Smarter way of doing this?

[Mel Wilson]

In article <401e4d57$0$295$edfadb0f at dread12.news.tele.dk>,
Max M <maxm at mxm.dk> wrote:
>I have written this function, "choose_by_probability()"
>
>It takes a list of probabilities in the range 0.00-1.00, and it must
>then randomly select one of the probabilities and return it's index.
>
>The idea is to have two lists, one with a value, and another with a
>probability. The value then gets randomly selected from its probability.
>
>values = ['item 1','item 2','item 3',]
>probabilities = [0.66, 0.33, 0.16]
>
>print values[choose_by_probability(probabilities)]
> >>'item 1'
>
>etc...
>
>I just wondered if anybody has a better way of doing it, as this seems
>nastily squared to me?

My usual way is:


import random

def choose_by_probability (values, probabilities):
    cumul = 0.0
    for prob, item in zip (probabilities, values):
        cumul += prob
        if prob > random.random()*cumul:
            selected = item
    return selected


def run_tests (n):
    D = {}
    for i in xrange (n):
        j = choose_by_probability (['item 1', 'item 2', 'item 3']
                , [0.66, 0.33, 0.17])
        D[j] = D.get (j, 0) + 1
    s = D.items()
    s.sort()
    for k, v in s:
        print k, float(v)/n
    print "Total:", sum([v for k, v in D.items()])

run_tests (10000)



        Regards.    Mel.