16 messages · wolfgang.siewert, Henrique Dallazuanna, Dimitris Rizopoulos +10 more
Try this: 0.7-0.3==0.4 (We get FALSE) 0.7-0.3<0.4 (We get TRUE) but 0.8-0.3==0.5 (TRUE) 0.8-0.3<0.5 (FALSE) Funny, he? There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal subtractions, that - we have the feeling - shouldt be solved. Best regards Sven & Wolfgang
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this is a (very) Frequently Asked Question; check: http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f Best, Dimitris
Try this: 0.7-0.3==0.4 (We get FALSE) 0.7-0.3<0.4 (We get TRUE) but 0.8-0.3==0.5 (TRUE) 0.8-0.3<0.5 (FALSE) Funny, he? There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal subtractions, that - we have the feeling - shouldt be solved. Best regards Sven & Wolfgang
Dimitris Rizopoulos Assistant Professor Department of Biostatistics Erasmus University Medical Center Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands Tel: +31/(0)10/7043478 Fax: +31/(0)10/7043014
wolfgang.siewert <wolfgang.siewert <at> gmail.com> writes:
There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal subtractions, that - we have the feeling - shouldt be solved.
Oh no, not that one again! This was lecture two in my first computer course in 1968, but it seems to be gone the way of the dodo since than. Dietr
wolfgang.siewert <wolfgang.siewert <at> gmail.com> writes:
There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal subtractions, that - we have the feeling - shouldt be solved.
Oh no, not that one again! This was lecture two in my first computer course in 1968, but it seems to be gone the way of the dodo since than.
What makes you think that the average useR has had any exposure to computer science courses? ;-) I bemoan the apparent inability of those asking such questions to use the resources provided to solve these problems for themselves... Gavin
Dietr
______________________________________________ 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.
Hi,
I bemoan the apparent inability of those asking such questions to use the resources provided to solve these problems for themselves...
Looking at all the people who quite obviously do NOT "read the posting guide and provide commented, minimal, self-contained, reproducible code", I wonder whether the mailing list could be configured to reply to each new mail (not replies in a thread) with an automated mail like this: "Have you read the posting guide and the FAQs? If you do not get a reply within two days, you may want to look at both and think about reformulating your query. Oh, and while you are at it, look through the archives, a lot of questions have already been asked and answered before." Just a thought, Stephan
Hi r-help-bounces at r-project.org napsal dne 20.04.2009 19:01:46:
wolfgang.siewert <wolfgang.siewert <at> gmail.com> writes:
There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal
subtractions,
that - we have the feeling - shouldt be solved.
Oh no, not that one again! This was lecture two in my first computer course in 1968, but it seems to be gone the way of the dodo since than.
Maybe that is because of Excel is so widespread now and gives expected results (it probably silently rounds all decimal numbers before calculation). Regards Petr
Dietr
______________________________________________ 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.
Petr PIKAL <petr.pikal <at> precheza.cz> writes:
Maybe that is because of Excel is so widespread now and gives expected results (it probably silently rounds all decimal numbers before calculation).
Marc Schwartz already reminded me of that one, and it's a good point to explicitly mention in lectures. I suggest to extend R by introducing %==% as "being Excellently equal". Dieter
r-help-bounces at r-project.org napsal dne 21.04.2009 10:00:06:
Petr PIKAL <petr.pikal <at> precheza.cz> writes:
Maybe that is because of Excel is so widespread now and gives expected
results (it probably silently rounds all decimal numbers before calculation).
Marc Schwartz already reminded me of that one, and it's a good point to explicitly mention in lectures. I suggest to extend R by introducing %==% as "being Excellently equal".
It helps but not in all cases
(0.7-0.3)-.4>0
[1] FALSE
(0.7-0.3)-.4<0
[1] TRUE
There always could be different issues with not exact representation of decimals. So educated user or internal rounding could help but I am not sure if later is desired. Regards Petr
Dieter
______________________________________________ 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.
Hi r-help-bounces at r-project.org napsal dne 20.04.2009 19:01:46:
wolfgang.siewert <wolfgang.siewert <at> gmail.com> writes:
There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal
subtractions,
that - we have the feeling - shouldt be solved.
