choosing random numbers with weights/probability?

Michael Hudson mwh21 at cam.ac.uk
Tue Jun 22 01:17:54 CEST 1999


kevinsl <kevinsl at yahoo.com> writes:

> I've been using the whrandom.choice routine and it's very
> useful. But is there any way to add weights or
> probabilities of being chosen to the items in the list?
> 
> example:
> 
> list=['one','two','three']
> item=whrandom.choice(list)
> 
> Is there any way to say that 'one' and 'two' have a 25%
> chance of being chosen, and 'three' has a 50% chance?

One very obvious, not very extensible, way:

list=['one','two','three','three']
item=whrandom.choice(list)

(NB: list is a builtin; using list as a variable name can lead to
confusion)

A not very efficient but more general method:

def weighted_choice(choices):
    tot = 0
    for w,v in choices:
        tot = tot + w
    d = random.random()*tot
    tot = 0
    for w,v in choices:
        tot = tot + w
        if tot > d:
            return v

list=[(0.25,'one'),(0.25,'two'),(0.5,'three')]
weighted_choice(list)

seems to work. Haven't tested it much, so please don't rely on it...

HTH
Michael


> I'm hoping there's already a module to do this... or else
> I'll be writing my own..
> 
> thanks,
> 
> Kevin




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