[R] an efficient pairwise matrix cell's comparison function

From: Diogo André Alagador <mcnda839_at_mncn.csic.es>
Date: Sun, 02 Mar 2008 13:11:25 +0100

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:  

       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 André Alagador
Biodiversity & Global Change Lab, Museo Nacional de Ciencias Naturales, CSIC, Madrid, España
Forest Research Centre, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Lisboa, Portugal  

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