Oh no, not that one again! This was lecture two in my first computer course in 1968, but it seems to be gone the way of the dodo since than.
Maybe that is because of Excel is so widespread now and gives expected results (it probably silently rounds all decimal numbers before calculation).
I don't have Excel, but I expect OpenOffice duplicates its bugs pretty well. And in OpenOffice I see all sorts of bugs due to this, e.g. examples where x = y and y = z but x != z, cases where I can calculate a number like 1 + 4.e-15 and get something different from 1, but if I enter it directly as 1.000000000000004, it gets changed to 1. So it only gives expected results in some tests, not others. Duncan Murdoch
On 21/04/2009 3:48 AM, Petr PIKAL wrote:
Hi r-help-bounces at r-project.org napsal dne 20.04.2009 19:01:46:
wolfgang.siewert <wolfgang.siewert <at> gmail.com> writes:
There is a way around: round(0.7-0.3,1)==0.4 (TRUE) Obviously there is a problem with some combinations of decimal
subtractions,
that - we have the feeling - shouldt be solved.
Oh no, not that one again! This was lecture two in my first computer course in 1968, but it seems to be gone the way of the dodo since than.
Maybe that is because of Excel is so widespread now and gives expected results (it probably silently rounds all decimal numbers before calculation).
I don't have Excel, but I expect OpenOffice duplicates its bugs pretty well. And in OpenOffice I see all sorts of bugs due to this, e.g. examples where x = y and y = z but x != z, cases where I can calculate a number like 1 + 4.e-15 and get something different from 1, but if I enter it directly as 1.000000000000004, it gets changed to 1. So it only gives expected results in some tests, not others. Duncan Murdoch
As Dieter noted from our offlist exchange, this had been discussed previously back in 2003. Just to refresh memories: https://stat.ethz.ch/pipermail/r-help/2003-June/034565.html https://stat.ethz.ch/pipermail/r-help/2003-June/034860.html OO.org has replicated Excel's behavior to a fault. Thus: Spreadsheet Use -> Brain to Porridge Just to update OO.org's behavior using version 3.0.1 on OSX: Formula: =4.145 * 100 + 0.5 Result: 415.00000000000000000000 Formula: =0.5 - 0.4 - 0.1 Result: 0.00000000000000000000 Formula: =(0.5 - 0.4 - 0.1) Result: 0.00000000000000000000 So nothing has changed in OO.org in five years. Somebody with Excel 2007 might want to try the 2nd and 3rd formula examples to see if using parens still makes a difference in the result as compared to the formula without the parens. FWIW, now that I am on OSX, I can add the following output using Numbers '09: Formula: =4.145 * 100 + 0.5 Result: 415.00000000000000000000 Formula: =0.5 - 0.4 - 0.1 Result: -2.77556E-17 Formula: =(0.5 - 0.4 - 0.1) Result: -2.77556E-17 It does look like R's behavior has changed since then. Using: R version 2.9.0 Patched (2009-04-18 r48348) on OSX: # This first example has changed. # Prior result was 414.99999999999994 > print(4.145 * 100 + 0.5, digits = 20) [1] 415 > formatC(4.145 * 100 + 0.5, format = "E", digits = 20) [1] "4.14999999999999943157E+02" > print(0.5 - 0.4 - 0.1, digits = 20) [1] -2.77555756156289e-17 > formatC(0.5 - 0.4 - 0.1, format = "E", digits = 20) [1] "-2.77555756156289135106E-17" What is interesting is that: > 4.145 * 100 + 0.5 == 415 [1] FALSE > (4.145 * 100 + 0.5) - 415 [1] -5.684342e-14 > all.equal(4.145 * 100 + 0.5, 415, 0) [1] "Mean relative difference: 1.369721e-16" So it would appear that in the first R example above, the print() function has changed in a material fashion. HTH, Marc Schwartz
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"Have you read the posting guide and the FAQs? If you do not get a reply within two days, you may want to look at both and think about reformulating your query. Oh, and while you are at it, look through the archives, a lot of questions have already been asked and answered before."
