132.2 Arrays

Learn how arrays store fixed-size collections of data and how they differ from Python lists

What is an array?

An array is a data structure that stores a collection of elements of the same type in a fixed-size, indexed format. Arrays are efficient for storing and processing large datasets, especially when the number of elements is known in advance.

While Python uses lists for most array-like operations, these are actually dynamic arrays that can hold different data types. True arrays (fixed in size and type) are used in lower-level or performance-sensitive contexts and are common in languages like C, Java, and Pascal.

To simulate arrays in Python, you can use:

• The array module (enforces type consistency)

• NumPy arrays (used for numerical computing)

Declaring a fixed-type array (using array module)

Python’s built-in array module allows you to create arrays with a specified type code.

Pseudocode

DECLARE grid AS ARRAY OF ARRAY OF INTEGER
SET grid TO [[1, 2], [3, 4]]
OUTPUT grid[1][0]

Python

Accessing and modifying array elements

Just like lists, arrays use zero-based indexing.

Pseudocode

DECLARE grid AS ARRAY OF ARRAY OF INTEGER
SET grid TO [[1, 2], [3, 4]]
OUTPUT grid[1][0]

Python

Fixed size and type

Once created, an array:

• Cannot grow or shrink (unlike a list)

• Must store only one data type (e.g. all integers)

This makes arrays efficient in memory use, especially in embedded systems or games.

Looping through an array

Arrays are often processed using for loops.

Pseudocode

DECLARE grid AS ARRAY OF ARRAY OF INTEGER
SET grid TO [[1, 2], [3, 4]]
OUTPUT grid[1][0]

Python

Multidimensional arrays

Arrays can have more than one dimension. In Python, this is often done using nested lists or NumPy arrays.

Pseudocode

DECLARE grid AS ARRAY OF ARRAY OF INTEGER
SET grid TO [[1, 2], [3, 4]]
OUTPUT grid[1][0]

Python

For high-performance applications, use NumPy's ndarray for true multidimensional arrays.

Why use an array in Python instead of a list?

While Python lists are more flexible and commonly used, typed arrays offer specific benefits in particular contexts. Here’s when you might use an array instead of a list:

1. Memory efficiency

Python lists store references to objects, meaning they can hold mixed types but use more memory per element.

Typed arrays (e.g. array.array('i', [...])) store values directly in memory in a contiguous block, similar to arrays in C. This is more memory-efficient, especially for large datasets of numbers.

2. Type enforcement

Arrays restrict all elements to a single data type, preventing accidental insertion of incompatible values.

pythonCopyEditimport array
scores = array.array('i', [10, 20, 30])
scores.append("forty")   # Error: incompatible type

In a list, this would silently succeed — potentially causing bugs later.

3. Performance

For numerical data, arrays provide faster performance for operations that involve:

• Iteration over large datasets

• Numerical calculations

• File I/O in binary format (e.g. image or sound data)

For even better performance, especially for maths-heavy operations, NumPy arrays (ndarray) are the preferred structure. They support:

• Fast vectorised operations

• Broadcasting

• Integration with scientific libraries

4. Interfacing with low-level code

When working with:

• Binary files (e.g. .wav, .bmp)

• Hardware devices

• C/C++ extensions

Typed arrays (array.array) or NumPy arrays offer better compatibility because they mirror C-style memory layout.

Summary: list vs array

Feature	list	array.array (or numpy.ndarray)
Stores	Any type	One fixed type only
Size	Dynamic	Fixed (or resizable with type checks)
Memory use	Higher	Lower (more compact storage)
Performance	Adequate	Faster for numeric processing
Use case	General-purpose	Numeric data, file I/O, interfacing, memory

Conclusion

Use a list when:

• You need flexibility (e.g. mixed types, dynamic structure)

• You’re building general-purpose applications

Use an array when:

• You need speed, compact storage, or type control

• You're working with large amounts of numbers

• You're integrating with binary formats or low-level systems

Key concepts

• An array stores a fixed-size collection of elements of the same type

• Arrays use zero-based indexing for access

• Python lists behave like dynamic arrays, but typed arrays can be created with modules like array or numpy

• Arrays are useful when memory and speed are critical, especially for large, numerical datasets