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Python: Getting a Fair Flip out of an Unfair Coin

If you have an unfair coin (i.e. one that favors heads or tails), how do generate a fair flip (i.e. one that doesn't favor heads or tails)? My buddy Hy Carrinski and I came up with the following algorithm:
"""Get a fair flip out of an unfair coin."""

from collections import defaultdict
from random import random

FLIP_RATIO = 0.7


def flip_unfair_coin():
"""Flip an unfair coin. Return a boolean."""
return random() > FLIP_RATIO


def create_fair_flip():
"""Generate a fair flip. Return a boolean."""
while True:
flip = flip_unfair_coin()
if flip != flip_unfair_coin():
return flip


# Demonstrate that it works.

if __name__ == '__main__':
results = defaultdict(int)
for i in xrange(1000000):
results[create_fair_flip()] += 1
percentage = (results[False] / float(results[False] + results[True])) * 100
print "Percentage heads:", percentage

Comments

Dirk Bergstrom said…
It works, but I don't really understand why (that's what happens when you don't learn statistics). An explanation would be nice...

While we're at it, there's a typo in the script: you write "Prove" but you merely demonstrate.
It works because even on an unfair coin, the chance of a heads followed by a tails is the same as the chance of a tails followed by a head. This algorithm only returns a value when one result is followed by the other. Looking at the possible outcomes makes this pretty clear. If heads is .7 and tails is .3, then the four possible outcome probabilities are:

heads/heads = 0.49
heads/tails = 0.21
tails/heads = 0.21
tails/tails = 0.09

The algorithm throws away heads/heads and tails/tails and returns tails for heads/tails and heads for tails/heads - each with a probability of of 0.21 per attempt (with transparent re-tries for the rejected cases).
Luke Plant said…
Very neat!

@Dirk: it works because even biased coin flips are still independent. (if they are not e.g. if a person is controlling the flips, or the coin has been set to do a certain sequence, this method will fail).

Essentially, you are flipping the coin twice, and then only looking at the times when the two results are different. The first time you get that scenario, pick the first result (or last, it doesn't matter). The probability of getting Heads then Tails is the same as the probability of getting Tails then Heads (due to independence), so you get odds of 50% for the overall result being Heads or Tails.
Jeff Epler said…
You've reinvented the von Neumann Extractor, and Jean-Paul is right about the reasons it works for a biased but non-autocorrelated source.
jjinux said…
> While we're at it, there's a typo in the script: you write "Prove" but you merely demonstrate.

Updated. Thanks.
jjinux said…
> It works because...

Wow, great explanation!
jjinux said…
> You've reinvented the von Neumann Extractor, and Jean-Paul is right about the reasons it works for a biased but non-autocorrelated source.

Nice job providing the reference. I'm 100% okay with the fact that I came up with the same thing as Von Neumann ;)
Paddy3118 said…
We have a task on this very topic here: http://rosettacode.org/wiki/Unbias_a_random_generator, so you can see it solved in Python and other languages.

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