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How do I simulate flip of biased coin?

martineau
Pratik Deoghare

In unbiased coin flip H or T occurs 50% of times.

But I want to simulate coin which gives H with probability ‘p’ and T with probability ‘(1-p)’.

something like this:

def flip(p):
   '''this function return H with probability p'''
   # do something
   return result

>> [flip(0.8) for i in xrange(10)]
[H,H,T,H,H,H,T,H,H,H]

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Answer

random.random() returns a uniformly distributed pseudo-random floating point number in the range [0, 1). This number is less than a given number p in the range [0,1) with probability p. Thus:

def flip(p):
    return 'H' if random.random() < p else 'T'

Some experiments:

>>> N = 100
>>> flips = [flip(0.2) for i in xrange(N)]
>>> float(flips.count('H'))/N
0.17999999999999999  # Approximately 20% of the coins are heads

>>> N = 10000
>>> flips = [flip(0.2) for i in xrange(N)]
>>> float(flips.count('H'))/N
0.20549999999999999  # Better approximation