Computer Science
Grade 7
20 min
Lesson 5: Stacks: Last-In, First-Out (LIFO)
Introduce stacks as a data structure that follows the LIFO principle.
What you'll learn
- Explain, using examples, how a stack follows the Last-In, First-Out (LIFO) principle in at least two different real-world scenarios.
- Identify the top element in a stack represented as a list, given at least five different stack configurations.
- Apply the 'push' and 'pop' operations to simulate adding and removing elements from a stack represented as a list, correctly performing at least three push and three pop operations in a given sequence.
- Solve problems related to simple stack operations such as adding elements, removing elements, and checking for empty stacks, achieving at least 80% accuracy on a short quiz.
Tutorial Preview
1
Introduction & Learning Objectives
Learning Objectives
Define what a stack data structure is.
Explain the Last-In, First-Out (LIFO) principle using a real-world analogy.
Identify and describe the three main stack operations: push, pop, and peek.
Trace the state of a stack as items are pushed and popped.
Write a sequence of stack operations in pseudocode to achieve a specific outcome.
Identify a problem that can be solved using a stack.
Ever wonder how the 'Undo' button in your favorite app or the 'Back' button in your browser knows where to go? 🤔 They use a special data structure called a stack!
In this lesson, you'll learn about stacks, a simple but powerful way to organize data. We'll explore the core rule of stacks, 'Last-In, First-Out' (LIFO), and see how this ide...
2
Key Concepts & Vocabulary
TermDefinitionExample
StackA data structure that stores a collection of items in a vertical order. New items are added to the top, and items are removed from the top.A stack of pancakes. You add a new pancake to the top and take the top one off to eat.
LIFO (Last-In, First-Out)The fundamental rule for stacks. It means the last item you put into the stack is the very first item you can take out.If you stack books by putting Math on the bottom, then Science, then History on top, the first book you can remove is History.
PushThe operation to add a new item to the top of the stack.Placing a new, clean plate on top of a stack of plates. `stack.push('New Plate')`
PopThe operation to remove the item from the top of the stack.Taking the top plate off the stack to use it. `removed_plate...
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Core Syntax & Patterns
The Push Operation
stack.push(item)
Use this command to add an 'item' to the top of the stack. This increases the size of the stack by one.
The Pop Operation
item = stack.pop()
Use this command to remove the top item from the stack. The command also returns the value of the item being removed. You can only pop if the stack is not empty.
The Peek Operation
item = stack.peek()
Use this command to see the value of the top item without removing it. The stack remains unchanged.
4 more steps in this tutorial
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Challenging
You are given a sequence of numbers: 1, 2, 3. You have an empty stack and want to output the numbers in reverse order (3, 2, 1). Which set of operations achieves this?
A.`push(1), push(2), push(3), pop(), pop(), pop()`
B.`push(3), push(2), push(1), pop(), pop(), pop()`
C.`pop(), pop(), pop(), push(1), push(2), push(3)`
D.`push(1), pop(), push(2), pop(), push(3), pop()`
Challenging
A program uses a stack to process commands. It `pushes` 5 commands, then `pops` 3 commands, then `pushes` 2 more. If it then tries to `pop` 6 times in a row, what will happen on the 5th `pop`?
A.It will successfully pop the last remaining item
B.It will pop a default 'null' value
C.It will cause a 'stack underflow' error
D.It will reverse the order of the remaining items
Challenging
You have two empty stacks, S1 and S2. You perform the following: `S1.push("Apple")`, `S1.push("Banana")`, `S2.push(S1.pop())`, `S1.push("Cherry")`. What is the final state of S1 and S2?
A.S1: ["Apple", "Cherry"], S2: ["Banana"]
B.S1: ["Apple"], S2: ["Banana", "Cherry"]
C.S1: ["Cherry"], S2: ["Apple", "Banana"]
D.S1: ["Apple", "Banana"], S2: ["Cherry"]
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Lesson 5: Stacks: Last-In, First-Out (LIFO) is a Grade 7 Computer Science lesson on ExcelOS.
What will I learn in Lesson 5: Stacks: Last-In, First-Out (LIFO)?
You'll be able to: Explain, using examples, how a stack follows the Last-In, First-Out (LIFO) principle in at least two different real-world scenarios; Identify the top element in a stack represented as a list, given at least five different stack….
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How many practice questions are included with Lesson 5: Stacks: Last-In, First-Out (LIFO)?
This lesson includes 27 practice questions across multiple difficulty levels, each with instant feedback and explanations.