🐍 2D Lists

list

Good to know

2D Lists are simply lists that contain other lists. Useful for grids, tables, matrices, or game boards.

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Creating a 2D List

number_grid = [
    [1, 2, 3],   # row 0
    [4, 5, 6],   # row 1
    [7, 8, 9]    # row 2
]

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Building a 2D List Dynamically

col = []
for _ in range(3):
    col.append(0)
 
grid = []
for _ in range(3):
    grid.append(col.copy())   # use copy() to avoid shared references

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Manipulate 2D List

Accessing Values

Use two indexes: [row][column].

number_grid[2][1]   # → 8
# row 2: [7, 8, 9]
# column 1 in that row → 8

Important: Avoid Shared Lists

a = [0,0,0]
bad_grid = [a] * 3    # ❌ all rows reference the SAME list
 
bad_grid[0][1] = 9
print(bad_grid)
# → [[0,9,0], [0,9,0], [0,9,0]]

Always use copy() or list comprehension:

good_grid = [a.copy() for _ in range(3)]   # ✔ independent rows

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Looping Through a 2D List

Find here how to loop through a dictionary: Looping Through a 2D List