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From: Warren <ve3wwg@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: S'Is_nan or S'Is_inf?
Date: Fri, 16 Jul 2010 22:35:14 +0000 (UTC)
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Date: 2010-07-16T22:35:14+00:00
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Dmitry A. Kazakov expounded in
news:1tbp3geoa5yna$.171cmlfdrbm98$.dlg@40tude.net: 

> with Ada.Text_IO;  use Ada.Text_IO;
> procedure IEEE is -- Only if Float is IEEE!
>    Zero : Float := 0.0;
>    Inf  : Float := 1.0 / Zero;
>    NaN  : Float := 0.0 / Zero;
> begin
>    Put_Line ("Valid    " & Boolean'Image (Inf'Valid));
>    Put_Line ("In range " & Boolean'Image (Inf <= Float'Last));
>    Put_Line ("Self NaN " & Boolean'Image (NaN = NaN));
> end IEEE;
> 
> On an IEEE machine it could print 3x FALSE.

To detect NaN then, this seems to work:

    function Is_Nan(F : Float) return Boolean is
    begin
        if not F'Valid then
            return not ( Is_Infinity(F) or Is_Neg_Infinity(F) );   
        else
            return False;
        end if;
    end Is_Nan;

where Is_Infinity(F) etc. is implemented as suggested. Essentially
if not valid, but not one of the infinities(+ or -), then it 
must be NaN. This works with Gnat on Cygwin. I'll need
to test it on other platforms, but hopefully most if not all
IEEE platforms will support this.

Warren