As I say every time someone brings this up, there are currently ~130 printed pages of FAQs. Reading all that seems a rather large burden on the novice poster. Hadley
I'd be happy if everyone made a minimal effort to solve their problems before posting. There may be 130 pages of printed FAQs, but the html file is fully searchable. It would be easier to browse with a good ToC, but it's not that overwhelming to _look to see if there's a FAQ related to your problem_. I think pointing people to Rseek is probably more useful than sending them directly to the FAQs. Sarah
"Have you read the posting guide and the FAQs? If you do not get a reply within two days, you may want to look at both and think about reformulating your query. Oh, and while you are at it, look through the archives, a lot of questions have already been asked and answered before."
As I say every time someone brings this up, there are currently ~130 printed pages of FAQs. ?Reading all that seems a rather large burden on the novice poster. Hadley -- http://had.co.nz/
Sarah Goslee http://www.functionaldiversity.org
"Have you read the posting guide and the FAQs? If you do not get a reply within two days, you may want to look at both and think about reformulating your query. Oh, and while you are at it, look through the archives, a lot of questions have already been asked and answered before."
As I say every time someone brings this up, there are currently ~130 printed pages of FAQs. Reading all that seems a rather large burden on the novice poster.
There is a ToC and HTML pages are searchable. Your point raises an important issue though; R is the product of voluntary offers of time and expertise and despite the large array of talented, clever individuals from a wealth of backgrounds contributing untold improvements and extensions to R, we (the R community) haven't really `solved` this documentation issue and the problem of getting people to actually read the damned stuff before firing off an email claiming x or wanting help with y, or developers to buy in and provide information in new ways. Either people don't have the time to implement a better system or we haven't come up with a better system? Maybe this is just human nature and we have to live with it? I hope Stephan's suggestion was in jest - flooding the world with yet more email traffic seems counter-productive. Instead of the list going on about these issues every couple of months or so, is there any scope for interested parties getting together and coming up with suggestions for how to improve/change things, *and*, more importantly, offers to help get it implemented? I know there are discussions/work on going to improve the design of the R website and CRAN. Now might be a good time to try and effect improvements that will help the community. How do we raise the profile of resources already out there - several people feel the Task Views aren't as well advertised as they could be for example? Do we really need a new 'type' of documentation or resource? How do we improve what we already have? G
Hadley
%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% Dr. Gavin Simpson [t] +44 (0)20 7679 0522 ECRC, UCL Geography, [f] +44 (0)20 7679 0565 Pearson Building, [e] gavin.simpsonATNOSPAMucl.ac.uk Gower Street, London [w] http://www.ucl.ac.uk/~ucfagls/ UK. WC1E 6BT. [w] http://www.freshwaters.org.uk %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 197 bytes Desc: This is a digitally signed message part URL: <https://stat.ethz.ch/pipermail/r-help/attachments/20090422/79fadff2/attachment-0002.bin>
"MS" == Marc Schwartz <marc_schwartz at me.com>
on Tue, 21 Apr 2009 08:06:46 -0500 writes:
>>> Hi
>>> r-help-bounces at r-project.org napsal dne 20.04.2009 19:01:46:
>>>> wolfgang.siewert <wolfgang.siewert <at> gmail.com> writes:
>>>>
>>>>> There is a way around: round(0.7-0.3,1)==0.4
>>>>> (TRUE)
>>>>>
>>>>> Obviously there is a problem with some combinations of decimal
>>> subtractions,
>>>>> that - we have the feeling - shouldt be solved.
>>>> Oh no, not that one again! This was lecture two in my first computer
>>>> course in 1968, but it seems to be gone the way of the dodo since
>>>> than.
>>> Maybe that is because of Excel is so widespread now and gives
>>> expected results (it probably silently rounds all decimal numbers
>>> before calculation).
