2D Arrays

A 2D array is a list of lists. The outer list holds the rows; each inner list is one row of values. Python does not have a built-in array type the way Cambridge pseudocode does — instead we use nested lists.

matrix = [
    [1, 2, 3],   # Row 0
    [4, 5, 6],   # Row 1
    [7, 8, 9]    # Row 2
]

print(matrix)
# Output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

There are six ways to fill a 2D array. Which one you choose depends on whether you already know the values, or whether you need a blank grid of a fixed size to fill in later.

1 — Static (values known in advance)

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

2 — Empty array (no data yet)

matrix = []  # add rows later with matrix.append(...)

3 — Fixed size, filled with zeros

rows = 3
cols = 4
matrix = [[0 for _ in range(cols)] for _ in range(rows)]

4 — Fixed size, any default value

rows = 2
cols = 3
matrix = [["-" for _ in range(cols)] for _ in range(rows)]

5 — Fill from user input

rows = 2
cols = 3
matrix = []
for i in range(rows):
    row = []
    for j in range(cols):
        value = int(input("Enter value: "))
        row.append(value)
    matrix.append(row)

6 — Fill by reading a file

matrix = []
with open("data.txt", "r") as file:
    for line in file:
        row = [int(x) for x in line.strip().split(",")]
        matrix.append(row)

To get one value you give the row index first, then the column index: matrix[row][col]. Both start counting at 0.

matrix = [
    [1, 2, 3],   # Row 0
    [4, 5, 6],   # Row 1
    [7, 8, 9]    # Row 2
]

print(matrix[0][1])   # row 0, col 1 -> 2
print(matrix[2][2])   # row 2, col 2 -> 9

Assign a new value straight to the element using its two indices. This overwrites just that one cell — everything else stays exactly as it was.

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

matrix[1][1] = 50   # change the middle element 5 -> 50
print(matrix)
# Output: [[1, 2, 3], [4, 50, 6], [7, 8, 9]]

The outer loop walks down the rows. For each row, the inner loop walks across its columns. After finishing a row, we print a newline to start the next one on a fresh line.

matrix = [[1, 2, 3], [4, 50, 6], [7, 8, 9]]

for row in matrix:          # each inner list
    for element in row:     # each value in that row
        print(element, end=" ")
    print()                 # newline after each row

# Output:
# 1 2 3
# 4 50 6
# 7 8 9

To work on whole rows, loop with an index i so you can grab the row as matrix[i] and use Python's sum() on it.

matrix = [[1, 2, 3], [9, 9, 9], [4, 0, 1]]

max_sum = -1
max_row = -1
for i in range(len(matrix)):
    row_sum = sum(matrix[i])
    if row_sum > max_sum:
        max_sum = row_sum
        max_row = i

print("Row", max_row, "has the highest sum:", max_sum)
# Output: Row 1 has the highest sum: 27

Each line of the file becomes one row. Split the line on its separator, convert if needed, then append the row to the array.

# scores.txt:
# Musarrat,80,90,70
# Naib,85,75,95
# Mohar,78,88,82

data = []
with open("scores.txt", "r") as file:
    for line in file:
        parts = line.strip().split(",")
        name = parts[0]
        marks = [int(x) for x in parts[1:]]
        data.append([name] + marks)

print(data)
# Output: [['Musarrat', 80, 90, 70], ['Naib', 85, 75, 95], ['Mohar', 78, 88, 82]]

A school stores student names and three subject marks (Math, English, Science) in a text file scores.txt. Each line is Name,Math,English,Science:

Musarrat,80,90,70
Tarnima,85,75,95
Aymaan,78,88,82
Mehrin,92,85,79

Mark scheme

Fifteen exam-style 2D array tasks. Each shows only the question — click Hint for the thought process, or Help for the worked solution.