Probability selection algorithm

[Noah]

Paul Rubin <phr-n2003b at NOSPAMnightsong.com> wrote in message news:<7xhebnscrj.fsf at ruckus.brouhaha.com>...
> Paul Rubin <phr-n2003b at NOSPAMnightsong.com> writes:
> ...
> Again, this depends on your probability list adding up to 1.
> Otherwise, add up the probabilities and scale everything appropriately.

It also depends on the probability list being sorted, but 
I may have implied that the list is sorted in my original post.

random.random() does what you want from urand().

I wrote the following demo based on your code.
This seems to do everything I need. Thanks!
Let me know if you see any mistakes.

Yours,
Noah

#!/usr/bin/env python
import random
from pprint import pprint

def make_random_probabilities_table ():
    """This creates a table of random probabilities that add up to 1.0.
        The table is a random size from 1 through 10.
        The table is sorted.
    """
    t = []
    count = random.randint (0, 9)
    total = 1.0
    for c in range (count):
        r = random.uniform (0.0, total)
        t.append(r)
        total = total - r
    t.append (total)
    t.sort ()
    return t

def random_select (problist):
    """This selects one of the odds from the problist table weighted
        on the probabilities in the table.
        The table must be sorted.
    """
    remaining = 1.0
    r = random.random()
    for i in range(len(problist)):
        p = problist[i]
        if r * remaining < p:
            return (r, i, p)
        remaining -= p

def main ():
    odds = make_random_probabilities_table ()
    (r, i, p) = random_select (odds)

    pprint (odds)
    print 'random number =', r
    print 'index =', i
    print 'probability =', p

if __name__ == '__main__':
    main ()