On Apr 2, 6:07 pm, Steven D'Aprano <st...@remove-this- cybersource.com.au> wrote: > On Fri, 02 Apr 2010 12:35:55 -0700, Mensanator wrote: > >> If you want an exact result when multiplying arbitrary fractions, you > >> need to avoid floats and decimals and use Fractions: > > >> >>> Fraction(1, 2)**2 > > >> Fraction(1, 4) > > > Where do you get that from? > > Where do I get what from? Fraction? Oops, sorry about that. > > In Python2.6: > > >>> from fractions import Fraction
Ok, thanks. I've been using gmpy to do rational arithmetic: >>> import gmpy >>> gmpy.mpq(1,2)**2 mpq(1,4) But I don't have a lot of call for it. > > In older Pythons, there was a demo module Demo/classes/Rat.py but it may > not be installed on your system. Seehttp://bugs.python.org/issue1682 > > If you meant, where did I get the statement about exact results from, > both float and Decimal are fixed precision numbers. float precision is > fixed by the operating system and/or hardware; Decimal precision can be > arbitrarily chosen by the caller, but having made that choice, > calculations are rounded to that precision. Only Fraction gives exact > results for any arbitrary rational number. Yes, rationals are handy sometimes. > > -- > Steven -- http://mail.python.org/mailman/listinfo/python-list
