Card 17: 2D Lists

Module 7: Expert Techniques

1 The Hook

A normal list is a line. A 2D List is a Grid.

Think of:

Spreadsheets (Rows & Columns)
Cinema Seats (Row A, Seat 5)
Game Maps (X, Y coordinates)

2 Worked Example

Accessing a Tic-Tac-Toe grid.

# A list OF lists

board = [

["X", "O", "X"], # Row 0

["O", "X", "O"], # Row 1

["X", "O", "X"] # Row 2

]

# board[ROW][COL]

print(board[1][1]) # Prints Middle "X"

Changing the Grid

We can update a specific "cell" in the grid.

board = [ ["X", "O"], ["O", "X"] ]

board[0][1] = "WIN" # Change Row 0, Col 1

print(board) # [ ["X", "WIN"], ["O", "X"] ]

PREDICT

What gets printed?

temps = [
[15, 20],
[18, 22]
]
print(temps[1][0])

Remember: Row First, then Column. Start at 0.

Interactive Editor

main.py

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
# Print 5
print(matrix[1][1])
# Print 7
print(matrix[2][0])

Terminal Output

> _

BRONZE (MODIFY)

🥉 Tic Tac Toe

1. The board has an empty spot "-" at Row 1, Column 1.
2. Update that spot to be "X".
3. Print the board.

bronze.py

board = [
    ["O", "X", "O"], 
    ["X", "-", "O"],
    ["O", "O", "X"]
]
# Fix the line below:
board[...][...] = "X"
print(board)

> _

SILVER (FIX)

🥈 Out of Bounds

1. We want to grab 99 from the grid.
2. The code tries `grid[0][2]`. BUT Row 0 only has index 0 and 1!
3. Fix the coordinates to find 99.

silver.py

grid = [
    [10, 20],
    [50, 99]
]
print(grid[0][2]) # Error!

> _

GOLD (MAKE)

🥇 Vertical Maths

1. nums = [ [1, 10], [2, 20], [3, 30] ].
2. Goal: Sum the second column (Index 1) only.
3. Loop through every row.
4. Add row[1] to a total. (Should be 10+20+30 = 60).

gold.py

nums = [
    [1, 10],
    [2, 20],
    [3, 30]
]
total = 0
for row in nums:
    # Add index 1 of the row
    ...
print(total)

> _

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