On 12/9/2006 8:29 AM, 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:
You should read the FAQ item "Why doesn't R think these numbers are equal?" at http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f Duncan Murdoch > >> (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 > > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel