numpy array: IndexError: too many indices for array

Question

This works:

>>> a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])
>>> a[: , 2]
array([ 3,  7, 11])

This doesn\'t

Answer 1

The first array has shape (3,4) and the second has shape (3,). The second array is missing a second dimension. np.array is unable to use this input to construct a matrix (or array of similarly-lengthed arrays). It is only able to make an array of lists.

>>> a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])

>>> print(a)
[[ 1  2  3  4]
 [ 5  6  7  8]
 [ 9 10 11 12]]

>>> print(type(a))



>>> b = np.array([[1,2,3,4], [5,6,7,8], [9,10,11]])

>>> print(b)
[list([1, 2, 3, 4]) list([5, 6, 7, 8]) list([9, 10, 11])]

>>> print(type(b))

So they are both Numpy arrays, but only the first can be treated as a matrix with two dimensions.