Re: Spurious long double accuracy
Re: Spurious long double accuracy
- Subject: Re: Spurious long double accuracy
- From: Greg Guerin <email@hidden>
- Date: Sun, 10 Dec 2006 10:17:34 -0700
Tommy Nordgren wrote:
>The predefined compiler macro __LDBL_EPSILON__ is defined to be
>4.94066e-324 Recomputing this constant from definition gives the same
>value. Now the formal definition for epsilon in respect to floating point
>calculation, is the smallest positive floating point value that when added
>to 1.0 gives a value that is not 1.0 with machine precision. I wonder what
>is going on here, since both defined and computed values of this constant
>implies a accuracy in floating point calculations with approximately 300
>more significant digits than available in long double format.
It's a direct consequence of the machine representation. See this
reference, under the "Epsilon" heading:
<http://publib.boulder.ibm.com/infocenter/pseries/v5r3/topic/com.ibm.aix.genprogc/doc/genprogc/128bit_long_double_floating-point_datatype.htm>
The text above that heading is also worth reading. In essence, long double
doesn't have a fixed precision. The precision floats depending on value.
I easily found that page googling for these keywords:
long double C 128 bit
-- GG
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