Nested Lists — How They Work

A nested list is a list that contains other lists. It's how we represent grids, tables, and matrices in Python.

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Creating a Nested List

# A 3×3 grid
grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

Think of it as a table with rows and columns:

	Col 0	Col 1	Col 2
Row 0	1	2	3
Row 1	4	5	6
Row 2	7	8	9

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Accessing Elements

Use two indices — first the row, then the column:

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print(grid[0])       # [1, 2, 3]  (entire first row)
print(grid[0][0])    # 1          (row 0, col 0)
print(grid[1][2])    # 6          (row 1, col 2)
print(grid[2][1])    # 8          (row 2, col 1)

How to read it: grid[row][col]

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Changing Values

grid = [
    [0, 0, 0],
    [0, 0, 0],
    [0, 0, 0]
]

grid[1][1] = 5    # Set the center to 5
grid[0][2] = 9    # Set top-right to 9

# grid is now:
# [0, 0, 9]
# [0, 5, 0]
# [0, 0, 0]

---

Looping Through a Nested List

Print Every Element

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in grid:
    for item in row:
        print(item, end=" ")
    print()

Output:

1 2 3
4 5 6
7 8 9

Using Index Numbers

Sometimes you need to know the row and column number:

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for r in range(len(grid)):
    for c in range(len(grid[r])):
        print(f"grid[{r}][{c}] = {grid[r][c]}")

---

Getting Dimensions

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

rows = len(grid)         # 3 (number of rows)
cols = len(grid[0])      # 3 (number of columns)
print(f"{rows} rows × {cols} cols")

---

Common Pattern: Creating a Grid

# Create a 3×4 grid filled with zeros
rows = 3
cols = 4

grid = []
for r in range(rows):
    row = []
    for c in range(cols):
        row.append(0)
    grid.append(row)

---

Common Pattern: Searching a Grid

grid = [
    [".", ".", "X"],
    [".", "X", "."],
    ["X", ".", "."]
]

for r in range(len(grid)):
    for c in range(len(grid[r])):
        if grid[r][c] == "X":
            print(f"Found X at row {r}, col {c}")

---

Common Pattern: Summing All Values

grid = [
    [10, 20, 30],
    [40, 50, 60],
    [70, 80, 90]
]

total = 0
for row in grid:
    for num in row:
        total += num

print(f"Total: {total}")  # Total: 450

---

Real-World Example: Tic-Tac-Toe Board

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

# Print the board nicely
for row in board:
    print(" | ".join(row))
    print("-" * 9)

# Check if a cell is empty
r, c = 1, 0
if board[r][c] == ".":
    board[r][c] = "O"
    print(f"Placed O at row {r}, col {c}")

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Key Takeaways

• Nested lists = lists inside lists (2D grids)
• Access with two indices: grid[row][col]
• Loop with nested for loops: outer = rows, inner = columns
• len(grid) = number of rows, len(grid[0]) = number of columns
• Common uses: game boards, tables, maps, matrices