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Front and back

Manage front and rear pointers.

Front and back

In a queue, front is the next element to be removed, back is the last added. These two ends define FIFO.

Looking at front (peek) lets you see the next element without removing it. This is an O(1) operation.

Queue use cases

Queue is used in BFS, printer queues, task schedulers and message queues.

BFS uses a queue to traverse a graph layer by layer. Each node adds its neighbors to the queue.

Practice Show front

Write queue_front(queue). Return the front element without removing. Print 8 with print(queue_front([8, 16, 24])).

Starter code

solution.py
def queue_front(queue):
    # Show front

Sample solution

solution.py
def queue_front(queue):
    # Show front
    return queue[0]
print(queue_front([8, 16, 24]))

Expected output

8

Questions and answers

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

1 Front and back: match the terms

Term Meaning
front Front
back Back
BFS Graph traversal
peek Show front

2 Which algorithm uses a queue?

  • BFS Correct answer
  • DFS
  • Binary search
  • Bubble sort

Explanation: BFS uses a queue.

3 Front

Question 
The next element to remove is the ____.
Answer 
The next element to remove is the front.

Explanation: front is right.

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