library ( bigmemory )
library ( biganalytics )
options ( bigmemory.typecast.warning = FALSE )

A <- big.matrix (5000 , 5000 , type ="char", init =0)
# Fill the matrix by randomly picking 20% of the positions for a 1.
x <- sample (1:5000 , size =5000 , replace = TRUE )
y <- sample (1:5000 , size =5000 , replace = TRUE )
for (i in 1:5000) {
	A[x[i],y[i]] <- 1
}
# Get the location in RAM of the pointer to A.
desc <- describe (A)
# Write it to disk .
dput (desc , file ="A.desc ")
sums <- colsum (A, 1:20)