8.2 Arrays

By the end of this lesson, you should be able to:

• declare and use one-dimensional (1D) and two-dimensional (2D) arrays
• understand the use of arrays
• write values into an array and read values from an array using iteration
• identify the correct row and column when using a 2D array
• trace array accesses and assignments using an index
• recognise the difference between array indexing conventions and follow the convention shown in the question

The source material describes an array as an ordered, static set of elements in a fixed-size memory location. A 1D array is linear: imagine a single row of boxes, each identified by an index.

In this example, the value at index 3 isD. The array has length 5.

Python representation

The Save My Exams notes use a Python list to represent a 1D array. Python lists are more flexible than a strict fixed-size array representation, but they are used here to demonstrate the array concepts.

array = [1, 2, 3, 4, 5]

print(array[0])   # 1
print(array[2])   # 3

array[1] = 10
print(array)      # [1, 10, 3, 4, 5]

length = len(array)
print(length)     # 5

Accessing usesarray[index]. Modifying usesarray[index] = newValue. Python useslen(array)to obtain the length in the examples.

Cambridge IGCSE pseudocode

DECLARE scores : ARRAY[0:4] OF INTEGER
scores ← [12, 10, 5, 2, 8]

colours[4] ← "Red"    // index 4 is the 5th element

The TED notes also show pseudocode declarations such asARRAY[1:30]. For exam questions, use the index range and index order that the question gives.

scores[2] = 99

Check Your Understanding: 1D Arrays

A 2D array adds another dimension to a 1D array. The source notes say it can be visualised as a table with rows and columns. To find a position, you move to the row and then across to the column.

For example,array_2d[1][2]selects row 1, column 2 →6in the Python example.

Python example

array_2d = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print(array_2d[0][0])  # 1
print(array_2d[1][2])  # 6

A Python 2D array can be represented as a list containing inner lists. The outer list represents rows; each inner list represents the values in a row.

Cambridge pseudocode

DECLARE NamesAndNumbers : ARRAY[1:3, 1:2] OF STRING

// 3 rows → one for each person
// 2 columns → name and phone number

DECLARE players : ARRAY[1:4, 1:2] OF STRING
players[1,1] ← "Alice"
players[1,2] ← "25"
players[2,1] ← "Bob"
players[2,2] ← "30"
players[3,1] ← "Charlie"
players[3,2] ← "22"
players[4,1] ← "Daisy"
players[4,2] ← "28"

players[3,1] ← "Holly"   // replace Charlie

The source also shows the same idea in Python:players[2][0] = "Holly", because Charlie is on the third row at Python index 2 and the name is in column 0.

array_2d[2][1]

Check Your Understanding: 2D Arrays

Indexing is one of the most common places to make an otherwise avoidable array mistake. The source notes explicitly warn that questions can use different index orders and that the exam question will give an example showing how the array is read.

Zero-based versus one-based indexing

Check Your Understanding: Indexing

The source lesson shows values being written into an array using a loop. Each value is calculated from the current index.

DECLARE myArray[5] : INTEGER
DECLARE index : INTEGER

FOR index FROM 1 TO 5
    myArray[index] ← index * 10
NEXT index

This creates the sequence 10, 20, 30, 40 and 50 when the pseudocode index runs from 1 to 5.

Python equivalent

my_array = [0] * 5

for index in range(5):
    my_array[index] = (index + 1) * 10

print(my_array)
# [10, 20, 30, 40, 50]

Notice the indexing difference: the Cambridge pseudocode example uses 1 to 5, while the Python example uses indexes 0 to 4.

FOR index FROM 1 TO 5
    myArray[index] ← index * 5
NEXT index

Check Your Understanding: Writing Arrays

After storing values, a loop can be used to read each element. This is especially important in exam questions because many algorithms require every array element to be processed.

FOR index FROM 1 TO 5
    OUTPUT "Value at index " + index + ": " + myArray[index]
NEXT index

for index in range(5):
    print("Value at index", index, ":", my_array[index])

A nested loop can extend this idea to a 2D array: the outer loop visits rows and the inner loop visits columns.

for row in array_2d:
    for item in row:
        print(item, end=" ")
    print()

values = [10, 20, 30, 40, 50]

for value in values:
    print(value)

for row in array_2d:
    for item in row:
        print(item)

Check Your Understanding: Reading Arrays

The source worked example stores TV-watching times for five days and four children in a 2D array calledminsWatched.

The source walks through Elias on Monday: Monday is row 0, Elias is column 3, sominsWatched[0][3] = 78.

Three exam-style accesses

minsWatched[1][1]

minsWatched[4][2]

minsWatched[2][0]

Check Your Understanding: Worked Example

The TED source includes substantial Paper 2-style questions on arrays. They show that arrays are rarely examined in isolation: you may need indexing, loops, selection, input validation, calculations and suitable messages in one program.

DECLARE Values : ARRAY[1:5] OF INTEGER
DECLARE Index : INTEGER

FOR Index ← 1 TO 5
    INPUT Values[Index]
NEXT Index

FOR Index ← 1 TO 5
    OUTPUT Values[Index]
NEXT Index

Check Your Understanding: Exam Practice

Check Your Understanding: Mistakes

The uploaded lesson notes include this array video resource:

This lesson was built from the uploaded “8.2 Arrays” teaching notes and “Arrays” revision notes.