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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Cannot summate small float values
Date: Sat, 20 Nov 2010 15:49:16 +0200
Organization: Tidorum Ltd
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Date: 2010-11-20T15:49:16+02:00
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tolkamp wrote:
> When I summate Float values smaller then 1.0E-6 then the summation is
> not done.
> 
> Code Example:
> 
> X, Dx : Float;
> X := 0.0;
> Dx := 1.0E-7;
> lwhile X <  1.0 loop
>     X = X + Dx;
>     Float_Io.Put(X, 3,9,0); New_Line;
> end loop;

Certainly the addition is done. Your program (after some small syntactic 
corrections) prints:

   0.000000100
   0.000000200
   0.000000300
   0.000000400
   0.000000500
   0.000000600
   0.000000700

and so on. If your program prints out something else, please show the 
source code of your whole program, exactly as you compile and run it. 
Don't re-type it into your message.

However, when X approaches 1.0, at some point the addition of 1.0E-7 may 
be lost in round-off, since it is close to the precision limit of the 
Float type, relative to 1.0. On my system (Debian, Gnat) the X variable 
does reach 1.0 and the program stops.

What are you really trying to do? There are probably safer and more 
accurate ways of doing it.

Here is the program that I used:

with Ada.Text_IO;
with Ada.Float_Text_IO;

procedure Sums
is
    use Ada.Text_IO, Ada.Float_Text_IO;
    X, Dx : Float;
begin
    X := 0.0;
    Dx := 1.0E-7;
    while X <  1.0 loop
        X := X + Dx;
        Put(X, 3,9,0); New_Line;
    end loop;
end Sums;

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .