Indexing and slicing arrays

Introduzione a NumPy

Izzy Weber

Core Curriculum Manager, DataCamp

Indexing 1D arrays

array = np.array([2, 4, 6, 8, 10])
array[3]

Indexing elements in 2D

A sudoku game with a single number selected in the third row of the fifth column

sudoku_game[2, 4]

Indexing rows in 2D

A sudoku game with the first row selected

sudoku_game[0]

array([0, 0, 4, 3, 0, 0, 2, 0, 9])

Indexing columns in 2D

A sudoku game with the fourth column selected

sudoku_game[:, 3]

array([3, 0, 0, 0, 0, 0, 0, 5, 9])

Slicing 1D arrays

array = np.array([2, 4, 6, 8, 10])
array[2:4]

array([6,  8])

Slicing 2D arrays

A sudoku game with the middle nine squares selected - columns four to six and rows four to six

sudoku_game[3:6, 3:6]

array([[0, 0, 2],
       [0, 0, 7],
       [0, 8, 3]])

Slicing with steps

A sudoku game with only the corners of the middle nine boxes selected

sudoku_game[3:6:2, 3:6:2]

array([[0, 2],
       [0, 3]])

Sorting arrays

Before sorting: An unsorted sudoku game

np.sort(sudoku_game)

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

Axis order

graphic showing that axis zero is the direction along rows and axis one is the direction along columns

Graphic showing that the number one looks like a column

Sorting by axis

np.sort(sudoku_game)

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

np.sort(sudoku_game, axis=0)

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

Let's practice!

Introduzione a NumPy