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From: "Stuart" <stuart@0.0>
Newsgroups: comp.lang.ada
References: <4a12ffa3$0$2853$ba620e4c@news.skynet.be>
Subject: Re: Conversion from floating point to signed 16 bits 
Date: Wed, 20 May 2009 09:29:56 +0100
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Date: 2009-05-20T09:29:56+01:00
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"Olivier Scalbert" <olivier.scalbert@algosyn.com> wrote in message 
news:4a12ffa3$0$2853$ba620e4c@news.skynet.be...
> Hello,
>
> My problem:
> I need to convert an "analogic" value which can vary from 0.0 to 1.0 into 
> a "discrete" value which is a signed 16 bits integer.
>
> My implementation:
> -----------------------------
> with Ada.Text_IO;
>
> procedure convert is
>     type Analog_Value is digits 10 range 0.0 .. 1.0;
>     type Signed_16    is range -32768 .. 32767;
>     type Unsigned_16  is range      0 .. 65535;
>
>     function Cv(Value: Analog_Value) return Signed_16 is
>         U16: Unsigned_16;
>     begin
>         U16 := Unsigned_16(65535.0 * Value);
>         return Signed_16(U16 - 32768);
>     end Cv;
>
>     procedure Put(S16: Signed_16) is
>     begin
>         Ada.Text_IO.Put_Line(Signed_16'image(S16));
>     end put;
> begin
>    Put(Cv(0.00)); -- Must be -32768
>    Put(Cv(0.25)); -- Must be -16384
>    Put(Cv(0.50)); -- Must be      0
>    Put(Cv(0.75)); -- Must be  16383
>    Put(Cv(1.00)); -- Must be  32767
> end convert;
> -----------------------------
>
> My question:
> Is there an other way to do this in Ada (Representation ? Other parts of 
> Ada I do not know ?)

Extending your [reasonable] assumptions about the base type of Analog_Value 
you could dispense with the Unsigned_16 altogether:

   function Cv(Value: Analog_Value) return Signed_16 is
   begin
      return Signed_16(65535.0*Value - 32768.0);
   end Cv;

If you really wanted to you could formally codify the assumptions by 
declaring a working base, then make Analog_Value a subtype of that:

   type Working_Base is digits 10 range -32768.0 .. 65535.0;
   subtype Analog_Value is Working_Base range 0.0 .. 1.0;

Regards
   Stuart