Python: Differentiating between row and column vectors

You can make the distinction explicit by adding another dimension to the array.

>>> a = np.array([1, 2, 3])>>> aarray([1, 2, 3])>>> a.transpose()array([1, 2, 3])>>> a.dot(a.transpose())14

Now force it to be a column vector:

>>> a.shape = (3,1)>>> aarray([[1],       [2],       [3]])>>> a.transpose()array([[1, 2, 3]])>>> a.dot(a.transpose())array([[1, 2, 3],       [2, 4, 6],       [3, 6, 9]])

Another option is to use np.newaxis when you want to make the distinction:

>>> a = np.array([1, 2, 3])>>> aarray([1, 2, 3])>>> a[:, np.newaxis]array([[1],       [2],       [3]])>>> a[np.newaxis, :]array([[1, 2, 3]])

Use double [] when writing your vectors.

Then, if you want a row vector:

row_vector = array([[1, 2, 3]])    # shape (1, 3)

Or if you want a column vector:

col_vector = array([[1, 2, 3]]).T  # shape (3, 1)

The vector you are creating is neither row nor column. It actually has 1 dimension only. You can verify that by

• checking the number of dimensions myvector.ndim which is 1
• checking the myvector.shape, which is (3,) (a tuple with one element only). For a row vector is should be (1, 3), and for a column (3, 1)

Two ways to handle this

• create an actual row or column vector
• reshape your current one

You can explicitly create a row or column

row = np.array([    # one row with 3 elements   [1, 2, 3]]column = np.array([  # 3 rows, with 1 element each    [1],    [2],    [3]])

or, with a shortcut

row = np.r_['r', [1,2,3]]     # shape: (1, 3)column = np.r_['c', [1,2,3]]  # shape: (3,1)

Alternatively, you can reshape it to (1, n) for row, or (n, 1) for column

row = my_vector.reshape(1, -1)column = my_vector.reshape(-1, 1)

where the -1 automatically finds the value of n.