tasks a-s

Task/Probabilistic-choice/Python/probabilistic-choice.py

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+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)