Pick items from list with probability based upon property of list member ?

[Mel]

southof40 wrote:

> I have list of of N Vehicle objects - the only possible vehicles are
> cars, bikes, trucks.
>
> I want to select an object from the list with a probability of : cars
> 0.7, bikes 0.3, trucks 0.1.
>
> I've currently implemented this by creating another list in which each
> car object from the original list appears 7 times, each bike 3 times
> and each truck once. I then pick at random from that list.

This seems about right.  It's like a lottery where the more likely
winners have more tickets, but all tickets are the same.  Pick one to
pick the winner.

There's a very expensive, but flexible technique that effectively gives
some tickets a better chance than others.  You have to examine each
ticket individually, so this algorithm burns random numbers like
kindling:



import random

#===============================
def weighted_choice (candidates, weight):
    chosen = None
    total = 0
    for c in candidates:
        w = weight (c)
        total += w
        if random.randint (0, total-1) < w:
            chosen = c
    return chosen
#===============================
    
def test_weight (c):
    return {'c':7, 'b':3, 't':1}[c]
    
def item_count (s, target):
    return sum (1 for x in s if x==target)
    
test_candidates = 'c'*100 + 'b'*100 + 't'*100

for i in xrange (10):
    test = [weighted_choice (test_candidates, test_weight) for k in xrange (100)]
    for x in 'cbt':
        print x, item_count (test, x), '\t',
    print




	Mel.