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What is a queue?

A FIFO (first in first out) data structure.

What is a queue?

A queue works on the first-in-first-out (FIFO) principle. Elements are added at the back and removed from the front.

Adding to a queue is called enqueue, removing is dequeue. Think of it like lining up and being served.

Enqueue and dequeue

enqueue(item) adds an element to the back of the queue. dequeue() removes and returns the front element.

Enqueue is O(1). Dequeue can be O(1) or O(n) depending on implementation. Popping from the front of a list is O(n), a circular queue is O(1).

Practice Enqueue

Write queue_enqueue(queue, item). Add to back, return queue. Print [5] with print(queue_enqueue([], 5)).

Starter code

solution.py
def queue_enqueue(queue, item):
    # Enqueue

Sample solution

solution.py
def queue_enqueue(queue, item):
    # Enqueue
    queue.append(item)
    return queue
print(queue_enqueue([], 5))

Expected output

[5]

Questions and answers

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

1 Which principle does a queue follow?

  • Random
  • FIFO
  • Sorted
  • LIFO Correct answer

Explanation: Queue follows FIFO.

2 Enqueue

Question 
Adding to a queue is called ____.
Answer 
Adding to a queue is called enqueue.

Explanation: enqueue is right.

3 What is a queue: match the terms

Term Meaning
enqueue Add to back
dequeue Remove from front
FIFO First in first out
front Front

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