Source code in the python programming language

import random, bisect

def probchoice(items, probs):
  '''\
  Splits the interval 0.0-1.0 in proportion to probs
  then finds where each random.random() choice lies
  '''
  
  prob_accumulator = 0
  accumulator = []
  for p in probs:
    prob_accumulator += p
    accumulator.append(prob_accumulator)
    
  while True:
    r = random.random()
    yield items[bisect.bisect(accumulator, r)]

def probchoice2(items, probs, bincount=10000):
  '''\
  Puts items in bins in proportion to probs
  then uses random.choice() to select items.
  
  Larger bincount for more memory use but
  higher accuracy (on avarage).
  '''
  
  bins = []
  for item,prob in zip(items, probs):
    bins += [item]*int(bincount*prob)
  while True:
    yield random.choice(bins)
      
      
def tester(func=probchoice, items='good bad ugly'.split(),
                    probs=[0.5, 0.3, 0.2],
                    trials = 100000
                    ):
  def problist2string(probs):
    '''\
    Turns a list of probabilities into a string
    Also rounds FP values
    '''
    return ",".join('%8.6f' % (p,) for p in probs)
  
  from collections import defaultdict
   
  counter = defaultdict(int)
  it = func(items, probs)
  for dummy in xrange(trials):
    counter[it.next()] += 1
  print "\n##\n## %s\n##" % func.func_name.upper()  
  print "Trials:              ", trials
  print "Items:               ", ' '.join(items)
  print "Target probability:  ", problist2string(probs)
  print "Attained probability:", problist2string(
    counter[x]/float(trials) for x in items)

if __name__ == '__main__':
  items = 'aleph beth gimel daleth he waw zayin heth'.split()
  probs = [1/(float(n)+5) for n in range(len(items))]
  probs[-1] = 1-sum(probs[:-1])
  tester(probchoice, items, probs, 1000000)
  tester(probchoice2, items, probs, 1000000)