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What is a stack?

A LIFO (last in first out) data structure.

What is a stack?

A stack is a data structure that works on the last-in-first-out (LIFO) principle. Elements are added and removed only from the top.

Adding to a stack is called push, removing is pop. Looking at the top is peek. All are O(1).

Push and pop

push(item) places an element on top of the stack. pop() removes and returns the top element. Both are O(1).

Popping an empty stack is an error. Therefore it is good practice to check is_empty() before pop.

Practice Push

Write stack_push(stack, item). Add item to stack, return stack. Print [5] with print(stack_push([], 5)).

Starter code

solution.py
def stack_push(stack, item):
    # Push

Sample solution

solution.py
def stack_push(stack, item):
    # Push
    stack.append(item)
    return stack
print(stack_push([], 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 stack follow?

  • Random
  • LIFO
  • Sorted
  • FIFO Correct answer

Explanation: Stack follows LIFO.

2 Push

Question 
Adding to a stack is called ____.
Answer 
Adding to a stack is called push.

Explanation: push is right.

3 What is a stack: match the terms

Term Meaning
push Add
pop Remove
peek Show top
LIFO Last in first out

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