Arrays

Overview

An array is a data structure that stores a collection of elements, typically of the same data type, in a contiguous block of memory. Arrays are used to efficiently store and access multiple values using a single variable name and an index. You need to know arrays up to 3 dimensions, as they are commonly used for organising data.

What is an Array?

• Definition: An array is a collection of elements, all of the same data type, identified by an index or set of indices.
• Indexing:
  • Arrays are zero-indexed in many programming languages (e.g., Python, Java, C++), meaning the first element is accessed with index 0.
  • Example: In an array arr = [10, 20, 30], arr[0] refers to 10.
• Fixed Size: Arrays are typically of a fixed size, meaning the number of elements is determined when the array is created.

Types of Arrays

One-Dimensional Array

A single row of elements.

grades = [85, 90, 78, 92]

Two-Dimensional Array

Consists of rows and columns, like a table or grid.

chessboard = [
    [1, 0, 0, 1],  # Row 1
    [0, 1, 1, 0],  # Row 2
    [1, 1, 0, 0],  # Row 3
    [0, 0, 1, 1]   # Row 4
]

Three-Dimensional Array

Represents data in three dimensions, such as multiple layers of 2D grids.

temperature = [
    [[15, 16], [20, 21]],  # Day 1: Locations 1, 2
    [[18, 19], [22, 23]]   # Day 2: Locations 1, 2
]

Uses of Arrays

• Storing Data: Arrays are used to store collections of related data, such as lists of names, scores, or positions in a game.
• Organising Complex Data: Multi-dimensional arrays help organise more complex data, such as images (2D arrays) or 3D models (3D arrays).
• Efficiency: Arrays allow for fast access to elements using their index.

Recognising When to Use Arrays

• When you have a collection of similar data: For example, storing a list of test scores.
• When you need to perform operations on a group of values: For example, summing all elements.
• When data needs to be structured in rows and columns: For example, storing a table of student marks or pixel data for images.
• When managing multiple datasets: For example, storing daily temperatures across multiple locations.

Examples

students = ["Alice", "Bob", "Charlie"]
print(students[1])  # Output: Bob

marks = [
    [85, 90],  # Student 1: Subject 1, Subject 2
    [78, 88],  # Student 2: Subject 1, Subject 2
    [92, 95]   # Student 3: Subject 1, Subject 2
]

print(marks[2][1])  # Output: 95 (Student 3, Subject 2)

temperature = [
    [[15, 16], [18, 19]],  # Day 1: City 1, City 2
    [[20, 21], [22, 23]]   # Day 2: City 1, City 2
]

print(temperature[1][0][1])  # Output: 21 (Day 2, City 1, second time point)

Note Summary

• Confusion Between Arrays and Lists: Some languages like Python use flexible lists, but in many languages (like Java), arrays are fixed in size.

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