[R] Counting elements in 2 rows of a matrix

[Fredy Mejía]

Dear R-users:

Sorry to bother so late with this question, which surely has simple answer.
   I'm working with matrices that contain either "1" or "0", for example:

   [,1] [,2] [,3] [,4] [,5]
A    1    0    1    0    0
B    0    1    0    1    0
C    1    0    1    0    0
D    1    1    1    0    1

I want to count the number of "1" common to, say, A and B, the number of 
"1" that appear only in A and the number of "1" that appear only in B.  
Please let me know if there's a simple way of doing this without the 
complicated for...next loops that have come across my mind.

Fredy Mejía
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