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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 20:42:44 -0600
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Date: 2010-01-05T20:42:44-06:00
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Leslie wrote:

> 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?
> 
        Gak! Never mind, that's the function call. :-p
> Leslie