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From: "Stuart" <stuart@0.0>
Newsgroups: comp.lang.ada
References: <4a12ffa3$0$2853$ba620e4c@news.skynet.be>
 <4a13baef$1_1@glkas0286.greenlnk.net>
Subject: Re: Conversion from floating point to signed 16 bits 
Date: Wed, 20 May 2009 09:48:34 +0100
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Date: 2009-05-20T09:48:34+01:00
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"Stuart" <stuart@0.0> wrote in message 
news:4a13baef$1_1@glkas0286.greenlnk.net...

> "Olivier Scalbert" <olivier.scalbert@algosyn.com> wrote in message 
> news:4a12ffa3$0$2853$ba620e4c@news.skynet.be...

>> 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.
...
>> 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;
...
>> begin
...
>>    Put(Cv(0.50)); -- Must be      0
...
>> end convert;


> 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;

Ooops!

Unfortunately, due to Ada rounding rules this fails your requirement for 
Cv(0.50) as it returns -1.

In my defence I plead that we got very annoyed by the overhead of Ada 
rounding in a similar scenario that we rigged our conversions and don't see 
this problem.  You can might also weedle  your way around the effect of the 
rounding by trying something like:

function Cv(Value: Analog_Value) return Signed_16 is
begin
    return Signed_16(65535.0*Value - 32767.9999);
end Cv;

You could probably _derive_ a value to replace -32768.0 by consideration of 
the rounding effect over the range of inputs 0.0 .. 1.0.

Regards
   Stuart