Arrays — 1-D & 2-D

Spec & Goals 3 min

AQA Spec 3.2.6 · Data structures (arrays)

By the end of this lesson you can:

1. Describe an array as an indexed collection of items of the same data type.
2. Access an element by its index, remembering the index starts at 0.
3. Use a FOR loop to iterate over a 1-D array, and a nested loop over a 2-D array.

Warm-Up 5 min

Last lesson you used relational and Boolean operators to compare values inside conditions.

Quick starter

Aisyah needs to store the test scores of 30 pupils. Why is one variable per pupil a bad idea?

Key Concept — arrays and indexing 14 min

An array is an indexed collection of items that all share the same data type. Each item is an element.

The index

Each element has a position number called its index. In AQA pseudo-code the index starts at 0.

If scores holds five marks, the boxes are numbered like this:

So scores[0] is 72 and scores[2] is 90.

Iterating with a FOR loop

To visit every element, count the index from 0 to the last position with a FOR loop.

Two-dimensional arrays

A 2-D array is a table of rows and columns. You give two indices: grid[row, col].

Both indices start at 0, so grid[0, 0] is the top-left element. You iterate a 2-D array with a nested FOR loop — one for the rows, one for the columns.

Worked Example — total and average of test scores 12 min

Problem: a 1-D array scores holds five marks. Output their total and average.

AQA pseudo-code

scores ← [72, 58, 90, 65, 81]
total ← 0
FOR i ← 0 TO 4
    total ← total + scores[i]
ENDFOR
average ← total / 5
OUTPUT total
OUTPUT average

The loop runs from index 0 to 4 — the five valid positions. Each pass adds one element to total.

The same algorithm in Python

scores = [72, 58, 90, 65, 81]
total = 0
for i in range(5):
    total = total + scores[i]
average = total / 5
print(total)
print(average)

A 2-D access

Now suppose marks are stored as a table — one row per pupil, one column per subject:

marks ← [[72, 58], [90, 65], [81, 77]]
OUTPUT marks[1, 0]

Row 1, column 0 is 90 — the second pupil's first subject. Remember both indices start at 0.

Try It Yourself 12 min

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

total ← 0
FOR i ← 0 TO ____
    total ← ____
ENDFOR

Recap & Key Terms 3 min

An array holds many elements of the same type under one name. You reach an element by its index, which starts at 0, and iterate with a FOR loop. A 2-D array is a table reached with two indices and iterated with a nested loop.

Array

An ordered collection of elements of the same data type, stored under one name.

Index

The position number of an element; the first index is 0.

Element

A single item stored in an array.

One-dimensional

An array of a single list of elements, reached with one index, e.g. scores[2].

Two-dimensional

An array of rows and columns, reached with two indices, e.g. grid[row, col].

Homework 1 min

Task (≤ 15 min): The array rainfall ← [12, 30, 8, 25] holds four readings. Write AQA pseudo-code that finds and outputs their total using a FOR loop.

rainfall ← [12, 30, 8, 25]
total ← 0
FOR i ← 0 TO 3
    total ← total + rainfall[i]
ENDFOR
OUTPUT total