[GRASS-user] Re: Strange r.mapcalc results

[Hermann Peifer]

Glynn Clements wrote:
> Hermann Peifer wrote:
> 
>> Thanks indeed for the enlightening. I am new to the mapcalc business and 
>> my obviously wrong (AWK-based) thinking was that mapcalc would do a lazy 
>> evaluation of the conditions, i.e.  "1 && 1 || something" would always 
>> be 1, whatever "something" was. Now I know better.
> 
> r.mapcalc never does "short-circuit" evaluation. This is true for if()
> and ?: as well as &&,||,&&&,|||.
> 
> && and || propagate nulls, while &&& and ||| return non-null where
> possible. All four operators are symmetric, so e.g. "null() ||| 1" and
> "1 ||| null()" both evaluate to 1.
> 

Thanks once again.

It might be worth adding this hint into the r.mapcalc documentation at: 
http://grass.itc.it/grass64/manuals/html64_user/r.mapcalc.html

Hermann