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From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga.tech
Subject: Re: C question
Message-ID: <8810112106.AA14840@cory.Berkeley.EDU>
Date: 11 Oct 88 21:06:22 GMT
Article-I.D.: cory.8810112106.AA14840
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 26

:I have stumbled onto one of those C mystery problems, using Lattice 4.01.
:
:INT_MAX is a #define for the largest int.  In my code I have
:
:     r1 = 1.0/(double)INT_MAX
:
:and
:
:     double im = INT_MAX;
:     r2 = 1.0/im;
:
:Guess what?  r1 and r2 get different values.  How come?
:
: .. only FFP

	The reason is that the first line, 1.0/(double)INT_MAX is a 
constant expression which is evaluated at compile time, using the 
maximum precision available to the compiler.

	The second line is evaluated run-time by the FFP library.  Since
the FFP representation of the maximum-sized integer looses some
accuracy (you cannot represent a 32 bit-sized non-power-of-2 integer
with a 32 bit FFP quantity without some error), the evaluation comes
out slightly flawed.

						-Matt