[R] Logical inconsistency
Gustavo Carvalho
gustavo.bio+R at gmail.com
Wed Dec 10 19:52:08 CET 2008
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 <Stephan.Kolassa at gmx.de> 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.Â And I've solved the problem by
>> rounding the variables before taking the difference between them.
>>
>> Thanks to all who replied.
>>
>> Emma JaneÂ
>>
>>
>>
>> ________________________________
>> From: Greg Snow <Greg.Snow at imail.org>
>>
>> .com.br>; Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk at idi.ntnu.no>; Chuck
>> Cleland <ccleland at optonline.net>
>> Cc: R help <R-help at stat.math.ethz.ch>
>> 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.Â 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).Â 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
>> greg.snow at imail.org
>> 801.408.8111
>>
>>
>>> -----Original Message-----
>>> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
>>> 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
>>>>
>>>> Â FALSE
>>>
>>> Emma Jane
>>>
>>>
>>>
>>>
>>> ________________________________
>>> From: Bernardo Rangel Tura <tura at centroin.com.br>
>>> To: Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk at idi.ntnu.no>
>>> Cc: R help <R-help at stat.math.ethz.ch>
>>> 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:Â 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.Â this explains why a
>>>> system with finite precision arithmetic, such as r, will fail to be
>>>> logically correct in certain cases.Â 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.Â 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
>>>
>>>
>>>
>>> Â Â Â Â [[alternative HTML version deleted]]
>>
>>
>> [[alternative HTML version deleted]]
>>
>>
>>
>> ------------------------------------------------------------------------
>>
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>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> 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.
>
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