[GRASS-user] v.dissolve and r.cost problems

[Moritz Lennert]

On 02/04/09 16:58, Markus Neteler wrote:
> On Thu, Apr 2, 2009 at 4:51 PM, Moritz Lennert
> <mlennert at club.worldonline.be> wrote:
>> On 02/04/09 16:00, Markus Neteler wrote:
>>> On Thu, Apr 2, 2009 at 1:43 PM, Markus Neteler <neteler at osgeo.org> wrote:
>>>> On Wed, Apr 1, 2009 at 6:05 PM, Paolo Cavallini <cavallini at faunalia.it>
>>>>> v.dissolve input=regiao at nuovo layer=1 column=REGIAO output=reg_diss
>>>>> dbmi: Protocol error
> ...
>>> Offlist we figured it out:
>>> If the "REGIAO" column field value is empty, then it fails. Perhaps
>>> only happening with char columns. Somewhere here:
>>>
>>>
>>> http://trac.osgeo.org/grass/browser/grass/branches/releasebranch_6_4/db/drivers/dbf/dbfexe.c#L644
>> My wild guess would be that the problem is rather in the cmp_row_asc
>> function, where you have on line 481:
>>
>>    case DBF_CHAR:
>>        c1 = tbl->rows[*row1].values[cur_cmp_ocol].c;
>>        c2 = tbl->rows[*row2].values[cur_cmp_ocol].c;
>>        return (strcmp(c1, c2));
>>        break;
>>
>> Don't know how strcmp handles empty values. Maybe you could add something
>> like
>>
>> fprinf(stdout, "c1 = %s, c2 = %s\n", a, b);
>>
>> before the return statement to see the state of the respective character
>> variables ?
> 
> Very good :)
> 
> echo "SELECT cat, REGIAO FROM regiao ORDER BY REGIAO" | db.select
> D0/0: c1 = ALTO PORTUGAL, c2 = ALTO PORTUGAL
> D0/0: c1 = NOROESTE CISMONTANO, c2 = ALTO PORTUGAL
> D0/0: c1 = NOROESTE CISMONTANO, c2 = ALTO PORTUGAL
> D0/0: c1 = NOROESTE CISMONTANO, c2 = ALTO PORTUGAL
> D0/0: c1 = (null), c2 = ALTO PORTUGAL
> dbmi: Protocol error
> 
> BAD happens when Null is found here.

And that's arguably a correct behaviour as how should NULL be handled in 
such a comparison ? Don't know how it is dealt with in other DBMS...

So, before we fix this in any way, we should probably agree to how to 
handle it. One possible way would be to check for nulls and if there are 
any to assign a return value according to our choice as to where to sort 
nulls to. Don't know how to check for an uninitialised char variable in 
C, but I imagine that it should be possible.

Moritz