>>
>> I don't have Excel, but I expect OpenOffice duplicates its bugs
>> pretty well. And in OpenOffice I see all sorts of bugs due to this,
>> e.g. examples where x = y and y = z but x != z, cases where I can
>> calculate a number like 1 + 4.e-15 and get something different from
>> 1, but if I enter it directly as 1.000000000000004, it gets changed
>> to 1.
>>
>> So it only gives expected results in some tests, not others.
>>
>> Duncan Murdoch
MS> As Dieter noted from our offlist exchange, this had been discussed
MS> previously back in 2003. Just to refresh memories:
MS> https://stat.ethz.ch/pipermail/r-help/2003-June/034565.html
MS> https://stat.ethz.ch/pipermail/r-help/2003-June/034860.html
MS> OO.org has replicated Excel's behavior to a fault. Thus:
MS> Spreadsheet Use -> Brain to Porridge
MS> Just to update OO.org's behavior using version 3.0.1 on OSX:
MS> Formula: =4.145 * 100 + 0.5 Result: 415.00000000000000000000
MS> Formula: =0.5 - 0.4 - 0.1 Result: 0.00000000000000000000
MS> Formula: =(0.5 - 0.4 - 0.1) Result: 0.00000000000000000000
MS> So nothing has changed in OO.org in five years. Somebody with Excel
MS> 2007 might want to try the 2nd and 3rd formula examples to see if
MS> using parens still makes a difference in the result as compared to the
MS> formula without the parens.
MS> FWIW, now that I am on OSX, I can add the following output using
MS> Numbers '09:
MS> Formula: =4.145 * 100 + 0.5 Result: 415.00000000000000000000
MS> Formula: =0.5 - 0.4 - 0.1 Result: -2.77556E-17
MS> Formula: =(0.5 - 0.4 - 0.1) Result: -2.77556E-17
MS> It does look like R's behavior has changed since then. Using:
MS> R version 2.9.0 Patched (2009-04-18 r48348)
MS> on OSX:
MS> # This first example has changed.
MS> # Prior result was 414.99999999999994
>> print(4.145 * 100 + 0.5, digits = 20)
MS> [1] 415
>> formatC(4.145 * 100 + 0.5, format = "E", digits = 20)
MS> [1] "4.14999999999999943157E+02"
>> print(0.5 - 0.4 - 0.1, digits = 20)
MS> [1] -2.77555756156289e-17
>> formatC(0.5 - 0.4 - 0.1, format = "E", digits = 20)
MS> [1] "-2.77555756156289135106E-17"
MS> What is interesting is that:
>> 4.145 * 100 + 0.5 == 415
MS> [1] FALSE
>> (4.145 * 100 + 0.5) - 415
MS> [1] -5.684342e-14
>> all.equal(4.145 * 100 + 0.5, 415, 0)
MS> [1] "Mean relative difference: 1.369721e-16"
MS> So it would appear that in the first R example above, the print()
MS> function has changed in a material fashion.
Yes ((though not with *my* vote...)).
However, be aware that such calculations *are* platform
dependent, and IIUC, you are now using OS X wheras you've used
another platform previously, so some of the differences you see
may not be from changes in R, but from changes in the platform
you use.
Back to the topic of print():
Actually, also format(<numeric>) has changed similarly to my chagrin.
In older versions of R, you could ask it to give "too many" digits,
but now it gives "too few" even for maximal 'digits'.
{There is a good reason - which I don't recall - for the new behavior}
With as.character() it was worse (in older R versions): it gave
sometimes too little digits, sometimes too many, whereas now it
is at least consistently giving "too little".
But the effect is that in ch <- as.character(x) ,
ch may contain duplicated entries even for unique x,
e.g., for x <- c(1, 1 + 4e-16)
BTW, one alternative to {"my"} formatC() is sprintf(),
and if you are really interested: The latest changes (in 2.10.0 R-devel),
ensuring unique factor levels actually now make use of
sprintf("%.17g", .)
instead of as.character(.) exactly in order to ensure that
different numbers map to different strings necessarily.
BTW, we are way off topic for R-help, being in R-devel realm,
but as this thread has started here, we may keep it...
Martin Maechler, ETH Zurich
MS> HTH,
MS> Marc Schwartz