Re: Strings and NSRanges

On 20 Nov 2006, at 08:24, Richard Frith-Macdonald wrote:

>
> On 19 Nov 2006, at 15:45, Graham J Lee wrote:
>
>> So I'm doing this:
>>
>>>   if ([self allowsFloats] && (NSNotFound != [useFormat  
>>> rangeOfString:@"." ].location))
>>>     {
>>>       decimalPlaceRange = [useFormat rangeOfString: @"." options:  
>>> NSBackwardsSearch];
>>>       while ([placeHolders characterIsMember: [useFormat  
>>> characterAtIndex: NSMaxRange(decimalPlaceRange)]])
>>>         {
>>>           decimalPlaceRange.length++;
>>>           if (NSMaxRange(decimalPlaceRange) == [useFormat length])
>>>             break;
>>>         }
>>>       decimalPlaces=decimalPlaceRange.length;
>>>       if (0 != decimalPlaces)
>>>         displayFractionalPart = YES;
>>>     }
>>
>> which doesn't work...decimalPlaces ends up being some garbage  
>> value such as 2412439 when the length of the string is only, say,  
>> 7.  For that to happen then the length of the *range* must have  
>> always been greater than the length of the string (so that the if 
>> (NSMaxRange(...)...) line never breaks the loop) and the loops  
>> *starts* by reading nonsense memory outside the string.  Why would  
>> that happen?
>
> It's hard to tell without a complete test program ... but I can  
> make a couple of observations.
>
> 1. the loop should not be able to start by 'reading nonsense memory  
> outside the string' because the -characterAtIndex: method should  
> raise an exception if asked to refer to an index outside the string.
> 2. if useFormat is nil then the code is totally broken since  
> sending a message to a nil object is defined to return nil ... but  
> where the method is expected to return a range this means that the  
> resulting behavior is undefined.  On sparc it will cause an  
> immediate crash, but on intel I think it probably just means the  
> returned range contains garbage ... which would explain your symptoms.
>
>
Got it in two that man :-).  I'd overlooked that _[posi|nega] 
tiveFormat don't get set to defaults in the -init method so could be  
nil.

Thanks,
Graham.
-- 
Graham J Lee
http://www.thaesofereode.info/