Convert the frame to its Numpy-array representation.
columns: list, optional, default:None
If None, return all columns, otherwise, returns specified columns.
values : ndarray
If the caller is heterogeneous and contains booleans or objects, the result will be of dtype=object. See Notes.
Return is NOT a Numpy-matrix, rather, a Numpy-array.
The dtype will be a lower-common-denominator dtype (implicit upcasting); that is to say if the dtypes (even of numeric types) are mixed, the one that accommodates all will be chosen. Use this with care if you are not dealing with the blocks.
e.g. If the dtypes are float16 and float32, dtype will be upcast to float32. If dtypes are int32 and uint8, dtype will be upcase to int32.
This method is provided for backwards compatibility. Generally, it is recommended to use ‘.values’.