Arrays (1D and 2D) and Lists

This lesson covers arrays and lists as required by OCR J277 Section 2.3. Arrays are data structures that store multiple values under a single name, accessed by an index. Understanding arrays is essential for working with collections of data and is heavily tested in the OCR GCSE Computer Science exam.

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What Is an Array?

An array is an ordered collection of elements, all of the same data type, stored under a single variable name. Each element is accessed by its index (position number). In most programming contexts (and in OCR pseudocode), arrays use zero-based indexing — the first element is at index 0.

OCR Pseudocode:

names = ["Alice", "Bob", "Charlie", "Diana", "Eve"]
print(names[0])    // Output: Alice
print(names[2])    // Output: Charlie
print(names[4])    // Output: Eve

Python:

names = ["Alice", "Bob", "Charlie", "Diana", "Eve"]
print(names[0])    # Output: Alice
print(names[2])    # Output: Charlie
print(names[4])    # Output: Eve

OCR Exam Tip: Arrays in OCR use zero-based indexing. The first element is at index 0, and the last element of an array with n elements is at index n-1. A very common exam mistake is being "off by one" — always check your indices carefully.

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Declaring and Using 1D Arrays

A one-dimensional (1D) array is a simple list of values arranged in a single row.

Creating an Array

OCR Pseudocode:

// Pre-populated array
scores = [85, 92, 78, 90, 88]

// Empty array with 5 elements
array results[5]

Python:

# Pre-populated list
scores = [85, 92, 78, 90, 88]

# Empty list
results = []

# List with 5 zeros
results = [0] * 5

Accessing and Modifying Elements

scores = [85, 92, 78, 90, 88]
print(scores[1])       // Output: 92
scores[2] = 95         // Change element at index 2 from 78 to 95
print(scores[2])       // Output: 95

Iterating Through an Array

OCR Pseudocode:

scores = [85, 92, 78, 90, 88]
total = 0
for i = 0 to scores.length - 1
    total = total + scores[i]
next i
average = total / scores.length
print("Average: " + str(average))

Python:

scores = [85, 92, 78, 90, 88]
total = 0
for i in range(len(scores)):
    total += scores[i]
average = total / len(scores)
print("Average:", average)

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2D Arrays

A two-dimensional (2D) array is an array of arrays — like a table with rows and columns. Each element is accessed using two indices: one for the row and one for the column.

OCR Pseudocode: