code: public long na_FeatureCount() {
      if( _featsWithNa!=-1 ) return _featsWithNa;
      long cnt=0;
  
      for(int i=0;i<_cols.length;++i) {
        if( !_cols[i]._ignored && !_cols[i]._response && _cols[i]._percentNA!=0) {
          cnt += 1;   // check if const columns along with user ignored columns are included in ignored
        }
      }
      return (_featsWithNa=cnt);
    }
docs: count of features with nas