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From: tolkamp <f.tolkamp@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Cannot summate small float values
Date: Sun, 21 Nov 2010 13:06:47 -0800 (PST)
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Date: 2010-11-21T13:06:47-08:00
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On 20 nov, 14:49, Niklas Holsti <niklas.hol...@tidorum.invalid> wrote:
> tolkamp wrote:
> > When I summate Float values smaller then 1.0E-6 then the summation is
> > not done.
>
> > Code Example:
>
> > X, Dx : Float;
> > X :=3D 0.0;
> > Dx :=3D 1.0E-7;
> > lwhile X < =A01.0 loop
> > =A0 =A0 X =3D X + Dx;
> > =A0 =A0 Float_Io.Put(X, 3,9,0); New_Line;
> > end loop;
>
> Certainly the addition is done. Your program (after some small syntactic
> corrections) prints:
>
> =A0 =A00.000000100
> =A0 =A00.000000200
> =A0 =A00.000000300
> =A0 =A00.000000400
> =A0 =A00.000000500
> =A0 =A00.000000600
> =A0 =A00.000000700
>
> 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
> =A0 =A0 use Ada.Text_IO, Ada.Float_Text_IO;
> =A0 =A0 X, Dx : Float;
> begin
> =A0 =A0 X :=3D 0.0;
> =A0 =A0 Dx :=3D 1.0E-7;
> =A0 =A0 while X < =A01.0 loop
> =A0 =A0 =A0 =A0 X :=3D X + Dx;
> =A0 =A0 =A0 =A0 Put(X, 3,9,0); New_Line;
> =A0 =A0 end loop;
> end Sums;
>
> --
> Niklas Holsti
> Tidorum Ltd
> niklas holsti tidorum fi
> =A0 =A0 =A0 =A0. =A0 =A0 =A0@ =A0 =A0 =A0 .

Thank you your reaction.
Using your procedure Sums I found out that when the start value of X <
0.24 the summation works correct with Dx =3D 1.0E-8
When start X > 0.25 the summation remains 0.250000000.