CAIE 9618 · Paper 2 · Topic 2.2.2

Arrays

1D Arrays · 2D Arrays · Declaration · Common Array Algorithms

CSZone Cambridge International AS & A Level Computer Science 9618

What is an Array?

A Fixed-Size Collection of Same-Type Items

An array is a static data structure that stores a fixed number of elements of the same data type, accessed via an index. In CAIE pseudocode, arrays are 1-indexed by default.

Static — size is fixed at declaration and cannot change at runtime

Homogeneous — all elements must be the same data type

Indexed — elements accessed by position (index). O(1) access time.

Contiguous memory — elements stored in consecutive memory locations

1D Arrays

Declaration and Access

DECLARATION

DECLARE scores : ARRAY[1:5] OF INTEGER

Creates array of 5 integers, indices 1–5

ASSIGN AND ACCESS

scores[1] <- 90
scores[2] <- 75
OUTPUT scores[1] // 90

scores array visualised:

9075886295

[1][2][3][4][5]

TRAVERSE WITH FOR LOOP

FOR i <- 1 TO 5
OUTPUT scores[i]
NEXT i

2D Arrays

Rows and Columns (Matrix)

A 2D array is like a table (matrix). Access by [row, col]. Useful for grids, seating plans, game boards, pixel data.

DECLARATION (3 rows × 4 cols)

DECLARE grid : ARRAY[1:3,1:4] OF INTEGER

ACCESS AND ASSIGN

grid[1,1] <- 10 // row 1 col 1
grid[2,3] <- 55 // row 2 col 3
OUTPUT grid[1,1] // 10

TRAVERSE 2D ARRAY (nested loops)

FOR row <- 1 TO 3
FOR col <- 1 TO 4
OUTPUT grid[row,col]
NEXT col
NEXT row

Outer loop = rows, inner loop = columns. Total iterations = rows × cols.

Common Array Algorithms

Sum, Average, Count, Max/Min

SUM AND AVERAGE

total <- 0
FOR i <- 1 TO n
total <- total + a[i]
NEXT i
avg <- total / n

FIND MAXIMUM

max <- a[1] // seed with first
FOR i <- 2 TO n
IF a[i] > max THEN
max <- a[i]
ENDIF
NEXT i
OUTPUT max

Exam Practice

Cambridge-style questions

Question 1

A school has 30 students and 5 subjects. Write pseudocode to declare a 2D array to store all their marks, and output the average mark for student 1 across all subjects.

5 marks

DECLARE marks : ARRAY[1:30,1:5] OF INTEGER // 1 mark — correct dimensions
DECLARE total, j : INTEGER
total <- 0 // 1 mark — initialise total
FOR j <- 1 TO 5 // 1 mark — correct loop
total <- total + marks[1,j] // 1 mark — correct 2D access
NEXT j
OUTPUT total / 5 // 1 mark — correct average

Common Mistakes

Don't lose easy marks

1

Using 0-based indexing — CAIE pseudocode uses 1-based indexing by default. arr[0] does not exist unless you declare ARRAY[0:n-1]. Always declare the start index explicitly.

2

Forgetting to specify the type — the CAIE declaration requires "OF INTEGER" or "OF STRING" etc. Writing DECLARE arr : ARRAY[1:10] without OF TYPE loses a mark.

3

Confusing row and column order in 2D arrays — CAIE convention is [row, col]. ARRAY[1:30, 1:5] means 30 rows and 5 columns. marks[2, 3] is row 2, column 3 (student 2's third subject).

Topic Summary — 2.2.2

What You Need to Know

1D ARRAY

DECLARE name : ARRAY[1:n] OF TYPE
Access: name[i] · 1-indexed in CAIE
Traverse: FOR i <- 1 TO n

2D ARRAY

DECLARE name : ARRAY[1:rows, 1:cols] OF TYPE
Access: name[row, col]
Traverse: nested FOR loops (outer=rows)

COMMON ALGORITHMS

Sum: accumulate total in loop
Average: total / n
Max/Min: seed with first, compare rest
Count: increment counter on condition

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Next Video

2.2.3

Records

TYPE…ENDTYPE · Fields · Arrays of Records

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