char[] annoyance...

[Regan Heath]

On Mon, 10 Apr 2006 11:58:09 +1000, Derek Parnell <derek at psych.ward> wrote:
> On Mon, 10 Apr 2006 12:23:06 +1200, Regan Heath wrote:
>
>> Take this code:
>>
>> void main()
>> {
>> 	//..open a file, read line for line, on each line:
>>
>> 	for(int i = 0; i < line.length-2; i++) {
>> 		if (line[i..i+2] != "||") continue;
>> 		//..etc..
>> 	}
>> }
>>
>> There is a subtle bug. On all lines with a length of 0 or 1 it will give
>> the following error:
>>
>> Error: ArrayBoundsError line_length.d(6)
>>
>> The problem is of course the statement "i < line.length-2". line.length  
>> is
>> unsigned, and when you - 2 from an unsigned value.. well lets just say
>> that it's bigger than the actual length of the line - 2.
>>
>> Of course there are plently of other ways to code this, perhaps using
>> foreach, but that's not the point. The point is that this code  _can_ be
>> written and on the surface looks fine. Not even -w (warnings) spots the
>> signed/unsigned problem. At the very least can we get a warning for  
>> this?
>
> I too have tripped up on this 'bug' and it is very annoying and  
> surprising.
>
> However your approach to the problem might need to change...as you state  
> -
> "On all lines with a length of 0 or 1 it will give the following  
> error..."
> - and this is because the test that you are performing is only applicable
> to lines with two or more characters in them ... so make that condition a
> part of the algorithm ...
>
>   if (line.length >= 2) {
>     for(int i = 0; i < line.length-2; i++) {
>       if (line[i..i+2] != "||") continue;
>       //..etc..
>     }
>   }
>   else {
>      // do something for short lines...
>   }

Thanks Derek, that's exactly what I did.

The point of this post isn't to get help with one specific problem but  
rather to ask whether there is a solution to the underlying problem  
behaviour. I suspect this behaviour will be the source of many bugs to  
come and I wonder if there is a way to avoid them.

Regan