# [FRPythoneers] Weighted List

Sue Giller climber at hydrosphere.com
Tue May 6 14:42:38 MDT 2003

```We have developed an in-house set of distributions, and one of
them is a discrete weighted one that sounds similar to what you
want.  It is assumed that the weightings must sum to 1 when
definining the distribution.

HTH

Code follows:

import whrandom,random,math

class distribution:
""" base class for all distributions """
def __call__(self):
# used when class called as function
return self.sample()

def __getitem__(self, i):
# return ith item - which is generated by the sample() function
return self.sample()

class discrete(distribution):
""" returns set values based on their weights """
# inputs are a tuple of 1 or more sets of wt:value pairs
def __init__(self, pairList):
pairs = pairList[:]
sum = 0.0
self._values = []
self._weights = []
for pair in pairs:
wt, value = pair
if wt < 0 or wt > 1.0:
raise AttributeError, "weight outside 0 and 1 range: %s" %
`pair`
sum += wt
self._weights.append(sum)
self._values.append(value)
if abs(sum - 1.0) > .001:
raise AttributeError, "weights don't sum to 1.0: %.3f" % sum

def sample(self):
# generate random value between 0 and 1 and see what value to
return
weight = whrandom.random()
for ii in range(len(self._weights)):
bin = self._weights[ii]
if weight > bin:
continue
return self._values[ii]
# what happens if we get here?

'for testing.  If you run a histogram on the output for the following, it
shows the distribution to match the weightings.
print 'test discrete'
wts = [(.0375, .0089), (.025, .009), (.05, .0092),
(.15, .0097), (.25, .0105,), (.25, .012),
(.15, .0141), (.05, .0164), (.0375, .0181)
]
dist = discrete(wts)
for i in range(100):
print dist()

```

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