From: Sebastian Hungerecker <sepp2k@...>
Date: 2008-10-28T02:15:32+09:00
Subject: Re: float equality

guille lists wrote:
> I'm a bit lost here, shouldn't (0.1) and (1 - 0.9) be equals regarding
> the == operator?

No. The result of 1 - 0.9 using floating point math is not actually 0.1. In 
irb it is displayed as 0.1, but that's only because Float#inspect rounds. 
Using printf you can see that the result of 1-0.9 actually is 0.09999lots:
>> printf "%.30f", 1-0.9
0.099999999999999977795539507497
0.1 itself isn't actually 0.1 either - it's 
0.100000000000000005551115123126...
Because of this you should not check two floats for equality (usually you want 
to check for a delta or not use floats at all). This is so because of the 
inherent inaccuracy of floating point maths. See this for more information:
http://docs.sun.com/source/806-3568/ncg_goldberg.html

HTH,
Sebastian
-- 
Jabber: sepp2k@jabber.org
ICQ: 205544826