[R] merge large matrices

[Stefan Mischke]

Dear List

I have two large matrices A and B. Both have the same dimensions, let's 
say 20k x 30k. About half the cells of B are missing. Now I'm looking 
for an efficient way to merge them, so that the missing values in B are 
replaced by the corresponding values of A.

Matrix A
	[,1]	[,2]	[,3]
[1,]	1	2	3
[2,]	4	5	6

merged with Matrix B

	[,1]	[,2]	[,3]
[1,]	10	NA	NA
[2,]	NA	50	60

equals

	[,1]	[,2]	[,3]
[1,]	10	2	3
[2,]	4	50	60

One way to do this, is  of course looping through all the cells, 
checking for NAs and then replacing them with the corresponding values. 
But this is way too slow for my application. There must be a more 
efficient way.
Does R provide any functions for this?

Cheers
Stefan