On Nov 22, 2007 4:04 AM, Steven D'Aprano <[EMAIL PROTECTED]> wrote: > Is there a simple, elegant way in Python to get the next float from a > given one? By "next float", I mean given a float x, I want the smallest > float larger than x. > > Bonus points if I can go in either direction (i.e. the "previous float" > as well as the next). > > Note to maths pedants: I am aware that there is no "next real number", > but floats are not reals. > -- > Steven
You could use the library functions for floating-point math in mpmath (http://code.google.com/p/mpmath/), which support directed rounding. Load a floating-point number, add a tiny number and round in the wanted direction, then convert back to a Python float: >>> from mpmath.lib import * >>> eps = (1, -2000, 1) # 2**-2000, smaller than any finite positive IEEE 754 double >>> a = from_float(1.0, 53, ROUND_HALF_EVEN) # note: exact >>> to_float(fadd(a, eps, 53, ROUND_UP)) 1.0000000000000002 >>> to_float(fsub(a, eps, 53, ROUND_DOWN)) 0.99999999999999989 This currently probably doesn't work if the numbers are subnormal, however. Fredrik -- http://mail.python.org/mailman/listinfo/python-list
