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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Conversion from floating point to signed 16 bits
Date: Tue, 19 May 2009 12:37:46 -0700 (PDT)
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Date: 2009-05-19T12:37:46-07:00
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On May 19, 11:51=A0am, Olivier Scalbert <olivier.scalb...@algosyn.com>
wrote:
> 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
> =A0 =A0 =A0type Analog_Value is digits 10 range 0.0 .. 1.0;
> =A0 =A0 =A0type Signed_16 =A0 =A0is range -32768 .. 32767;
> =A0 =A0 =A0type Unsigned_16 =A0is range =A0 =A0 =A00 .. 65535;
>
> =A0 =A0 =A0function Cv(Value: Analog_Value) return Signed_16 is
> =A0 =A0 =A0 =A0 =A0U16: Unsigned_16;
> =A0 =A0 =A0begin
> =A0 =A0 =A0 =A0 =A0U16 :=3D Unsigned_16(65535.0 * Value);
> =A0 =A0 =A0 =A0 =A0return Signed_16(U16 - 32768);
> =A0 =A0 =A0end Cv;
>
> =A0 =A0 =A0procedure Put(S16: Signed_16) is
> =A0 =A0 =A0begin
> =A0 =A0 =A0 =A0 =A0Ada.Text_IO.Put_Line(Signed_16'image(S16));
> =A0 =A0 =A0end put;
> begin
> =A0 =A0 Put(Cv(0.00)); -- Must be -32768
> =A0 =A0 Put(Cv(0.25)); -- Must be -16384
> =A0 =A0 Put(Cv(0.50)); -- Must be =A0 =A0 =A00
> =A0 =A0 Put(Cv(0.75)); -- Must be =A016383
> =A0 =A0 Put(Cv(1.00)); -- Must be =A032767
> end convert;
> -----------------------------
>
> My question:
> Is there an other way to do this in Ada (Representation ? Other parts of
> Ada I do not know ?)

If your problem is that you're getting a Constraint_Error when the
subtraction "U16 - 32768" has a negative result, you can convert U16
to some larger integer type before doing the subtraction:

  return Signed_16(Signed_32(U16) - 32768);

                              -- Adam