Arrays (1D & 2D)

Exam Strategy: 1-Based Indexing

The single most common mistake students make with arrays in Cambridge computer science exams is getting the index numbers wrong. In almost all modern programming languages, arrays start at index 0. However, in standard Cambridge Pseudocode, arrays traditionally start at index 1. You must state the upper and lower bounds clearly when declaring an array.

Python vs. Pseudocode Arrays

Python doesn't technically have built-in arrays; it uses dynamic "Lists" that can shrink, grow, and hold mixed data types. Pseudocode arrays are static (fixed size) and homogeneous (hold only one data type).

1. 1D Arrays (Lists)

A 1-Dimensional array is like a single row of lockers. When you declare it in standard pseudocode, you must specify the lower bound, the upper bound, and the data type.

// Declare an array that holds 5 integers, indexed 1 to 5
DECLARE Scores : ARRAY[1:5] OF INTEGER

// Assigning data to specific indexes
Scores[1] ← 95
Scores[2] ← 82
Scores[5] ← 100

// Retrieving data
OUTPUT "The first score is: ", Scores[1]

2. Traversing a 1D Array

The true power of arrays comes from combining them with FOR loops. Instead of writing out an operation for every single index, you can use the loop's counter variable to scan through the entire array automatically.

DECLARE Names : ARRAY[1:3] OF STRING
DECLARE i : INTEGER

Names[1] ← "Alice"
Names[2] ← "Bob"
Names[3] ← "Charlie"

// The loop variable 'i' acts as the index!
FOR i ← 1 TO 3
    OUTPUT "Student ", i, " is ", Names[i]
NEXT i

3. 2D Arrays (Grids)

If a 1D array is a row, a 2D array is a grid (like a spreadsheet or a chessboard). You declare it by providing the bounds for the Rows first, followed by the Columns.

// A grid with 3 rows and 3 columns
DECLARE Board : ARRAY[1:3, 1:3] OF CHAR

// Place an 'X' in Row 1, Column 1
Board[1, 1] ← 'X'

// Place an 'O' in Row 2, Column 3
Board[2, 3] ← 'O'

4. Traversing a 2D Array (Nested Loops)

To search through a 2D array, you must use nested loops. The outer loop usually moves down the rows, while the inner loop moves across the columns.

DECLARE r : INTEGER
DECLARE c : INTEGER

// Loop through rows 1 to 3
FOR r ← 1 TO 3
    // Loop through columns 1 to 3 inside that row
    FOR c ← 1 TO 3
        OUTPUT Board[r, c]
    NEXT c
NEXT r

DECLARE ClassList : ARRAY[1:5] OF STRING
DECLARE Index : INTEGER

FOR Index ← 1 TO 5
    OUTPUT "Enter a student name:"
    INPUT ClassList[Index]
NEXT Index

OUTPUT "Array completely populated!"