From: Daniel Carrera <dcarrera@...>
Date: 2003-03-29T17:14:58+09:00
Subject: Re: Weighted random selection -- how would you do this?

On Sat, Mar 29, 2003 at 05:04:32PM +0900, Phil Tomson wrote:

> >event = events.each do |event, weight|
> >	val -= weight; 
> >	return event if val < 0
> >end
>
> Hmmm.... Wouldn't this be biased by order?
> 

Would it?
This is my thinking, perhaps you can help me see if I'm wrong:

Example:
Take two events with weights:
event1 => 3
event2 => 7

Pick a random number from 0 to 3+7=10
There is a 30% chance that the number will be between 0 and 3 and there is 
a 70% chance that the number will be between 3 and 10.

By selecting an event based on which range the random number lies in I 
should get the biased probability that I desire.

I think I should replace the "val < 0" in my code by "val <= 0".
Is this the error you were referring to?  Or is there a problem with my 
reasoning?

-- 
Daniel Carrera
Graduate Teaching Assistant.  Math Dept.
University of Maryland.  (301) 405-5137
--
distrain: distrain (di-STRAYN) verb tr., intr.
   To seize the property in order to force payment for damages, debt, etc.