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

```