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From: stefan-lucks@see-the.signature
Newsgroups: comp.lang.ada
Subject: Re: Efficient Sequential Access to Arrays
Date: Thu, 26 Jul 2012 21:21:09 +0200
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Date: 2012-07-26T21:21:09+02:00
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On Thu, 26 Jul 2012, Keean Schupke wrote:

> What is the literal for 'invalid', IE:
> 
> X : Index := #INVALID -- we should be able to initialise to invalid.

Of course, you cannot *assign* an invalid value (out of the range of the 
(sub-)type of X) to X, at least not without Unchecked_Conversion or any 
other dirty tricks.

But you can use pragma Normalize_Scalars to ensure X is set to invalid:
<http://en.wikibooks.org/wiki/Ada_Programming/Pragmas/Normalize_Scalars>. 

There is also pragma Initialize_Scalars, but I believe it is 
implementation defined for gnat, not standard Ada. 

> if X = #INVALID -- I want to be able to test for invalid.

As already answered by Brian Drummond, this is "not X.Valid". 



-- 
---- Stefan.Lucks (at) uni-weimar.de, University of Weimar, Germany  ----
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------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------