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From: Leslie <jlturriff@centurytel.net>
Newsgroups: comp.lang.ada
Subject: Re: How to convert a string containing two hex digits to a character?
Date: Tue, 05 Jan 2010 20:39:00 -0600
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Date: 2010-01-05T20:39:00-06:00
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Hibou57 (Yannick Duchêne) wrote:

> You may first convert from two 4 bits digits, that is, convert
> "7" and "C".
> 
> 16#7#; -- 7
> 16#C#; -- 12
> 
> To get 16#7C# from 16#7# and 16#C#, just do ((16#7# * 16) +
> 16#C#)
> 
> To get 16#7# and 16#C#, first get '7' and 'C' from the string,
> you may do something like the following (providing your string
> is named S, and it is an ASCII string) :
> 
>    High_Order_Digit_Position : constant Positive := S'First;
>    Low_Order_Digit_Position : constant Positive :=
>    Positive'Succ
> (High_Order_Digit_Position);
>    High_Order_Digit_Representation : constant Character := S
> (High_Order_Digit_Position);
>    Low_Order_Digit_Representation : constant Character := S
> (Low_Order_Digit_Position);
> 
> To get a digit Natural value from its Character representation,
> you may do something like the following (providing your
> character is stored in a Character named C) :
> 
>    case C is
>       when '0' .. '9' =>
>          Digit_Value := 0 + (Character'Pos (C) - Character'Pos
>          ('0'));
>       when 'A' .. 'F' =>
>          Digit_Value := 10 + (Character'Pos (C) - Character'Pos
> ('A'));
>       when 'a' .. 'f' =>
>          Digit_Value := 10 + (Character'Pos (C) - Character'Pos
> ('a'));
>       when others =>
>          raise Program_Error;
>    end case;
> 
> Wrap the latter in a function like “ function
> Hexadecimal_Digit_Value (C : Character) return Natural; ”
> 
> Then do :
> 
>    Hexadecimal_Base : constant Natural := 16;
>    High_Order_Digit : constant Natural :=
>    Hexadecimal_Digit_Value
> (High_Order_Digit_Representation);
>    Low_Order_Digit : constant Natural :=
>    Hexadecimal_Digit_Value
> (Low_Order_Digit_Representation);
>    Number : constant Natural := (High_Order_Digit *
>    Hexadecimal_Base)
> + Low_Order_Digit;

        Okay, wait now; where did Hexadecimal_Digit_Value come
from? It should be declared outside of the function, and is
different than Decimal_Value, is that right?

Leslie