From: Justin Collins <justincollins@...>
Date: 2010-11-17T17:16:04+09:00
Subject: Re: Thread problem

On 11/16/2010 11:50 PM, Jaeyong Lee wrote:
> Hi all,
>
> I learning ruby thread and faced to thread problem.
>
> Here's my test code that sum 1 to 1000.
>
> --
> threads = []
>
> sum = 0
>
> 1.upto(1000) { |external|
>    threads<<  Thread.new(external) { |local|
>      sleep(rand(0.01))
>      sum = sum + local
>    }
> }
>
> threads.each() { |thread| thread.join() }
>
> puts sum
> --
>
> I expected '500500' but output is smaller than. (ex: 499864)
>
> Please let me know what problem is.
>
>    

http://en.wikipedia.org/wiki/Race_condition#Computing

You are updating a shared variable "simultaneously" in separate threads, 
which is a very bad idea.

-Justin