[GRASS5] How to code NULL in sites lists?

[Markus Neteler]

Hi Eric, hi all,

On Wed, Oct 11, 2000 at 06:30:48PM -0700, Eric G . Miller wrote:
> On Wed, Oct 11, 2000 at 06:36:01PM +0100, Markus Neteler wrote:
> <snip>
> > Maybe we should allow two values:
> > 
> > -9999 (for all data coming from ESRI world)
> > NAN  (but this is a strings?!)
> 
> I should clarify.  NAN is used for floating point (double) variables
> while resident in memory. Then:
> 
> 	if (isnan(my_double)) { /* It's NULL */ }
> 	else {  /* do work */ }
> 
> For strings:
> 	if (string[0] == '\0') { /* Empty string */ }
> 
>     or 
>     	if (strlen(string) == 0) { /* Again */ }
> 
> Granted, in some contexts an empty string is not the same thing as a
> NULL value, but I think it'll be easier to code around rather than
> library functions returning a NULL pointer for a string attribute.  I
> don't know...
> 
> As far as the ESRI sentinel value -9999, it's an integer.  I don't know
> if they use such a thing for their floating point grids.  I don't see
> any reason to carry this around in the site file.  Import or conversion
> routines might want to look for it and do an appropriate translation.
> I'm not sure where this fits in in relation to sites...
If we can just have "nothing" for a int/float value, it would be o.k.
Then the user, if using ESRI world data, can just use a text editor
to replace the "-9999" by "". If the s.in.ascii could manage that?
But probably a parameter "nv" comparing to r.in.ascii would be better:

      nv   String representing NULL value data cell
           default: *

Then we don't run into problems if several columns of floats with
partly missing values are there.
 
> I think this change in behavior should be postponed to GRASS 5.1.  I
> suspect it will cause lots of code breakage (maybe I'm wrong?).  It
> depends how important people think this is.  It's hovering around the
> middle in my mind.
Ok. We postpone it. Bugfixing is more important at time.

Markus

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