Two-dimensional arrays

Programming basics

💾 What is a Two-Dimensional Array?

A two-dimensional array is a contiguous block of memory that stores elements in rows and columns. It is essentially an array of arrays — each element in the outer array is itself an inner array.

In most high-level programming languages (such as Python or Java), a 2D array is indexed using two indices:

• The first index represents the row
• The second index represents the column

✅ Accessing Elements

To access an element in a 2D array, use the following syntax:

array[row][column]

Example (Python):

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

To access the number 6, you would write:

matrix[1][2]

Explanation:

• matrix[1] returns the second row: [4, 5, 6]
• matrix[1][2] returns the third element in that row: 6

🔄 Updating Elements

To update an element in a 2D array, use the same syntax on the left-hand side of an assignment:

matrix[2][0] = 99

This updates the first element of the third row to 99. The new array is:

[
    [1, 2, 3],
    [4, 5, 6],
    [99, 8, 9]
]

🔁 Traversing a 2D Array

To process every element in a 2D array, use nested loops:

for row in range(len(matrix)):
    for col in range(len(matrix[row])):
        print(matrix[row][col])

This prints every element in row-major order (left-to-right, top-to-bottom).

🧠 Common Mistakes

Mistake	Explanation
Mixing up row and column	Accessing matrix[column][row] by mistake gives incorrect results or errors.
Using invalid indices	Accessing matrix[3][0] in a 3×3 matrix (0-indexed) will raise an IndexError.
Assuming all rows are the same length	In Python, inner arrays can be jagged. Always use len(matrix[row]) instead of assuming a fixed length.

🧪 Practice Task

Given the following array:

grades = [
    ["Alice", 85],
    ["Bob", 78],
    ["Clara", 92]
]

• What does grades[2][0] return?
• Change Bob's grade to 80.
• Loop through the array and print each student name and grade.

A video

This video has some references to Java, but it should help you understand the very basics of 2d arrays : https://www.youtube.com/watch?v=nySZfYSVspo

Standards

• Construct algorithms using pre- defined sub-programmes, one- dimensional arrays and/or collections.

A unit of abstract mathematical system subject to the laws of arithmetic.

Develop information in a diagrammatic or logical form.