how to get rid of this double for loop?

["Danni Wang" <wangdn@uclink4.berkeley.edu>]

Happy Halloween, Group,

I used a double for loop to replace NAs in the matrix with a new value. My
matrix is 20000 by 30. It is very slow to run the loop. I don't know how to
use apply() and is.na() to make it faster. Could you give some suggestions?

My code:

for (i in 1:nrow(data)){
   for (j in 1:ncol(data){
        if (is.na(data[i,j])) { data[i,j] <- data[i-1,j] }
    }
}

Thanks very much,

Danni