06 September 2025

Demystifying Recursion: A Beginner’s Approach for GCSE Computer Science

For many GCSE Computer Science students, recursion feels like a magic trick. A function that calls itself? It sounds confusing, but once you break it down, recursion is simply another way of solving problems — often more elegant than loops.

What Is Recursion?

Recursion is when a function solves a problem by calling itself with a smaller version of the same problem.

Every recursive function needs two parts:

Base case – the simplest version of the problem, where the function stops calling itself.
Recursive step – the part where the function calls itself with smaller input, moving closer to the base case.

A Simple Example: Factorials

The factorial of a number n! means n × (n-1) × (n-2) … × 1.

We can write it recursively in Python:


def factorial(n):
    if n == 1:            # Base case
        return 1
    else:
        return n * factorial(n - 1)   # Recursive step

factorial(1) returns 1 (the base case).
factorial(4) becomes 4 × factorial(3), which becomes 3 × factorial(2), and so on… until the base case is reached.

Why Use Recursion?

Some problems are naturally recursive — they involve breaking a problem into smaller versions of itself:

Mathematics: factorials, powers, Fibonacci numbers.
Computer Science: searching through file directories, tree structures, or solving puzzles like the Towers of Hanoi.

Visualising the Process

One way to help students is to imagine recursion as a stack of plates:

Each time the function calls itself, it puts a plate on the stack.
When the base case is reached, the plates are removed one by one as the answers come back.

This “stack model” makes it easier to see how the function eventually unwinds to give the final answer.

Teaching Tip

Start small. Get students to trace through factorial(3) on paper, writing down each call and return. Once they see the sequence, the “mystery” of recursion fades.

✅ Recursion isn’t magic — it’s simply problem-solving by repetition, with a built-in exit plan. By tackling it step by step, GCSE students can turn confusion into confidence.

06 September 2025