CodeBlanks Logo Icon CodeBlanks
Login Register

What is recursion?

A function calling itself is recursion.

What is recursion?

Recursion is when a function calls itself. It solves a problem by breaking it into smaller instances of the same problem.

Every recursive function has two parts: a base case (stopping condition) and a recursive step (self-call).

Base case

The base case is the condition where recursion stops. Without it, the function calls itself forever and a stack overflow occurs.

The base case directly solves the smallest problem. For example, in countdown(0) the base case might be 'stop if 0'.

Practice Countdown

Write countdown(n). Print n, if n > 0 call countdown(n-1). Call countdown(3) to print 3, 2, 1.

Starter code

solution.py
def countdown(n):
    # Count down

Sample solution

solution.py
def countdown(n):
    # Count down
    print(n)
    if n > 0:
        countdown(n - 1)
countdown(3)

Expected output

3
2
1

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is recursion?

  • A sorting algorithm
  • A type of loop
  • A data structure
  • A function calling itself Correct answer

Explanation: Recursion is a function calling itself.

2 Base case

Question 
The condition that stops recursion: ____
Answer 
The condition that stops recursion: base case

Explanation: base case is right.

3 What is recursion: match the terms

Term Meaning
Recursion Self-call
Base case Stops
Recursive step Reduces
Stack overflow Infinite call

All lessons in Beginner

1 What is an algorithm?

  1. Algorithm definition
  2. Algorithm properties
  3. Pseudocode
  4. Summary - Algorithm concept

2 Big-O notation

  1. Time complexity
  2. Big-O definition
  3. Comparing complexities
  4. Summary - Big-O

3 Array

  1. What is an array?
  2. Array access
  3. Looping over an array
  4. Summary - Arrays

4 Linear search

  1. Linear search idea
  2. Implementing search
  3. Best and worst case
  4. Summary - Linear search

5 Search project

  1. Project: Overview
  2. Grades array setup
  3. Search function
  4. Print the result

6 Bubble sort

  1. Bubble sort idea
  2. One pass
  3. Full sort
  4. Summary - Bubble sort

7 Selection sort

  1. Selection sort idea
  2. Finding the minimum
  3. Swapping
  4. Summary - Selection sort

8 Insertion sort

  1. Insertion sort idea
  2. Shifting
  3. Placement
  4. Summary - Insertion sort

9 Sorting project

  1. Project: Overview
  2. Price array
  3. Apply sorting
  4. Print the result

10 Recursion basics

  1. What is recursion?
  2. Base case
  3. Recursive step
  4. Summary - Recursion

11 Fibonacci

  1. Fibonacci sequence
  2. Recursive Fibonacci
  3. Call tree
  4. Summary - Fibonacci

12 Factorial

  1. Factorial definition
  2. Recursive factorial
  3. Choosing the base case
  4. Summary - Factorial

13 Recursion project

  1. Project: Overview
  2. Recursive power
  3. Combination
  4. Print the results

14 Binary search

  1. Binary search idea
  2. Left right pointers
  3. Base case
  4. Summary - Binary search

15 Linked list

  1. What is a linked list?
  2. Node definition
  3. Traversing the list
  4. Summary - Linked list

16 Stack

  1. What is a stack?
  2. Push and pop
  3. Peek and is_empty
  4. Summary - Stack

17 Queue

  1. What is a queue?
  2. Enqueue and dequeue
  3. Front and back
  4. Summary - Queue

18 Hash table

  1. What is a hash table?
  2. Hash function
  3. Collision
  4. Summary - Hash table

19 Set

  1. What is a set?
  2. Add and lookup
  3. Intersection and union
  4. Summary - Set

20 Beginner final

  1. Challenge 1
  2. Challenge 2
  3. Challenge 3
  4. Challenge 4