an efficient pairwise matrix cell's comparison function

A <- matrix(sample(0:2, 25, TRUE), ncol=5)
B <- matrix(1:25, ncol=5)
C <- ifelse(A == 0, 0, B)
A

B

C

To all,



I am undergoing an analysis involving big matrices of about 30000x200 which
I have to handle in a more efficient way. So I would like some advice to
build such efficient function to deliver the following result:



-          starting with 2 matrices of the same dimension (eg. A and B)



      0  0  3  5                      6  0  0  5

A=   0  0  6  4              B=   0  4  3  5

            0  0  5  0                      1  0  0  9



-          the function should deliver a C matrix (same dimension too),
where at each position C(i,j), compares A and B.

     if A(i,j)=0, than C(i,j)=0,

     if A(i,j)!=0, than C(i,j)=B(i,j)



     6  0  0  5

C= 0  0  3  5

 0  0  0  0



Although not an expert I could build a function with 2 cycles (reading
columns and rows) which is not quick. Maybe you can help me in this
"challenge".



Much thanks in advance,




Diogo Andr? Alagador
Biodiversity & Global Change Lab, Museo Nacional de Ciencias Naturales,
CSIC, Madrid, Espa?a
Forest Research Centre, Instituto Superior de Agronomia, Universidade
T?cnica de Lisboa, Lisboa, Portugal


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