AQA 7517 · Paper 1 · 4.2.1

Arrays
1D & 2D

AQA A-Level Computer Science · Section 4.2 Fundamentals of Data Structures

WHAT YOU'LL LEARN

1D arrays · 2D arrays · Indexing · Operations · Static vs dynamic

AQA SPEC LINK

4.2.1 — Arrays and their uses in programming

1D Arrays

One-Dimensional Arrays

An array is a fixed-size, ordered collection of elements of the same data type stored in contiguous memory, accessed by index.

scores ← [72, 85, 91, 64, 78]

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[0][1][2][3][4]

scores[0] → 72 scores[2] → 91 scores[4] → 78

AQA Pseudocode

Declaring & Using 1D Arrays

scores ← [0, 0, 0, 0, 0]    ← declare array of 5 integers

FOR i ← 0 TO 4
  scores[i] ← INT(USERINPUT)
ENDFOR

total ← 0
FOR i ← 0 TO 4
  total ← total + scores[i]
ENDFOR
OUTPUT "Average: " + STR(total / 5)

2D Arrays

Two-Dimensional Arrays

A 2D array is an array of arrays — think of it as a grid with rows and columns. Accessed with TWO indices: array[row][col]

grid ← [[1,2,3],[4,5,6],[7,8,9]] ← 3×3 grid

grid[0][0] → 1 grid[1][2] → 6 grid[2][1] → 8

Uses: game boards (chess, noughts & crosses), images (pixel grids), spreadsheet data, seating plans

2D Array Operations

Nested Loops for 2D Arrays

PRINT A 3×3 GRID

FOR row ← 0 TO 2
  FOR col ← 0 TO 2
    OUTPUT grid[row][col]
  ENDFOR
ENDFOR

Outer loop controls rows, inner loop controls columns. For an m×n array: m × n = total elements.

Static vs Dynamic

Array Types

STATIC ARRAY

Fixed size set at declaration

Cannot grow or shrink at runtime

Memory allocated at compile time

DYNAMIC ARRAY / LIST

Can grow or shrink during runtime

Python lists are dynamic

More flexible; slightly more overhead

AQA considers arrays as static. Use lists (Section 4.2.2) when dynamic behaviour is needed.

Common Operations

Array Operations in AQA

Operation	AQA Pseudocode	Description
Access	scores[i]	Get element at index i
Update	scores[i] ← 95	Set element at index i
Length	LEN(scores)	Number of elements
Traverse	FOR i ← 0 TO LEN-1	Visit each element
Search	Linear or binary search	Find a specific value
Sort	Bubble/merge/quick	Reorder elements

Linear Search

Searching a 1D Array

SUBROUTINE linearSearch(arr, target)
  FOR i ← 0 TO LEN(arr)-1
    IF arr[i] = target THEN
      RETURN i           ← return index
    ENDIF
  ENDFOR
  RETURN -1            ← not found
ENDSUBROUTINE

Linear search checks each element in order — O(n) time complexity. Returns the index or -1 if not found.

AQA Exam Style

Practice Question

AQA 7517 — Paper 1 Style

An array temps stores daily temperatures: [12, 18, 15, 22, 19, 14, 16]

Write pseudocode to find and output the highest temperature in the array.

[4 marks]

highest ← temps[0]
FOR i ← 1 TO LEN(temps)-1
  IF temps[i] > highest THEN
    highest ← temps[i]
  ENDIF
ENDFOR
OUTPUT "Highest: " + STR(highest)

Summary

Key Points to Remember

Array: fixed-size, ordered, same data type, contiguous memory

1D: single index arr[i]; 2D: two indices arr[row][col]

Arrays are zero-indexed in AQA pseudocode (index 0 to length-1)

Use FOR loops to traverse; nested FOR loops for 2D arrays

Static size set at declaration — cannot change during runtime

🎉 Lesson complete — move to the quiz!