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From: =?UTF-8?Q?Bj=c3=b6rn_Lundin?= <b.f.lundin@gmail.com>
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Subject: Fixed vs float and precision and conversions
Date: Tue, 7 Jul 2020 23:10:20 +0200
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Hi !

I've for years run an interface towards extenal part on a raspberry pi
that communicates with JSON over http (JSONRPC2)

the versions are

bnl@pibetbot:~ $ gnatls -v

GNATLS 6.3.0
Copyright (C) 1997-2016, Free Software Foundation, Inc.
...

bnl@pibetbot:~ $ uname -a
Linux pibetbot 4.14.79-v7+ #1159 SMP Sun Nov 4 17:50:20 GMT 2018 armv7l 
GNU/Linux
bnl@pibetbot:~ $ cat /etc/deb
debconf.conf    debian_version
bnl@pibetbot:~ $ cat /etc/deb
debconf.conf    debian_version
bnl@pibetbot:~ $ cat /etc/debian_version
9.4


I have found this reliable but suddenly I have got some rounding 
troubles. Or I perhaps just discovered it now.


I have a fixed type
   type Fixed_Type is delta 0.001 digits 18;

but JSON does not support that. So I get floats instead.


I use gnatcoll.json as parser by the way


   procedure Get_Value(Container: in    Json_Value;
                       Field    : in     String;
                       Target   : in out Fixed_Type;
                       Found    :    out Boolean ) is
     Tmp : Float := 0.0;
   begin
     if Container.Has_Field(Field) then
       Tmp := Container.Get(Field);
       Found := True;
       Target := Fixed_Type(Tmp);
     else
       Found := False;
     end if;
   end Get_Value;


The message (JSON) contains a value 5.10 (in a float), but that is 
converted (sometimes I think) to the fixed_type variable with value 
5.099. This gets me into trouble further down in the code.

So - What should I do instead?

should I express my Fixed_Type in another way?

I need to be able to express 6.5 % (0.065) which I could not with
type Fixed_Type is delta 0.01 digits 18;




-- 
Björn