From: Robert Klemme <shortcutter@...>
Date: 2009-06-06T20:25:11+09:00
Subject: Re: rounding errors?

On 06.06.2009 05:46, s.ross wrote:
> The result of the calculation is undefined in pretty much any  
> language. The loose definition of the expression is "return a value  
> 100 times a value approximating 29 and truncated to an integer using  
> the algorithm of your choice." So, you claim the *only* right answer  
> must be the one yielded by Lisp.

Agreed.  And, btw, with BigDecimal even Ruby comes up with more 
"natural" results:

require 'bigdecimal'
x = BigDecimal.new "0.29"
x *= 100
puts x
printf "%f %10.8f\n", x, x

->

0.29E2
29.000000 29.00000000

> However, the correct answer is: The  
> result of evaluating this expression is implementation-specific.

Actually, this is only half true: if the implementation is IEEE-754 
standards conform then it must yield certain results - regardless of 
implementation.  I believe nowadays most if not all math libraries 
conform to the standard so you should expect to see the same results 
from all implementations.  I would assume that differences are mostly 
due to numeric data type (see your C example) and conversion for 
printing (e.g. how many places are output by default).

http://en.wikipedia.org/wiki/IEEE_754
http://grouper.ieee.org/groups/754/

> The moral of the story is not "use Lisp". The moral is use floating  
> point numbers with great care.

Exactly.

Kind regards

	robert

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