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From: f.couperin
Subject: [BUG REPORT] Reading LongReal
Date: 25 Mar 2001, 11:54:55

Frank Heckenbach wrote :

[...]
> So, why is B slighly inaccurate? Well, if you look at the file, it
> contains the text:
>
>  1.030000000000000e+02
>
> And that's exactly what ReadLn sees. I.e., first the number 1.03
> which cannot be represented exactly since floating point numbers (at
> least IEEE and most other popular formats) are binary, not decimal,
> and 1.03 in binary is periodic.
>
> Afterwards, it finds `e+02' and multiplies the result by 100 which
> can be done exactly, but the error was already made.
[...]

Is this really computer dependent or does it depend of the program ?
I think that reading 1.030000000000000e+02 as
1030000000000000 * 10^(-15+2) would suppress the problem.
Wouldn't it be better ?

What I don't understand is that there is no problem if Readln reads
10.3e+01 in the file (10.3 isn't periodic in binary). And if it reads
103000e-03 (103000 IS periodic), the value is slightly wrong...
(I tested this strange behaviour with the program shown below.)

I'm wondering if there's a way to write a binary (or hexadecimal)
representation of the LongReal type in a text file, a clear and
understandable representation that could be read back by the program
without any difference with the original number.
I wrote LongReal numbers to a File Of LongReal, and I translated this
file into an hexadecimal representation, but the result was really
unclear to me and I didn't understand the logic behind those numbers.
I found and read this web page :
http://www.infobiogen.fr/docs/info/SUNWspro/pascal/lang_ref/ref_data.doc.htm
l
(about another Pascal compiler)
but it seems the GPC LongReal type is different from what is described
there.

Program Bug09;
Var
  A,B,C,D,E,F,G,H,I :  LongReal;
  TextFile : Text;
Begin
  Assign(TextFile,'tmp');
  Rewrite(TextFile);
  Writeln(TextFile,'1030000e-04');
  Writeln(TextFile,'103000e-03');
  Writeln(TextFile,'10300e-02');
  Writeln(TextFile,'1030e-01');
  Writeln(TextFile,'103');
  Writeln(TextFile,'10.3e+01');
  Writeln(TextFile,'1.03e+02');
  Writeln(TextFile,'0.103e+03');
  Writeln(TextFile,'0.0103e+04');
  Close(TextFile);
  Reset(TextFile);
  Readln(TextFile,A);
  Readln(TextFile,B);
  Readln(TextFile,C);
  Readln(TextFile,D);
  Readln(TextFile,E);
  Readln(TextFile,F);
  Readln(TextFile,G);
  Readln(TextFile,H);
  Readln(TextFile,I);
  Close(TextFile);
  Erase (TextFile);
  Writeln;
  Writeln('A = ',A:28); {Inaccurate}
  Writeln('B = ',B:28); {Inaccurate}
  Writeln('C = ',C:28); {OK}
  Writeln('D = ',D:28); {OK}
  Writeln('E = ',E:28); {OK}
  Writeln('F = ',F:28); {OK}
  Writeln('G = ',G:28); {Inaccurate}
  Writeln('H = ',H:28); {Inaccurate}
  Writeln('I = ',I:28); {Inaccurate}
End.

--
«Couperin»

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