On Nov 14, 3:20 am, "Fredrik Johansson" <[EMAIL PROTECTED]> wrote: > Dick Moores wrote: > > For 1234 ** 10.9, why the wrong result from mpmath.power()? > > > ======================================== > > #!/usr/bin/env python > > #coding=utf-8 > > from mpmath import * > > > mpf.dps = 32 > > > x = mpf(1234) > > y = mpf(10.9) > > > print power(x,y) > > print "4.9583278648155041477415234438717e+33" # from Windows calculator > > > """ > > output: > > 4.9583278648155166864966558721921e+33 > > 4.9583278648155041477415234438717e+33 > > """ > > ======================================== > > (Code is also at <http://python.pastebin.com/m72a277b8>) > > > Thanks, > > > Dick Moores > > Hi, > > When you create y, you first create an inexact Python float whose > value is actually 10.900000000000000355... and then pass it to mpf, > which just copies that value along with the error. To create an > accurate representation of a fractional number (integers are safe), > pass a string instead: > > >>> from mpmath import * > > >>> mpf.dps = 32 > >>> x = mpf(1234) > >>> y = mpf('10.9') > >>> print power(x,y) > > 4.9583278648155041477415234438719e+33 > > The last printed digit is wrong, which is to be expected in binary > arithmetic since 10.9 still cannot be represented exactly (with > mpf.dps = 32, the error is about 5e-033); computing a power amplifies > that error. You can temporarily increase the precision by a couple of > digits during calculations and then reduce it before printing the > final value if you want all digits to be right. > > I should update the mpmath documentation to be much clearer about all this. > > Fredrik
You could also use the Decimal module to keep things accurate. Some people like numpy or scipy for the more complex math problems they do. Mike -- http://mail.python.org/mailman/listinfo/python-list
