From: Tim Roberts <timr@...>
Date: 2012-04-12T15:21:16+09:00
Subject: Re: BigDecimal precision on square root

F�ldes L�szl� <lists@ruby-forum.com> wrote:

>What is the problem with this?:
>
>> BigDecimal("4").sqrt(10)
>=> #<BigDecimal:9c5a734,'0.1999999999 9999999998 66602351E1',36(36)>
>> BigDecimal("4").sqrt(11)
>=> #<BigDecimal:9c6457c,'0.2E1',9(45)>
>
>`sqrt`'s parameter should be the precision, and for this operation 4
>should suffice (or not?).

What are you complaining about?  You asked for 10 digits of precision, and
the answer you got is correct to 19 digits.

>Why BigDecimal.sqrt is imprecise and why do I have to set a 11 digit
>precision to get the proper value for sqrt(4)?

BigDecimal.sqrt is implemented via Newton-Raphson iteration.  That method
converges very quickly, but not immediately.  That's why the precision is a
parameter.

You got the correct answer, given the precision you requested.
-- 
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.