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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: 'Base
Date: Sat, 10 Dec 2005 08:52:45 +0100
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Date: 2005-12-10T08:52:45+01:00
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ada_student@yahoo.com wrote:

> Indefinite subtypes cannot be allowed under any circumstances.
> It sounds like a hack just inorder to allow one class of array
> object declarations into the language.

No at all. Ada is a higher level language then C/C++ or Java. It allows for
better abstraction  which is closer to real live. In real live arrays
seldom beginn with zero:

Weekdays 1 .. 7
Month 1 .. 31
Year 1 .. 366

Ada allows integer and array type for all those circumstances.

> How can a statically typed language allow a type whose size is not
> known at compile time?

Quite easily. The size is just calculated at run time and the memory needed
is then allocated staticly, on stack or on heap. The actual size becomes
part of the array itself.

In lower level languages this tedious work is dumped onto the programmer. 

> The Java array type certainly wins over Ada in this regard.

Really? How about:

type Historic_Data is array (-6000 .. 2005) of Data_Entry;

Java (and C/C++) only have zero based arrays. In my eye that allway looses
over abritary array bound. "Off by one error" is a well known term in those
languages. 

To represent an Ada array in Java you would need:

class Array_Type
   {
   private int First;
   private int Last;
   private Data Elements [];

   Data get_Element (int Index)
     {
     if (Index > First) then throw ....
     if (Index < Last) then throw ....
     return Data [Index - First];
     } 
   }

Ada arrays are not a primitive type - they are a quite complex and powerfull
type.

Performace? Ada allways outperform Java for a start. But Ada's build in
checks will also outperform C/C++ when used with all appropriate
"assert()"s in place.

For details on why read:

http://en.wikibooks.org/wiki/Computer_programming/Error_handling
http://en.wikibooks.org/wiki/Ada_Programming/Error_handling

Especially chapter 5 on "Design by Contract".

Martin
-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com