From: jzakiya <jzakiya@...>
Date: 2007-02-26T12:00:06+09:00
Subject: Re: extract a random number of items from an array

On Feb 25, 4:50 pm, Brian Candler <B.Cand...@pobox.com> wrote:
> 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.

But for n > a.size Option 1 fills result with valid values while for
Option 2 it fills it with nil. Problem didn't state that n always <=
a.size.