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:
> (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
