From: Robert Klemme <shortcutter@...>
Date: 2011-08-29T20:51:53+09:00
Subject: Re: srand(0) confusion

On Mon, Aug 29, 2011 at 1:20 PM, Samuel Mensah <sasogeek@yahoo.com> wrote:
> I'm fairly new to coding ruby and I'm using "Learn to program" by Chris
> Pine.
> After taking a look at the 'rand' method, I'm introduced to 'srand' and
> here's what I'm confused about...
>
> srand(123)
> puts rand(100)
> puts rand(100)
>
> ...will give me 2 random numbers which will repeat themselves for as
> many times as the program is run but according to the book "If you want
> to get different numbers again (like what happens if you
> never use srand), then just call srand(0)." I don't understand what it
> means cos this is what I tried...
>
> srand(0)
> puts rand(100)
> puts rand(100)
>
> ...and only the first 2 numbers changed after it was run again and they
> kept repeating every time it is run. what exactly does the book mean and
> how is it written in code?

I assume the documentation is wrong.  If you use "srand" instead of
"srand(0)" the behavior is as documented for the case that 0 is used,
i.e. the numbers really change.  With 0 I get a fixed sequence.

$ for i in `seq 1 5`; do ruby -e 'srand 0;p 10.times.map {rand(10)}'; done
[5, 0, 3, 3, 7, 9, 3, 5, 2, 4]
[5, 0, 3, 3, 7, 9, 3, 5, 2, 4]
[5, 0, 3, 3, 7, 9, 3, 5, 2, 4]
[5, 0, 3, 3, 7, 9, 3, 5, 2, 4]
[5, 0, 3, 3, 7, 9, 3, 5, 2, 4]

RKlemme@padrklemme2
/cygdrive/c/SCMws/RKlemme/JavaProducts_oslee_ngcp_dev_R3.1_vf_fiji_be4rb
$ for i in `seq 1 5`; do ruby -e 'srand;p 10.times.map {rand(10)}'; done
[5, 8, 6, 0, 1, 6, 9, 1, 3, 3]
[0, 8, 4, 1, 7, 1, 8, 8, 9, 5]
[4, 3, 3, 7, 1, 5, 2, 4, 4, 1]
[1, 1, 4, 5, 3, 2, 6, 2, 7, 0]
[1, 6, 8, 5, 0, 8, 4, 0, 4, 3]

Without calling srand we get random sequences as well:

RKlemme@padrklemme2
/cygdrive/c/SCMws/RKlemme/JavaProducts_oslee_ngcp_dev_R3.1_vf_fiji_be4rb
$ for i in `seq 1 5`; do ruby -e 'p 10.times.map {rand(10)}'; done
[7, 6, 6, 5, 1, 7, 8, 9, 2, 1]
[1, 8, 7, 5, 2, 3, 1, 6, 3, 8]
[0, 4, 8, 0, 9, 2, 5, 2, 1, 4]
[5, 7, 0, 9, 3, 8, 1, 4, 9, 4]
[2, 6, 5, 8, 1, 8, 4, 5, 4, 2]

Basically the "or zero" must be removed from the doc.

Kind regards

robert


-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/