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NNTP-Posting-Date: Wed, 13 Dec 2006 10:53:06 -0600
From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Comparing Floating Point Values
References: <1166026474.277616.267400@f1g2000cwa.googlegroups.com>
Date: Wed, 13 Dec 2006 17:51:05 +0100
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Date: 2006-12-13T17:51:05+01:00
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markp writes:
> I was wondering if there was a convention for comparing 2 floats
> dealing with significant digits. For example, if I had 2 variables (x
> and y) and wanted to see if they were equal in an if statement, is
> there a standard way to do a compare based on "x" significant digits?

The way we usually do this is by defining our own floating-point types
and redefine the "=" operator to deal with the precision appropriate
for the type and the application.  For example:

type Kilograms is digits 6 range 0.0 .. 600_000.0;

function "=" (L, R : Kilograms) is
   Epsilon : constant Kilograms := 0.1;
begin
   if L < R then
      return R - L < Epsilon;
   else
      return L - R < Epsilon;
   end if;
end "=";

The above uses an absolute precision; it would be trivial to use a
relative precision instead.

HTH

-- 
Ludovic Brenta.