selection based on percentage weighting

Question

I have a set of values, and an associated percentage for each:

a: 70% chance
b: 20% chance
c: 10% chance

I want to select a value (a, b, c) based

Answer 1

Take the list of and find the cumulative total of the weights: 70, 70+20, 70+20+10. Pick a random number greater than or equal to zero and less than the total. Iterate over the items and return the first value for which the cumulative sum of the weights is greater than this random number:

def select( values ):
    variate = random.random() * sum( values.values() )
    cumulative = 0.0
    for item, weight in values.items():
        cumulative += weight
        if variate < cumulative:
            return item
    return item # Shouldn't get here, but just in case of rounding...

print select( { "a": 70, "b": 20, "c": 10 } )

This solution, as implemented, should also be able to handle fractional weights and weights that add up to any number so long as they're all non-negative.