Hello, An alternative to round():
isTRUE(all.equal((2.3-1.3),1)) Regards, Gustavo. On Wed, Dec 10, 2008 at 3:12 PM, Stephan Kolassa <[EMAIL PROTECTED]> wrote: > Hi Emma, > > unfortunately, rounding variables before taking the difference will not > solve your problem, because the *rounded* variables are subject to the same > (effectively) random internal representation. Examples: > >> round(8.3,20)-round(7.3,20)>=1 > [1] TRUE >> round(2.3,20)-round(1.3,20)>=1 > [1] FALSE >> round(8.3,1)-round(7.3,1)>=1 > [1] TRUE >> round(2.3,1)-round(1.3,1)>=1 > [1] FALSE > > I'm afraid you will need to think about what you really need your comparison > for and whether this problem really affects your results. For example, if > you are doing a large number of such comparisons, the effect may only affect > a few of them and basically average out (sometimes going one way, sometimes > the other), so the end results may be stable. > > Good luck! > Stephan > > > emma jane schrieb: >> >> Thanks Greg, that does make sense. And I've solved the problem by >> rounding the variables before taking the difference between them. >> >> Thanks to all who replied. >> >> Emma Jane >> >> >> >> ________________________________ >> From: Greg Snow <[EMAIL PROTECTED]> >> >> .com.br>; Wacek Kusnierczyk <[EMAIL PROTECTED]>; Chuck >> Cleland <[EMAIL PROTECTED]> >> Cc: R help <[EMAIL PROTECTED]> >> Sent: Tuesday, 9 December, 2008 16:30:08 >> Subject: RE: [R] Logical inconsistency >> >> Some (possibly all) of those numbers cannot be represented exactly, so >> there is a chance of round off error whenever you do some arithmetic, >> sometimes the errors cancel out, sometimes they don't. Consider: >> >>> print(8.3-7.3, digits=20) >> >> [1] 1.000000000000001 >>> >>> print(11.3-10.3, digits=20) >> >> [1] 1 >> >> So in the first case the rounding error gives a value that is slightly >> greater than 1, so the greater than test returns true (if you round the >> result before comparing to 1, then it will return false). In the second >> case the uncertainties cancelled out so that you get exactly 1 which is not >> greater than 1 an so the comparison returns false. >> >> Hope this helps, >> >> -- >> Gregory (Greg) L. Snow Ph.D. >> Statistical Data Center >> Intermountain Healthcare >> [EMAIL PROTECTED] >> 801.408.8111 >> >> >>> -----Original Message----- >>> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] >>> project.org] On Behalf Of emma jane >>> Sent: Tuesday, December 09, 2008 7:02 AM >>> To: Bernardo Rangel Tura; Wacek Kusnierczyk; Chuck Cleland >>> Cc: R help >>> Subject: Re: [R] Logical inconsistency >>> >>> Many thanks for your help, perhaps I should have set my query in >>> context .... ! >>> >>> I'm simply calculating an indicator variable [0,1] based on the whether >>> the difference between two measured variables is > 1 or <=1. >>> >>> I understand the FAQ about floating point arithmetic, but am still >>> puzzled that it only apparently applies to certain elements, as >>> follows: >>> >>> 8.8 - 7.8 > 1 >>>> >>>> TRUE >>> >>> 8.3 - 7.3 > 1 >>>> >>>> TRUE >>> >>> However, >>> >>> 10.2 - 9.2 > 1 >>>> >>>> FALSE >>> >>> 11.3 - 10.3>1 >>>> >>>>  FALSE >>> >>> Emma Jane >>> >>> >>> >>> >>> ________________________________ >>> From: Bernardo Rangel Tura <[EMAIL PROTECTED]> >>> To: Wacek Kusnierczyk <[EMAIL PROTECTED]> >>> Cc: R help <[EMAIL PROTECTED]> >>> Sent: Saturday, 6 December, 2008 10:00:48 >>> Subject: Re: [R] Logical inconsistency >>> >>> On Fri, 2008-12-05 at 14:18 +0100, Wacek Kusnierczyk wrote: >>>> >>>> Berwin A Turlach wrote: >>>>> >>>>> Dear Emma, >>>>> >>>>> On Fri, 5 Dec 2008 04:23:53 -0800 (PST) >>>>> >>>>>> Please could someone kindly explain the following inconsistencies >>>>>> I've discovered__when performing logical calculations in R: >>>>>> >>>>>> 8.8 - 7.8 > 1 >>>>>> >>>>>>> TRUE >>>>>>> >>>>>> 8.3 - 7.3 > 1 >>>>>> >>>>>>> TRUE >>>>>>> >>>>> Gladly: FAQ 7.31 >>>>> http://cran.at.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R- >>> >>> th >>>>> >>>>> ink-these-numbers-are-equal_003f >>>>> >>>>> >>>> well, this answer the question only partially. this explains why a >>>> system with finite precision arithmetic, such as r, will fail to be >>>> logically correct in certain cases. it does not explain why r, a >>>> language said to isolate a user from the underlying implementational >>>> choices, would have to fail this way. >>>> >>>> there is, in principle, no problem in having a high-level language >>>> perform the computation in a logically consistent way. for example, >>>> bc is an "arbitrary precision calculator language", and has no >>> >>> problem >>>> >>>> with examples as the above: >>>> >>>> bc <<< "8.8 - 7.8 > 1" >>>> # 0, meaning 'no' >>>> >>>> bc <<< "8.3 - 7.3 > 1" >>>> # 0, meaning 'no' >>>> >>>> bc <<< "8.8 - 7.8 == 1" >>>> # 1, meaning 'yes' >>>> >>>> >>>> the fact that r (and many others, including matlab and sage, perhaps >>>> not >>>> mathematica) does not perform logically here is a consequence of its >>>> implementation of floating point arithmetic. >>>> >>>> the faq you were pointed to, and its referring to the goldberg's >>>> article, show that r does not successfully isolate a user from >>> >>> details >>>> >>>> of the lower-level implementation. >>>> >>>> vQ >>> >>> Well, first of all for 8.-7.3 is not equal to 1 [for computers] >>> >>>> 8.3-7.3-1 >>> >>> [1] 8.881784e-16 >>> >>> But if you use only one digit precision >>> >>>> round(8.3-7.3,1)-1 >>> >>> [1] 0 >>>> >>>> round(8.3-7.3,1)-1>0 >>> >>> [1] FALSE >>>> >>>> round(8.3-7.3,1)==1 >>> >>> [1] TRUE >>> >>> >>> So the problem is the code write and no the software >>> >>> -- >>> Bernardo Rangel Tura, M.D,MPH,Ph.D >>> National Institute of Cardiology >>> Brazil >>> >>> >>> >>>     [[alternative HTML version deleted]] >> >> >> [[alternative HTML version deleted]] >> >> >> >> ------------------------------------------------------------------------ >> >> ______________________________________________ >> 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. > ______________________________________________ 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.
