Numpy array dimension

Question

In [21]: a = np.array([1,2,3])

In [22]: a.shape
Out[22]: (3,)

What's the difference between (3,) and (3,1) in shape?

Why isn't the output (,3) or (1,3) ? This is a 1 by 3 matrix, right?

Answer 1

No. A 1x3 matrix would be a two-dimensional array (eg, np.array([[1, 2, 3]]) ). What you have is a one-dimensional array of length three. In mathematical terms, a 1D array roughly corresponds to a vector.