GCSE Computer Science

Python 2D lists — GCSE Computer Science

What is a 2D list?

A 2D list is a list of lists — each item in the outer list is itself a list. It's the standard way to represent a grid: a seating plan, a noughts-and-crosses board, a spreadsheet of scores. Instead of one long list, you get rows, and each row is a list of its own.

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

Indexing with two indices

To reach a single value in a 2D list, you need two indices: one for the row, one for the column — grid[row][col]. grid[1][2] means: go to row 1 (the second row, since indexing starts at 0), then column 2 (the third value in that row).

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

Iterating with nested loops

To visit every value in a 2D list, you need a loop inside a loop — one for the rows, one for the columns within each row.

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
for row in grid:
    for value in row:
        print(value)

The outer loop takes each row in turn. The inner loop then takes each value within that row. Together, they visit all nine values, row by row, left to right.

A worked example: a 3×3 grid

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
total = 0
for row_index in range(3):
    for col_index in range(3):
        total = total + grid[row_index][col_index]
print(total)   # 45

This does the same job as the row-by-row loop above, but using index numbers directly — useful when you need to know which row and column a value came from, not just the value itself.

What examiners actually test

2D list questions typically describe a grid — a seating plan, a set of scores — and ask you to retrieve, update, or sum values at specific positions, or to write nested loops that process every cell. Read the row/column order carefully in the question: grid[row][col] is consistent throughout Python, but it's easy to swap the two under exam pressure.