Re: Checking to see if a string is a letter

[Adam Beneschan <adam@irvine.com>]

On Thursday, April 5, 2012 10:12:42 AM UTC-7, deuteros wrote:
> 
> Alright, here's my function:
> 
>    function isVariable(token: in String) return Boolean is
>       ParserException : Exception;
>    begin
>       if(token'Length = 1) then
>          return (Is_Letter(token(1)));
>       end if;
> 
>       raise ParserException with ("Not a letter : " & token);
>      
>    end isVariable;
> 
> But I'm getting this warning:
> 
> warning: index for "token" may assume lower bound of 1
> warning: suggested replacement: "token'First + -1"

The warning hints at the problem; if something is declared as a "string" in Ada, the lower bound is not fixed at 1 but could be any positive integer.  You can get the lower bound with Token'First and the upper bound with Token'Last, so Is_Letter(Token(Token'First)) is what you want.  (The "suggested replacement" is wrong--it's off by one, in addition to being syntactically incorrect.  Someone needs to fix the compiler.)

Suppose that outside of isVariable, another procedure does this:

   Line : String (1..500);
...
   if isVariable (Line (362..362)) then blah-blah-blah...

then inside isVariable, Token'First and Token'Last will both be 362.  The caller is taking a *slice* of another string, and the bounds used for the slice become the bounds of the resulting string.
   
Also, note that in general, if S is a String, S(S'First) may raise an exception if S could be an empty (length 0) string.  In your example, you already ruled that out with the "if" statement.

Some other notes:
(1) You don't need parentheses around the expressions in the IF statement, the RETURN statement, or the RAISE statement, although they are harmless.  (2) The Ada fans on this newsgroup are going to roll their eyes at the names you picked (like "isVariable").  We tend to like putting underscores between words (or word-like parts) and capitalizing the first letter of each word, e.g. Is_Variable.  Also: you're probably already aware of this, but in case you aren't, Ada does *not* treat lower- and upper-case letters in identifiers as different, unlike C and some other languages.

                      -- Adam