From: Stefano Crocco <stefano.crocco@...>
Date: 2009-02-05T22:42:04+09:00
Subject: Re: Array#choice always produce the same sequence

Alle Thursday 05 February 2009, ruud grosmann ha scritto:

> > I've just stumbled upon a strange behaviour of Array#choice (using ruby
> > 1.8.7- p72). As far I understand, an_array.choice should be (almost) the
> > same as an_array[rand(an_array.size)]. Thus, the two following pieces of
> > code should be equivalent
> >
> > a = [1, 2, 3, 4]
> > p 10.times.map{a.choice}.join
> > p 10.times.map{a[rand(a.size)]}.join
> >
> > However, this doesn't seem to be the case. In particular, something like:
> >
> > ruby -e 'a = [1, 2, 3, 4]; p 10.times.map{a.choice}.join'
> >
> > always give the same result, while
> >
> > ruby -e 'a = [1, 2, 3, 4]; p 10.times.map{a[rand(a.size)]}.join'
> >
> > gives a different result every time the command is executed (which, in my
> > opinion, is the correct behaviour).
> >
> > It seems that to get the correct behaviour from choice, you need to call
> > srand
> > before using it. Does anyone know why this is necessary with choice but
> > not with rand? Is it the intended behaviour or a bug?
> >
> > Thanks
> >
> > Stefano

>Hi Stefano,
>
>it seems to work for me:
>
>irb(main):003:0> a = [1, 2, 3, 4]
>=> [1, 2, 3, 4]
>irb(main):004:0> p 10.times.map{a.choice}.join
>"4414312344"
>=> nil
>irb(main):006:0> p 10.times.map{a[rand(a.size)]}.join
>"4342411331"
>=> nil
>irb(main):007:0>  p 10.times.map{a.choice}.join
>"2333341212"
>=> nil
>irb(main):008:0> p 10.times.map{a.choice}.join
>"1312441122"
>=> nil
>irb(main):009:0>  p 10.times.map{a.choice}.join
>"2412444223"
>=> nil
>Z ruby -v
>ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]

Maybe I wasn't clear in explaining the issue. Within a single irb session (or 
ruby invocation in general) I, too, get different results at each invocation. 
The problem is that if I run the script twice, I get the same results for each 
call to choice (that is, the first call to choice gives the same result at 
each ruby invocation; the second call always gives the same result, which can 
be different from the first, and so on). A simpler example:

File: test.rb

p [1,2,3,4].choice

ruby test.rb
=> 4
ruby test.rb
=> 4
ruby test.rb
=> 4

And so on. If at the beginning of the script I add a call to Kernel#srand 
(without arguments), I get the correct behaviour, that is a different number 
every time the script is run.

Stefano