public long numRows() throws IOException, DMLException {
  if (mc.rowsKnown()) {
    return mc.getRows();
  }
 else {
    return (long)getScalarBuiltinFunctionResult("nrow");
  }
}

public long numRows ( ) throws IOException , DMLException { if ( mc . rowsKnown ( ) ) { return mc . getRows ( ) ; } else { return ( long ) getScalarBuiltinFunctionResult ( "nrow" ) ; } }
gets or computes the number of rows .
