Mark Dickinson <dicki...@gmail.com> added the comment: > I think ANY attempt to rely on eval(repr(x))==x is asking for trouble, > and it should probably be removed from the docs.
I disagree. I've read the paper you refer to; nevertheless, it's still perfectly possible to guarantee eval(repr(x)) == x in spite of the various floating-point details one has to deal with. In the worst case, assuming that C doubles are IEEE 754 binary64 format, for double - > string conversion one can simply cast the double to an array of 8 bytes, extract the sign, exponent and mantissa from those bytes, and then compute the appropriate digit string using nothing but integer arithmetic; similarly for the reverse operation: take a digit string, do some integer arithmetic to figure out what the exponent, mantissa and sign should be, and pack those values back into the double. Hey presto: correctly rounded string -> double and double -> string conversions! Add a whole bag of tricks to speed things up in common cases and you end up with something like David Gay's code. It *is* true that the correctness of Gay's code depends on the FPU being set to use 53-bit precision, something that isn't true by default on x87 FPUs. But this isn't hard to deal with, either: first, you make sure that you're not using the x87 FPU unless you really have to (all Intel machines that are newer than Pentium 3 have the SSE2 instruction set, which doesn't have the problems that x87 does); second, if you *really* have to use x87 then you figure out how to set its control word to get 53-bit precision and round-half-to-even rounding; third, if you can't figure out how to set the control word then you use a fallback something like that described above. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue1580> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
