Introduction to NumPy

NumPy arrays: creating them from lists and with zeros, ones and arange; their attributes ndim, shape, size, dtype and itemsize; and element-wise arithmetic.

Key ideas

NumPy (Numerical Python) is imported as np; an array holds items of a single data type, stored next to each other in memory.
np.array() turns a list into a 1-D array, and a list of equal-length lists into a 2-D array of rows and columns.
One float in the list makes every item a float and one string makes every item a string; dtype=float forces floats.
ndim gives the number of dimensions, shape the length along each, such as (3, 4), and size the total number of elements.
np.zeros((r, c)) and np.ones((r, c)) fill an array with 0s or 1s, float64 by default; np.arange(a, b, s) works like range and stops before b.
itemsize is the bytes one element takes (8 for float64, 4 for int32), so the elements take size × itemsize bytes in all.
Arithmetic on an array works on every element: an array * 2 doubles each item, while a list * 2 repeats the list.

Remember

Shape	2-D: (rows, columns); 1-D: (n,) with a comma
size	rows × columns: np.zeros((3, 4)).size → 12
dtype from the items	all whole numbers → int; any float → float; any string → string
itemsize	float64 = 8 bytes; int32 = 32 bits = 4 bytes
Total bytes	size × itemsize: np.ones((3, 5), dtype=np.int32) → 15 × 4 = 60
arange	np.arange(1, 10, 2) → 1, 3, 5, 7, 9; np.arange(6) → 0 to 5
Array and list	import numpy as np L = [2, 5, 7] A = np.array(L) print(L * 2) # [2, 5, 7, 2, 5, 7] print(A * 2) # [ 4 10 14]

Watch out

Writing shape as (columns, rows): it is (rows, columns).
Counting the stop value in arange: np.arange(0, 10, 2) ends at 8.
Taking zeros() items as whole numbers: they are float64, 8 bytes each.
Thinking an array with one string still holds numbers: every item becomes text.

Worked example

What are the elements of c? Write them separated by commas.
import numpy as np
a = np.array([24, 13, 17, 45])
b = np.array([32, 30, 33, 28])
c = a + b

Working: element by element: 24 + 32, 13 + 30, 17 + 33, 45 + 28

Answer: 56, 43, 50, 73