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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Beginners question
Date: Fri, 12 Nov 2021 17:24:17 +0200
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On 2021-11-12 16:32, uq5huo...@gmail.com wrote:
> Hi,
> 
> I'm beginning to play with Ada, and run into this
> 
> with Ada.Text_IO; use Ada.Text_IO;
> procedure Learn is
>     subtype Alphabet is Character range 'A' .. 'Z';
> begin
>     Put_Line ("Learning Ada from " & Alphabet'First & " to " & Alphabet'Last);
> end Learn;
> 
> Now I want to play a bit with the code ... and this fails and I don't get why and how to solve.
> 
> Put_Line ("Learning Ada from " & Alphabet'First & " to " & Alphabet'Last-1);
> or
> Put_Line ("Learning Ada from " & Alphabet'First & " to " & (Alphabet'Last-1));
> 
> Anyone more expirienced can explain this to my, please?


The Character type is not an integer type (unlike "char" in C), so you 
cannot subtract 1 from a Character (or an Alphabet value). Character is 
an enumeration type in Ada.

To get the Alphabet value immediately preceding Alphabet'Last, use the 
Pred ("predecessor") attribute function:

    Alphabet'Pred (Alphabet'Last)

If you need to take larger strides in the Alphabet enumeration, you can 
use the Pos and Val attribute functions. Pos returns the "position 
number" of its argument, and that is an integer. Val performs the 
opposite mapping. For example, to get the Alphabet value that is three 
positions before Alphabet'Last, you can do

    Alphabet'Val (Alphabet'Pos (Alphabet'Last) - 3)