Re: character literals

[agent@drrob1.com]

On Tue, 11 Feb 2014 18:03:18 -0800 (PST), adambeneschan@gmail.com
wrote:

>On Tuesday, February 11, 2014 5:34:20 PM UTC-8, ag...@drrob1.com wrote:
>
>> But now I don't know why this function is failing.  I want to convert
>> a character value to its integer value.  Modula-2 used the built-in
>> function ORD for this purpose.
>> 
>> This fails type checking
>> 
>>   I := Natural'Value(CH);
>
>To get the ORD of a character:
>
>    I := Character'Pos (Ch);   
>        -- short for "position", i.e. the position of the character literal in        
>        -- the list of enumeration literals, which for Character is just the
>        -- list of all 256 Extended ASCII characters in order
>
>If Ch is '0', then I will be 48 (the ASCII value of the character '0').
>
>To go the other way:
>
>    Ch := Character'Val (I);
>
>Natural'Value does something completely different.  It takes a string, parses it, and expects the result to be a "Natural".  It raises an exception if the string doesn't have the right format for a "Natural" or is an integer that's out of range.  
>
>In your code, Natural'Value should work fine if Ch is in the range '0' .. '9', because then S will be a validly formatted Natural.  The result will be in the range 0 to 9, not 48 to 57 as Character'Pos (and I think ORD) would give you.  (I don't know Modula-2, but ORD would return 48..57 in Pascal.)  If Ch is something outside that range, then Natural'Value will raise Constraint_Error.  If you think Ch is really in the range '0' .. '9', but you're still getting Constraint_Error on the Natural'Value call, then I'd need to see more of the code.
>
>                                -- Adam

Thanks, that clears it up for me.