Removing columns from big.matrix which have only one value

I have a very large binary matrix, stored as a big.matrix to conserve memory (it is over 2 gb otherwise - 5 million columns and 100 rows).

r <- 100
c <- 10000
m4 <- matrix(sample(0:1,r*c, replace=TRUE),r,c)
m4 <- cbind(m4, 1)
m4 <- as.big.matrix(m4)

I need to remove every column which has only one unique value (in this case, only 0s or only 1s). Because of the number of columns, I want to be able to do this in parallel.

How can I accomplish this while keeping the data compressed as a big.matrix? I can convert it into a df and loop over the columns looking for the number of unique values, but this takes too much RAM.

Thanks!

Put that in an .cpp file and source it with Rcpp::sourceCpp:

// [[Rcpp::depends(BH, bigmemory)]]
#include <bigmemory/MatrixAccessor.hpp>
#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
LogicalVector to_keep(SEXP bm_addr) {

  XPtr<BigMatrix> xptr(bm_addr);
  MatrixAccessor<double> macc(*xptr);

  size_t n = macc.nrow();
  size_t m = macc.ncol();

  double first_val;

  LogicalVector keep(m, false);

  for (size_t j = 0; j < m; j++) {
    first_val = macc[j][0];
    for (size_t i = 1; i < n; i++) {
      if (macc[j][i] != first_val) {
        keep[j] = true;
        break;
      }
    }
  }

  return keep;
}

/*** R
library(bigmemory)
r <- 100
c <- 10000
m4 <- matrix(sample(0:1,r*c, replace=TRUE),r,c)
m4 <- cbind(m4, 1)
m4 <- as.big.matrix(m4)
m4[, 1] <- 1
m4[, 2] <- 0

keep <- to_keep(m4@address)
m4.keep <- deepcopy(m4, cols = which(keep))
*/