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BigDecimal#sqrt

James Gray

12/22/2005 10:05:00 PM

I'm trying to understand the argument to BigDecimal#sqrt. I figured
it was for precision and it does seem to affect it, but I can't
understand the exactly relationship. Can anyone tell me what the
argument controls and how?

Thanks.

James Edward Gray II
6 Answers

James Gray

12/23/2005 4:11:00 AM

0

On Dec 22, 2005, at 4:04 PM, James Edward Gray II wrote:

> I'm trying to understand the argument to BigDecimal#sqrt. I
> figured it was for precision and it does seem to affect it, but I
> can't understand the exactly relationship. Can anyone tell me what
> the argument controls and how?

Let me try asking me question a different way... Is this the correct
way to get a printable sqrt() with 16 digits precision?

>> sprintf "%.16f", BigDecimal.new("66543").sqrt(16)
=> "257.9592991151890260"

James Edward Gray II

James Gray

12/23/2005 4:13:00 AM

0

On Dec 22, 2005, at 4:04 PM, James Edward Gray II wrote:

> I'm trying to understand the argument to BigDecimal#sqrt. I
> figured it was for precision and it does seem to affect it, but I
> can't understand the exactly relationship. Can anyone tell me what
> the argument controls and how?

Let me try asking my question a different way... Is this the correct
way to get a printable square root with N digits precision (N = 16 in
this example)?

>> sprintf "%.16f", BigDecimal.new("66543").sqrt(16)
=> "257.9592991151890260"

James Edward Gray II

vanekl

12/23/2005 12:26:00 PM

0

Gregory Brown

12/23/2005 5:07:00 PM

0

On 12/22/05, James Edward Gray II <james@grayproductions.net> wrote:
> I'm trying to understand the argument to BigDecimal#sqrt. I figured
> it was for precision and it does seem to affect it, but I can't
> understand the exactly relationship. Can anyone tell me what the
> argument controls and how?

This is the C source, if you haven't looked at it yet.

BigDecimal_sqrt(VALUE self, VALUE nFig)
{
ENTER(5);
Real *c, *a;
S_INT mx, n;

GUARD_OBJ(a,GetVpValue(self,1));
mx = a->Prec *(VpBaseFig() + 1);

n = GetPositiveInt(nFig) + VpDblFig() + 1;
if(mx <= n) mx = n;
GUARD_OBJ(c,VpCreateRbObject(mx, "0"));
VpSqrt(c, a);
return ToValue(c);
}

I don't fully understand what's going on with it, because my C is
pretty weak, but hopefully this will provide some insight from
someone.

mathew

12/28/2005 4:26:00 AM

0

James Edward Gray II wrote:
> Let me try asking me question a different way... Is this the correct
> way to get a printable sqrt() with 16 digits precision?
>
> >> sprintf "%.16f", BigDecimal.new("66543").sqrt(16)
> => "257.9592991151890260"

Yes.

I've written some documentation for BigDecimal which will hopefully be
checked in to Ruby some time.


mathew
--
<URL:http://www.pobox.com/...
My parents went to the lost kingdom of Hyrule
and all I got was this lousy triforce.

James Gray

12/28/2005 1:42:00 PM

0

On Dec 27, 2005, at 10:27 PM, mathew wrote:

> James Edward Gray II wrote:
>> Let me try asking me question a different way... Is this the
>> correct way to get a printable sqrt() with 16 digits precision?
>> >> sprintf "%.16f", BigDecimal.new("66543").sqrt(16)
>> => "257.9592991151890260"
>
> Yes.
>
> I've written some documentation for BigDecimal which will hopefully
> be checked in to Ruby some time.

Is it available online anywhere before then or could I trouble you to
send it to me privately?

James Edward Gray II