On 2/5/2008 7:21 AM, Eric Elguero wrote: > thank you to all who answered. > > >> 0+0.05+ > + 0.05+0.05+0.05+0.05+0.05+0.05+ > + 0.05+0.05+0.05+0.05+0.05+0.05+ > + 0.05+0.05+0.05+0.05+0.05+0.05 - 0.95 > [1] 3.330669e-16 > >> seq(0,1,0.05)[20] - 0.95 > [1] 1.110223e-16 > >> 0+19*0.05 - 0.95 > [1] 1.110223e-16 > > so this is the way seq calculates. I would have guessed > that addition was more accurate than multiplication, > but that is not the case.
I think they have equal accuracy on modern hardware; the problem is that you are performing so many additions, and the errors accumulate. > this one however bothers me: >> 19/20-0.95 > [1] 0 0.95 is almost certainly calculated as 95/100, so it shouldn't be too surprising that 19/20 gives the same answer. I wouldn't have bet on it, though. > I noticed this problem when I tried to extract rows of a matrix > according to whether values of some vector where in the set > (0,0.05,...,0.95,1), with something like x%in%seq(0,1,0.05) > Now I understand that I should not use this construction > unless x is of type integer. Would you agree? I would be a little less specific: don't use it unless you are sure the values on both sides of %in% can be computed exactly. That's true for integers, but it's also true for some floating point values, i.e. those that can be represented as fractions with modest powers of 2 in the denominator (0.25 = 1/4, 0.375 = 3/8, etc. "Modest" may be as large as 52, but I wouldn't recommend pushing it that far: you may run into inaccuracies when reading the numbers.) And of course, it's also true for character and logical values. Duncan Murdoch ______________________________________________ R-help@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.
