What is a 2D list in Python?

A list whose items are themselves lists — a list of rows. You access a cell with two indices: grid[row][col].

Which index comes first, row or column?

By convention: row first, column second. grid[2][0] is row 2, column 0.

Why nested for loops?

The outer loop moves through the rows, and the inner loop moves through the cells inside that row. Every cell in the grid gets visited exactly once.

Python 2D lists

A two-dimensional list — a list of lists — is Python's standard way to represent grids and tables such as a noughts-and-crosses board, a seating plan, or a class register. Every UK A Level Computer Science specification requires 2D array handling, and it appears in higher-tier GCSE questions too. This unit covers building a 2D list, indexing rows and columns with grid[row][col], and iterating with nested for loops. Every example runs in the browser. Perfect for classroom teaching, GCSE and A Level revision, and independent or home-schooled learners.

What you'll learn

Build a 2D list as a list of lists
Access a cell with grid[row][col]
Iterate over all cells with nested for loops
Iterate over rows and columns separately
Store and update grid data (game boards, tables, seating plans)

UK GCSE topic coverage

2D arrays
2D indexing
2D mutation
Nested iteration
Row algorithms
Column algorithms
2D linear search
Constructing 2D lists
Applied 2D lists

Worked example

Print a 3x3 grid, row by row

board = [
    ["X", "O", "X"],
    ["O", "X", "O"],
    ["X", "O", "X"],
]
for row in board:
    for cell in row:
        print(cell, end=" ")
    print()

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Lessons in this unit

Full interactive lessons with checked tasks are available to signed-in RunPy users. Unit 1 and a selection of exam-style questions are free with a RunPy account.

What is a 2D list?
Accessing elements
Modifying elements
Iterating a 2D list
Row operations
Column operations
Searching a 2D list
Building 2D lists from input
Composite grid algorithm

Frequently asked questions

What is a 2D list in Python?: A list whose items are themselves lists — a list of rows. You access a cell with two indices: grid[row][col].
Which index comes first, row or column?: By convention: row first, column second. grid[2][0] is row 2, column 0.
Why nested for loops?: The outer loop moves through the rows, and the inner loop moves through the cells inside that row. Every cell in the grid gets visited exactly once.