On Sun, Jan 23, 2011 at 11:13:11PM +0100, Marc Jekel wrote: > Hello R Fans, > > Another question for the community that really frightened me today. The > following logical comparison produces a "false" as output: > > t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1)) > tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1)) > > t == tt > > This is really strange behavior. Most likely this has something to do > how R represents numbers internally and the possible sensitivity of a > computer? Does anyone know when this strange behavior occurs and how to > fix it?
The number 0.7 has infinite expansion in binary 0.1011001100110011001100110011... so is rounded in the standard numeric data type, which is used for speed needed in complex computations. If you know in advance that the result has at most 2 decimal positions, then round(, digits=2) yields the correct comparison round(t, 2) == round(tt, 2) # [1] TRUE athough 0.2 is also not exactly representable. Both sides are rounded to the same representable number. See also http://rwiki.sciviews.org/doku.php?id=misc:r_accuracy for other examples. Petr Savicky. ______________________________________________ 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.
