# How do I sample randomly based on some probability(wightage)?

Mel mwilson at the-wire.com
Tue May 26 21:23:00 CEST 2009

Sumitava Mukherjee wrote:

> Hi all,
> I need to randomly sample from a list where all choices have weights
> attached to them. The probability of them being choosen is dependent
> on the weights.
> If say Sample list of choices are [A,B,C,D,E] and weights of the same
> are [0.895,0.567,0.765,0.890,0.60] when I draw (say 2) samples then I
> want the likeliness of them being chosen be in the order : D>A>C>E>B
>
> In short I mean if prob of a H is .9 and probability of T be 0.1 then
> if I draw 10 samples, 9 should be H and 1 should be T.
>
> I coudn't find a function in the module random that does so.
> Please can someone guide me how the above could be implemented [either
> through some function which exists and I don't know or pointers to
> some code snippets which does so]?

I've usually used something like (untested):

def chooser (objects, weights):
total = 0.0
for obj, weight in zip (objects, weights):
total += weight
if weight < total * random.random():
chosen = obj
return chosen

Works fine if runtime is not a great concern.

Mel.