Re: character literals

[adambeneschan@gmail.com]

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