• A figure in matplotlib means the whole window in the user interface.

Each array has attributes ndim (the number of dimensions), shape (the size of each dimension), and size (the total size of the array):

print("x3 ndim: ", x3.ndim)
print("x3 shape:", x3.shape)
print("x3 size: ", x3.size)

x3 ndim:  3
x3 shape: (3, 4, 5)
x3 size:  60

Another useful attribute is the dtype, the data type of the array (which we discussed previously in Understanding Data Types in Python):

print("dtype:", x3.dtype)

dtype: int64

Other attributes include itemsize, which lists the size (in bytes) of each array element, and nbytes, which lists the total size (in bytes) of the array:

print("itemsize:", x3.itemsize, "bytes")
print("nbytes:", x3.nbytes, "bytes")

itemsize: 8 bytes
nbytes: 480 bytes

In general, we expect that nbytes is equal to itemsize times size.

x = np.arange(10)
x

# array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

x[::2]  # every other element

# array([0, 2, 4, 6, 8])

x[1::2]  # every other element, starting at index 1

# array([1, 3, 5, 7, 9])

A potentially confusing case is when the step value is negative. In this case, the defaults for start and stop are swapped. This becomes a convenient way to reverse an array:

x[::-1]  # all elements, reversed

# array([9, 8, 7, 6, 5, 4, 3, 2, 1, 0])

x[5::-2]  # reversed every other from index 5

# array([5, 3, 1])