> On 23 Apr 2017, at 14:49 , J C Nash <profjcn...@gmail.com> wrote: > > > So equality in floating point is not always "wrong", though it should be used > with some attention to what is going on. > > Apologies to those (e.g., Peter D.) who have heard this all before. I suspect > there are many to whom it is new.
Peter D. still insists on never trusting exact equality, though. There was at least one case in the R sources where age-old code got itself into a condition where a residual terme that provably should decrease on every iteration oscillated between two values of 1-2 ulp in magnitude without ever reaching 0. The main thing is that you cannot trust optimising compilers these days. There is, e.g., no guarantee that a compiler will not transform (x_new + offset) == (x_old + offset) to (x_new + offset) - (x_old + offset) == 0 to (x_new - x_old) + (offset - offset) == 0 to.... well, you get the point. -pd -- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Office: A 4.23 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.
