Help using r.mapcalc

[K.A.MOHD NASIR]

Dear Grass users,

I need help in using r.mapcalc for my application. I would like to 
use r.mapcalc in assigning different equations to different 
categories within the same map coverage. For example, I would like 
assign eqn A to category 1, eqn B to category 2, eqn C to category 3 
and eqn D to category 4 within the same map coverage. How can I do 
this using r.mapcalc or is there any other grass comands that I can 
use for the purpose mentioned above.

Thanks for any help.

Cheers,
Kamarul