code: def get_column(columns, column_tys, index):
      
      weld_obj = WeldObject(encoder_, decoder_)
      columns_var = weld_obj.update(columns, tys=WeldVec(column_tys), override=False)
      if isinstance(columns, WeldObject):
          columns_var = columns.obj_id
          weld_obj.dependencies[columns_var] = columns
  
      weld_template = """
       map(
         %(columns)s,
         |elem: %(ty)s| elem.$%(index)s
       )
    """
  
      weld_obj.weld_code = weld_template % {"columns": columns_var,
                                            "ty": column_tys,
                                            "index": index}
      return weld_obj
docs: Get column corresponding to passed-in index from ptr returned
      by groupBySum.
  
      Args:
          columns (List<WeldObject>): List of columns as WeldObjects
          column_tys (List<str>): List of each column data ty
          index (int): index of selected column
  
      Returns:
          A WeldObject representing this computation