Lists of lists

Lists can hold lists, which gives you a grid. The first index picks the row and the second picks the column:

board = [["X", "O", "X"],
         ["O", "X", " "],
         [" ", " ", "O"]]
board[0][2]     # row 0, col 2 -> "X"
board[1][2] = "X"

The Visualizer draws nested lists as rows, so you can see the grid.

See it run

row = [0, 0]
grid = [row, row]
grid[0][0] = 9
print(grid)
grid = [[0, 0], [0, 0]]
grid[0][0] = 9
print(grid)

Two grids, built differently. Follow the arrows: how many separate rows are there?

Practice lists of lists

Exercise 1

Use board in the editor below.

Place "X" in the bottom-left corner.
Store the top-right cell in top_right.
Store the whole middle row in middle.

Exercise 2

Use the cinema hall in the editor below: 1 is a taken seat and 0 is free. Using loops (or sum on each row):

taken = the total number of taken seats
row_free = a list with the number of free seats in each row

Exercise 3

Check the noughts-and-crosses board in the editor below for a win: - column1 = the list of cells at column index 1 (the middle column, one cell from each row) - x_wins = True if every cell in column1 is "X" - diagonal = the top-left to bottom-right diagonal: board[0][0], board[1][1], board[2][2] Use loops or comprehensions so this would work on any 3×3 board.

The explanation and tasks are public. Running this lesson requires an account with course access.

Practice

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Visualizer Watch your code run line by line

Run your code, then choose Play to follow its execution.

Memory · names → values

Variables appear here as Python creates them.

Output & plots