučení >>> odpovědnosti: >>> https://www.precheza.cz/01-dovetek/ | This email and any documents >>> attached to it may be confidential and are subject to the legally >>> binding >>> disclaimer: https://www.precheza.cz/en/01-disclaimer/ >>> >>> >>> >>> ******************************************************************* >>> ************************************************* >>> >>> This message may contain confidential information. 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NHSmail is approved for exchanging >>> patient data and other sensitive information with NHSmail and other >>> accredited email services. >>> >>> For more information and to find out how you can switch, >>> https://portal.nhs.net/help/joiningnhsmail >> >> Osobní údaje: Informace o zpracování a ochraně osobních údajů obchodních >> partnerů PRECHEZA a.s. jsou zveřejněny na: >> https://www.precheza.cz/zasady-ochrany-osobnich-udaju/ | Information about >> processing and protection of business partner’s personal data are available >> on website: https://www.precheza.cz/en/personal-data-protection-principles/ >> Důvěrnost: Tento e-mail a jakékoliv k němu připojené dokumenty jsou důvěrné >> a podléhají tomuto právně závaznému prohláąení o vyloučení odpovědnosti: >> https://www.precheza.cz/01-dovetek/ | This email and any documents attached >> to it may be confidential and are subject to the legally binding disclaimer: >> https://www.precheza.cz/en/01-disclaimer/ >> >> >> >> ******************************************************************************************************************** >> >> This message may contain confidential information. If you are not the >> intended recipient please inform the >> sender that you have received the message in error before deleting it. >> Please do not disclose, copy or distribute information in this e-mail or >> take any action in relation to its contents. To do so is strictly prohibited >> and may be unlawful. Thank you for your co-operation. >> >> NHSmail is the secure email and directory service available for all NHS >> staff in England and Scotland. NHSmail is approved for exchanging patient >> data and other sensitive information with NHSmail and other accredited email >> services. >> >> For more information and to find out how you can switch, >> https://portal.nhs.net/help/joiningnhsmail >> >> ______________________________________________ >> R-help@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. > ______________________________________________ R-help@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.
