gistfile1.txt

// ----- count.c
// build with R CMD SHLIB count.c (or put it in the package, w/e)

#include <R.h>
#include <Rinternals.h>

SEXP colsum_which_eq0(SEXP nrows_, SEXP ncols_, SEXP J_)
{
  SEXP ret;
  const int nrows = INTEGER(nrows_)[0];
  const int ncols = INTEGER(ncols_)[0];
  const int *const restrict J = INTEGER(J_);
  const int len = LENGTH(J_);
  
  PROTECT(ret = allocVector(INTSXP, ncols));
  int *const restrict ret_ptr = INTEGER(ret);
  
  for (int j=0; j<ncols; j++)
    ret_ptr[j] = nrows;
  
  for (int k=0; k<len; k++)
  {
    int nnz = 0;
    int col = J[k];
    
    while (col == J[k] && k < len)
    {
      nnz++;
      k++;
    }
    
    ret_ptr[col-1] = nrows - nnz;
    
    if (nnz)
      k--;
  }
  
  UNPROTECT(1);
  return ret;
}





### count.r
# assumes the data is in COO/"simple triplet"/AIJ format. This is the easiest to convert
# to CSR/CSC if that's what you're really interested in (don't use CSR)
x = coop:::dense_stored_sparse_mat(100, 10, 0.1)
x[, 5] = 0
storage.mode(x) = "integer"
coo = slam::as.simple_triplet_matrix(x)

dyn.load("count.so")
.Call("colsum_which_eq0", coo$nrow, coo$ncol, coo$j)
colSums(x == 0L) # compare