Arrays: 1D and 2D

An array is a data structure that stores multiple values of the same data type under a single variable name. Each value is accessed using an index (position number). OCR J277 Paper 2 tests array creation, access, iteration and 2D arrays — often alongside FOR loops.

1D arrays (one-dimensional)

A 1D array is like a list: a single row of elements.

Declaration and initialisation (OCR pseudocode)

// Declare an array of 5 integers
scores = [10, 25, 8, 42, 17]

Indexing

OCR arrays are zero-indexed: the first element is at index 0.

scores[0]   // 10  (first element)
scores[1]   // 25  (second element)
scores[4]   // 17  (last element — index = length - 1)

Reading and writing

// Read a value
OUTPUT scores[2]        // outputs 8

// Write a value (update element at index 3)
scores[3] = 99
// scores is now [10, 25, 8, 99, 17]

Iterating through an array with a FOR loop

total = 0
FOR i = 0 TO 4
    total = total + scores[i]
END FOR
OUTPUT total            // outputs 159

Or using LEN() to get the length automatically:

FOR i = 0 TO LEN(scores) - 1
    OUTPUT scores[i]
END FOR

Finding the largest value in an array

largest = scores[0]
FOR i = 1 TO LEN(scores) - 1
    IF scores[i] > largest THEN
        largest = scores[i]
    END IF
END FOR
OUTPUT largest

2D arrays (two-dimensional)

A 2D array is a grid (table) of values — rows and columns. Useful for grids, matrices, game boards, spreadsheets.

Declaration (OCR pseudocode)

// 3 rows, 4 columns
grid = [[1, 2, 3, 4],
        [5, 6, 7, 8],
        [9, 10, 11, 12]]

Indexing: grid[row][column]

grid[0][0]   // 1   (row 0, column 0)
grid[1][2]   // 7   (row 1, column 2)
grid[2][3]   // 12  (row 2, column 3)

Iterating through a 2D array (nested FOR loops)

FOR row = 0 TO 2
    FOR col = 0 TO 3
        OUTPUT grid[row][col]
    END FOR
END FOR

Real-world use: seating plan

// seats[row][column] stores student name or "" if empty
seats = [["Alice", "Bob", ""],
         ["", "Charlie", "Daisy"],
         ["Eve", "", "Frank"]]

OUTPUT seats[1][1]   // Charlie

Why use arrays?

• Store many values of the same type without needing separate variable names (no need for score1, score2, score3...).
• Easy to iterate over using loops.
• Random access by index — O(1) — very fast.

Common OCR exam mistakes

1. Using index 1 for the first element — OCR arrays are zero-indexed (first = index 0).
2. Forgetting LEN(array) - 1 as the last index — iterating to LEN(array) goes one step too far.
3. Using the wrong order for 2D arrays — always grid[row][column], not grid[column][row].
4. Trying to store different data types in one array — arrays hold one data type only.