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Newsgroups: comp.lang.ada
Date: Wed, 22 Jan 2014 14:45:22 -0800 (PST)
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Subject: Re: How to round to the nearest fixed-point value?
From: adambeneschan@gmail.com
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Xref: news.eternal-september.org comp.lang.ada:18253
Date: 2014-01-22T14:45:22-08:00
List-Id: <comp.lang.ada>

On Wednesday, January 22, 2014 8:48:26 AM UTC-8, Natasha Kerensikova wrote:

> package Lib is
>    type High is delta 0.001 digits 9;
>    type Low is delta 0.01 digits 9;
>
>    function Convert (Value : High) return Low;
> end Lib;
> 
> package body Lib is
>    function Convert (Value : High) return Low is
>    begin
>       return Low'Round (Value);
>    end Convert;
> end Lib;

> with Ada.Text_IO;
> with Lib;
> procedure Testcase is
>    Raw_Value : constant Lib.High := 0.999;
>    Shown_Value : Lib.Low;
> begin
>    Ada.Text_IO.Put_Line (Lib.Low'Image (Lib.Low'Round (Raw_Value)));
>    Shown_Value := Lib.Convert (Raw_Value);
>    Ada.Text_IO.Put_Line (Lib.Low'Image (Shown_Value));
> end Testcase;

>  1.00
>  0.99

I'm pretty sure this is a compiler bug.  Since Low'Small is a multiple of High'Small, you can work around it by changing the definition of Convert to

    return Low (Value + Low'Small / 2);

(Low'Small is a universal real and therefore Low'Small/2 will be interpreted as a value of type High.  The type conversion to Low will truncate.)

                            -- Adam