a *= b not equivalent to a = a*b

[Chris Angelico]

On Fri, Aug 26, 2016 at 8:20 PM, mlz <mlzarathustra at gmail.com> wrote:
> I believe there are languages that preserve exact accuracy in this way for rational fractions. I don't know if Python is one of them.

It is, but only if you explicitly request it (due to the performance
impact). Just import it:

#!/usr/bin/python2
import fractions

import sys
FAIL= True if len(sys.argv)>1 else False

def bin(n,k):
    rs=1
    k=min(k,n-k)
    n = fractions.Fraction(n)

    for i in range(1,k+1):
        if FAIL: rs *= (n-(i-1))/i  # these should be the same,
        else: rs = rs * (n-(i-1))/i  #  but apparently are not
    return rs


for n in range(10):
    for k in range(n+1):
        print bin(n,k),
    print''


That's the only change you need. The arithmetic will then be done with
ratios of integers, and it'll be exact.

ChrisA