Re: Checking to see if a string is a letter

[Martin Dowie <martin@re.mo.ve.thedowies.com>]

deuteros <deuteros@xrs.net> wrote:
> On Tue 03 Apr 2012 04:26:40a, Simon Wright <simon@pushface.org> wrote in
> news:m2k41xi5kv.fsf@pushface.org: 
> 
>> deuteros <deuteros@xrs.net> writes:
>> 
>>> On Tue 03 Apr 2012 01:15:27a, Jeffrey Carter
>>> <spam.jrcarter.not@spam.not.acm.org> wrote in
>>> news:jle13q$ale$1@tornado.tornevall.net: 
>>> 
>>>> What do you mean by "contains a single letter"?
>>> 
>>> I mean the string contains a single letter and nothing more. For
>>> example:
>>> 
>>> a  - Legal
>>> A  - Legal
>>> aa - Illegal
>>> a1 - Illegal
>> 
>> Then for a start the length of the string needs to be 1.
>> 
>> If it is, the first (and only!) character needs to be a lower- or
>> upper-case letter. There are (at least) three ways of doing this:
>> 
>> * declare an array of Boolean indexed by Character, with the elements
>>   indexed by letters set to True and the others to False, and index by
>>   the character to be tested;
>> 
>> * declare two subtypes of character ("Character range 'a' .. 'z'", for
>>   instance) and check whether the character to be tested is 'in' either
>>   of the subtypes;
>> 
>> * use the standard library, Ada.Characters.Handling.Is_Letter (probably
>>   the easiest for you!)
> 
> 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"

See http://www.ada-auth.org/standards/12rm/html/RM-3-6-3.html to see how
String is defined.

From this you can see that token may not have a value at "token (1)",
perhaps it is indexed by 10..20.

If you know that all the strings you'll ever pass to this function will
start with an index = 1, then you could add a "pragma Assert (tokenFirst =
1);" at the start of the declarative part. An exception will then be raised
if this isn't true (assuming the correct compiler switches are selected).

NB: ParserException is declared locally, so isn't visible to any subprogram
that could attempt to catch it...you'd need to use "when others =>".

HTH
-- Martin



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