Return a flattened array.
A 1D array, containing the elements of the input, is returned. A copy is made only if needed.
Parameters :  a : array_like
order : {‘C’,’F’}, optional


Returns :  1d_array : ndarray

See also
Notes
In rowmajor order, the row index varies the slowest, and the column index the quickest. This can be generalized to multiple dimensions, where rowmajor order implies that the index along the first axis varies slowest, and the index along the last quickest. The opposite holds for Fortran, or columnmajor, mode.
Examples
If an array is in Corder (default), then ravel is equivalent to reshape(1):
>>> x = np.array([[1, 2, 3], [4, 5, 6]])
>>> print x.reshape(1)
[1 2 3 4 5 6]
>>> print np.ravel(x)
[1 2 3 4 5 6]
When flattening using Fortranorder, however, we see
>>> print np.ravel(x, order='F')
[1 4 2 5 3 6]