[GRASS-user] Problems with r.mapcalc

[Elisa Dal Maso]

Hi, I'm a PhD student in Italy and I am a new user of GRASS.
I have a problem in a weighted average with r.mapcalc.
There are 3 raster map (areas of species), with different extent but the
same resolution, area has value 1, outside is null *.
I would like to sum these areas with different weights (10, 9 and 4), then
divide for the sum of weights (23), generating a float, and I have used
this formula:

(10.*if(isnull(Fraxinus_excelsior_raster a Phalaen), 0,
1)+9.*if(isnull(Fraxinus_angustifolia_raster a Phalaen), 0, 1)
+4.*if(isnull(Fraxinus_ornus_raster a Phalaen), 0, 1))/23.

The resulting map have only 4 rectangles but it considers the sum of all
the map (null values comprised).

How could I resolve the problem?
Thanks a lot!!!

Elisa
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