Many thanks for pointing that out. Tom
On Sat, 09 Dec 2006 13:48:06 -0000, Peter Dalgaard <[EMAIL PROTECTED]> wrote: > Tom McCallum wrote: >> Hi, >> >> I am not sure if this is just me using R (R-2.3.1 and R-2.4.0) in the >> wrong way or if there is a more serious bug. I was having problems >> getting some calculations to add up so I ran the following tests: >> >> > Please read FAQ 7.31 and the reference therein. > > http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f > > (short answer: You can not represent thirds exactly in decimal nor > tenths in binary.) >>> (2.34567 - 2.00000) == 0.34567 <------- should be true >>> >> [1] FALSE >> >>> (2.23-2.00) == 0.23 <------- should be true >>> >> [1] FALSE >> >>> 4-2==2 >>> >> [1] TRUE >> >>> (4-2)==2 >>> >> [1] TRUE >> >>> (4.0-2)==2 >>> >> [1] TRUE >> >>> (4.0-2.0)==2 >>> >> [1] TRUE >> >>> (4.0-2.0)==2.0 >>> >> [1] TRUE >> >>> (4.2-2.2)==2.0 >>> >> [1] TRUE >> >>> (4.20-2.20)==2.00 >>> >> [1] TRUE >> >>> (4.23-2.23)==2.00 <------- should be true >>> >> [1] FALSE >> >>> (4.230-2.230)==2.000 <------- should be true >>> >> [1] FALSE >> >>> (4.230-2.230)==2.00 <------- should be true >>> >> [1] FALSE >> >>> (4.230-2.23)==2.00 <------- should be true >>> >> [1] FALSE >> >> I have tried these on both 64 and 32-bit machines. Surely R should be >> able to do maths to 2 decimal places and be able to test these simple >> expressions? The problem occurs as in the 16th decimal place junk is >> being placed by the FPU it seems. I have also tried: >> >> >>> (4.2300000000000000-2.230000000000000) == 2 >>> >> [1] FALSE >> >>> a <- (4.2300000000000000-2.230000000000000) >>> a == 2 >>> >> [1] FALSE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000000 >>> >> [1] FALSE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000004 <-- >>> correct when add 16th decimal place to 4 >>> >> [1] TRUE >> >>> (4.2300000000000000-2.230000000000000) == 2.00000000000000043 <-- >>> any values after the 16th decimal place mean that the expression is >>> true >>> >> [1] TRUE >> >>> (4.2300000000000000-2.230000000000000) == 2.000000000000000435 >>> >> [1] TRUE >> >> Also : >> >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000001 >>> >> [1] FALSE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000003 >>> >> [1] TRUE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000004 >>> >> [1] TRUE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000005 >>> >> [1] TRUE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000006 <-- 3,5 >>> I can understand being true if rounding occurring, but 6? >>> >> [1] TRUE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000007 >>> >> [1] FALSE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000008 >>> >> [1] FALSE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000009 >>> >> [1] FALSE >> >>> (4.2300000000000000-2.230000000000000) == 2.0000000000000010 >>> >> >> >> This is an example of junk being added in the FPU >> >>> formatC(a, digits=20) >>> >> [1] "2.0000000000000004441" >> >> I don't know if this is just a formatC error when using more than 16 >> decimal places or if this junk is what is stopping the equality from >> being true: >> >> >>> formatC(a, digits=16) >>> >> [1] " 2" >> >>> formatC(a, digits=17) <-- 16 decimal places, 17 significant figures >>> shown >>> >> [1] "2.0000000000000004" <-- the problem is the 4 at the end >> >> Obviously the bytes are divided between the exponent and mantissa in >> 16-16bit share it seems, but this doesn't account for the 16th decimal >> place behaviour does it? >> >> If any one has a work around or reason why this should occur it would >> be useful to know. >> >> what I would like is to be able to do sums such as (2.3456 - 2 ) == >> 0.3456 and get a sensible answer - any suggestions? Currently the >> only way is for formatC the expression to a known number of decimal >> places - is there a better way? >> >> Many thanks >> >> Tom >> >> >> > -- Dr. Thomas McCallum Systems Architect, Level E Limited ETTC, The King's Buildings Mayfield Road, Edinburgh EH9 3JL, UK Work +44 (0) 131 472 4813 Fax: +44 (0) 131 472 4719 http://www.levelelimited.com Email: [EMAIL PROTECTED] Level E is a limited company incorporated in Scotland. The c...{{dropped}} ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
