Re: Cannot summate small float values

[tolkamp <f.tolkamp@gmail.com>]

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 := 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
>        .      @       .

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 = 1.0E-8
When start X > 0.25 the summation remains 0.250000000.