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

[Glynn Clements]

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.

-- 
Glynn Clements <glynn at gclements.plus.com>