Creating and Indexing Arrays

There are several ways to make arrays. The most common:

import numpy as np
a = np.array([1, 2, 3])      # from a list
b = np.arange(0, 10, 2)      # like range: [0 2 4 6 8]
z = np.zeros(4)              # [0. 0. 0. 0.]
o = np.ones((2, 3))          # a 2x3 grid of ones

Every array has a dtype (the type of its elements) and a shape (its size along each dimension):

a = np.array([1, 2, 3])
print(a.dtype)   # int64
print(a.shape)   # (3,)   one dimension, length 3

grid = np.array([[1, 2, 3], [4, 5, 6]])
print(grid.shape)  # (2, 3)   2 rows, 3 columns

Indexing and slicing work like lists, and start at 0:

a = np.array([10, 20, 30, 40, 50])
print(a[0])     # 10
print(a[-1])    # 50
print(a[1:4])   # [20 30 40]   start included, stop excluded

For a 2D array, index with grid[row, col]:

print(grid[0, 2])   # 3  (row 0, column 2)
print(grid[1])      # [4 5 6]  (the whole second row)