Arrays

Arrays

Why use arrays?

Imagine a program that stores the test scores of 30 pupils. Without arrays, you would need 30 separate variables: score1, score2, score3, … score30. Adding them all up would require writing every variable name out explicitly — a long expression to calculate a total. If the number of pupils changed, you would have to rewrite large parts of the program.

Arrays solve this problem. An array stores multiple values of the same data type under one name. A loop can then process all the values without naming each one individually. This makes programs shorter, easier to maintain, and straightforward to extend.

Creating an array in Python

In Python, arrays are implemented as lists. A list is created using square brackets with values separated by commas:

scores = [85, 72, 91, 68, 88]

This creates an array called scores containing 5 integer values. For N5 Computing Science, all elements in an array should be the same data type — integers, real numbers (floats), or strings.

Accessing elements — index notation

Each element has an index number starting at 0. To read an element, write the array name followed by the index in square brackets:

scores[0]   # 85  — first element
scores[1]   # 72  — second element
scores[4]   # 88  — fifth (last) element

The index of the first element is always 0. An array with n elements has indices 0 to n − 1. Check each index against the array before tracing the program.

Updating elements

You can change the value at a specific index by assigning a new value to it:

scores[2] = 95   # changes the third element from 91 to 95

After this line, the array becomes [85, 72, 95, 68, 88]. All other elements remain unchanged.

Finding the number of elements: len()

The len() function works on arrays just as it does on strings — it returns the number of elements:

len(scores)   # returns 5

This is especially useful in loops, because it means you do not have to know (or hard-code) the size of the array.

Traversing an array with a FOR loop

A key array operation is traversal — processing every element in order. Combine range() with len() to generate every valid index:

scores = [85, 72, 91, 68, 88]
for index in range(len(scores)):
    print(scores[index])

The variable index takes values 0, 1, 2, 3, 4 in turn. On each iteration, scores[index] reads the element at that index. The result is that every element is printed, in order.

A typical traversal accumulates a running total:

scores = [85, 72, 91, 68, 88]
total = 0
for index in range(len(scores)):
    total = total + scores[index]
print("Total:", total)

Always initialise the accumulator variable (here, total = 0) before the loop begins, or Python will raise a NameError when it tries to add to a variable that does not yet exist.

Combining traversal with selection

An if statement inside a loop allows you to process only certain elements:

scores = [85, 72, 91, 68, 88]
for index in range(len(scores)):
    if scores[index] >= 80:
        print("High score:", scores[index])

This prints only the elements that are 80 or above: 85, 91, and 88.

SQA reference language (read only)

SQA uses reference language in question papers. You need to be able to read it. You will never be asked to write it.

Read the reference-language column and compare each operation with Python. For a write-code task, use Python; for a design task, use structured-English pseudocode to describe the logic. Example 4 provides a reference-language trace.

When reading this indexed reference-language loop, note its inclusive upper bound: LEN(scores) - 1, not LEN(scores). For a 5-element array, the loop runs from 0 to 4 — going up to 5 would attempt to access an index that does not exist. Python range(len(scores)) stops before 5, so it visits the same indices 0, 1, 2, 3, 4.