Pop Operation on Stack in DSA Typescript - Build from Scratch

Pop Operation on Stack

📖 Scenario: Imagine you have a stack of books. You can only take the top book off the stack. This is called the "pop" operation in a stack data structure.

🎯 Goal: You will create a stack using an array, set up a variable to hold the popped book, perform the pop operation, and then print the stack after popping.

📋 What You'll Learn

Create a stack array called bookStack with these exact books in order: 'Math', 'Science', 'History', 'Art'

Create a variable called poppedBook to store the book removed from the stack

Use the pop() method on bookStack and assign the result to poppedBook

Print the bookStack after the pop operation

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and managing function calls.

💼 Career

Understanding stack operations like pop is essential for software developers working with algorithms, data structures, and system design.

1

Create the stack array

Create an array called bookStack with these exact books in order: 'Math', 'Science', 'History', 'Art'

DSA Typescript

# Your code here

Hint

Use square brackets [] to create an array and separate items with commas.

2

Create the poppedBook variable

Create a variable called poppedBook and set it to null to prepare for the pop operation

DSA Typescript

const bookStack = ['Math', 'Science', 'History', 'Art'];
// Your code here

Hint

Use let to declare a variable that will change later.

3

Pop the top book from the stack

Use the pop() method on bookStack and assign the result to poppedBook

DSA Typescript

const bookStack = ['Math', 'Science', 'History', 'Art'];
let poppedBook: string | null = null;
// Your code here

Hint

The pop() method removes the last item from the array and returns it.

4

Print the stack after pop

Print the bookStack array after the pop operation using console.log(bookStack)

DSA Typescript

const bookStack = ['Math', 'Science', 'History', 'Art'];
let poppedBook: string | null = null;
poppedBook = bookStack.pop() || null;
// Your code here

Hint

Use console.log(bookStack) to see the stack after popping.

Practice

1. What does the pop() operation do in a stack?

easy

A. Clears all elements from the stack

B. Adds a new element to the bottom of the stack

C. Returns the bottom element without removing it

D. Removes and returns the top element of the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is on top.

2. Step 2: Define pop operation

   The pop operation removes and returns this top element from the stack.

3. Final Answer:

   Removes and returns the top element of the stack -> Option D

4. Quick Check:

   pop() removes top element = A [OK]

Hint: Pop always removes the last added item [OK]

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek
• Assuming pop removes bottom element

2. Which of the following is the correct TypeScript syntax to pop an element from a stack array named stack?

easy

A. pop(stack);

B. stack.remove();

C. stack.pop();

D. stack.delete();

Solution

1. Step 1: Recall array pop method in TypeScript

   In TypeScript, arrays have a built-in method pop() to remove the last element.

2. Step 2: Match correct syntax

   The correct syntax to pop from stack is stack.pop();. Other options are invalid methods.

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Array pop method = stack.pop() [OK]

Hint: Use .pop() method on array to remove top stack element [OK]

Common Mistakes:

• Using non-existent methods like remove() or delete()
• Calling pop as a function with stack as argument
• Confusing pop with other array methods

3. What will be the output after executing the following TypeScript code?

const stack = [10, 20, 30];
const popped = stack.pop();
console.log(popped);
console.log(stack);

medium

A. 30
[10, 20]

B. 20
[10, 20, 30]

C. 30
[10, 20, 30]

D. undefined
[10, 20]

Solution

1. Step 1: Understand pop() effect on array

   pop() removes and returns the last element. Here, 30 is removed and returned.

2. Step 2: Check console outputs

   First console.log prints popped value 30. Second prints updated stack [10, 20].

3. Final Answer:

   30 [10, 20] -> Option A

4. Quick Check:

   pop() returns last element and removes it [OK]

Hint: pop() returns last item and removes it from array [OK]

Common Mistakes:

• Expecting pop() to return second last element
• Thinking pop() does not modify the array
• Confusing pop() return value with stack after pop

4. Identify the error in the following TypeScript code that tries to pop an element from an empty stack:

const stack: number[] = [];
const popped = stack.pop();
console.log(popped);

medium

A. pop() returns undefined when stack is empty, no error

B. Runtime error because pop() on empty array is invalid

C. Syntax error due to missing type annotation

D. pop() removes first element instead of last

Solution

1. Step 1: Check pop() behavior on empty array

   pop() returns undefined if the array is empty; it does not throw an error.

2. Step 2: Verify code correctness

   Code runs without error and prints undefined.

3. Final Answer:

   pop() returns undefined when stack is empty, no error -> Option A

4. Quick Check:

   pop() on empty array = undefined [OK]

Hint: pop() returns undefined if stack is empty, no crash [OK]

Common Mistakes:

• Expecting runtime error on empty pop()
• Confusing pop() with shift() behavior
• Thinking pop() removes first element

5. Given a stack implemented as an array stack = [5, 10, 15, 20], write a TypeScript function popTwo that pops two elements and returns them as an array. What will be the returned value and the stack after calling popTwo(stack)?

hard

A. [15, 20], stack = [5, 10]

B. [20, 15], stack = [5, 10]

C. [20, 15], stack = [10, 5]

D. [10, 5], stack = [15, 20]

Solution

1. Step 1: Understand pop() order and stack state

   pop() removes last element first. First pop returns 20, second pop returns 15.

2. Step 2: Determine returned array and stack after two pops

   Returned array is [20, 15]. Stack after pops is [5, 10].

3. Final Answer:

   [20, 15], stack = [5, 10] -> Option B

4. Quick Check:

   Two pops remove last two elements in order [20, 15] [OK]

Hint: Pop twice returns last two elements in reverse order [OK]

Common Mistakes:

• Returning popped elements in wrong order
• Assuming stack order changes after pop
• Mixing up stack contents after pops