Signature Description Parameters
template<typename T>
StdDataFrame<T>
value_counts (const char *col_name) const;
It counts the unique values in the named column.
It returns a StdDataFrame of following specs:
  1. The index is of type T and contains all unique values in the named column.
  2. There is only one column named "counts" of type size_type that contains the count for each index row.
For this method to compile and work, 3 conditions must be met:
  1. Type T must be hashable. If this is a user defined type, you must enable and specialize std::hash.
  2. The equality operator (==) must be well defined for type T.
  3. Type T must match the actual type of the named column.
Of course, if you never call this method in your application, you need not be worried about these conditions.
T: Type of the named column