From: Brian Candler <B.Candler@...>
Date: 2007-02-26T06:50:00+09:00
Subject: Re: extract a random number of items from an array

On Mon, Feb 26, 2007 at 06:40:05AM +0900, Josselin wrote:
> >Are you trying to do pick 'n' elements from 'a' like lottery balls? i.e.
> >once a value has been picked once, it cannot be picked again?
> no, I need to put all the balls again for the next pick

Still not entirely clear, but I think one of these is what you mean:

Option 1: After picking one ball, you put that one ball back, then pick
another ball, put it back etc. You repeat this 'n' times to get your
selection of 'n' balls.

  result = []
  n.times { result << a[ rand(a.size) ] }

Clearly this allows for duplicates.

Option 2: You pick 'n' balls out of the bag (without replacement). Then you
put these balls back in, so that on the next run you pick another 'n' balls
from the original set.

  b = a.dup
  result = []
  n.times { result << b.slice!( rand(b.size) ) }

You don't get any duplicates within your selection of 'n' balls, but the
fact that you've picked a ball on one run doesn't prevent it from being
picked on a subsequent run.

Regards,

Brian.