Pop Operation on Stack in DSA Rust - Build from Scratch

Pop Operation on Stack

📖 Scenario: You are managing a stack of books. You want to remove the top book from the stack to read it.

🎯 Goal: Build a simple stack using a vector and perform a pop operation to remove the top item.

📋 What You'll Learn

Create a stack with exact elements: 10, 20, 30

Create a variable called popped_item to store the popped value

Use the pop() method on the stack to remove the top element

Print the stack 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 expression evaluation.

💼 Career

Understanding stack operations is fundamental for software development and technical interviews.

1

Create the stack

Create a vector called stack with the exact elements 10, 20, and 30 in that order.

DSA Rust

# Create a vector called stack with elements 10, 20, 30
# Your code here

Hint

Use vec![10, 20, 30] to create the stack.

2

Create a variable to hold the popped item

Create a mutable variable called popped_item and set it to None initially.

DSA Rust

let mut stack = vec![10, 20, 30];
# Create a mutable variable called popped_item and set it to None
# Your code here

Hint

Use Option<i32> type and set it to None.

3

Pop the top item from the stack

Use the pop() method on stack and assign the result to popped_item.

DSA Rust

let mut stack = vec![10, 20, 30];
let mut popped_item: Option = None;
# Pop the top item from stack and assign it to popped_item
# Your code here

Hint

Use stack.pop() to remove the last element.

4

Print the stack after pop

Print the stack vector to show its state after the pop operation.

DSA Rust

let mut stack = vec![10, 20, 30];
let mut popped_item: Option = None;
popped_item = stack.pop();
# Print the stack
# Your code here

Hint

Use println!("{:?}", stack); to print the vector.

Practice

1. What does the pop operation do in a stack data structure?

easy

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

B. Removes and returns the top element of the stack

C. Returns the bottom element without removing it

D. Clears all elements from 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 the top element, which is the last inserted item.

3. Final Answer:

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

4. Quick Check:

   Pop = remove top element [OK]

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

Common Mistakes:

• Thinking pop removes from bottom
• Confusing pop with push
• Assuming pop returns without removing

2. Which of the following is the correct Rust syntax to pop an element from a stack implemented as a Vec<T> named stack?

easy

A. let x = stack.pop_front();

B. let x = stack.remove();

C. let x = stack.pop();

D. let x = stack.delete();

Solution

1. Step 1: Recall Rust Vec methods

   Rust's Vec has a pop() method that removes and returns the last element.

2. Step 2: Check other options

   remove() requires an index, pop_front() is for other collections, delete() does not exist.

3. Final Answer:

   let x = stack.pop(); -> Option C

4. Quick Check:

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

Hint: Use stack.pop() to remove last element in Rust Vec [OK]

Common Mistakes:

• Using pop_front() which is not in Vec
• Calling remove() without index
• Using non-existent delete() method

3. What is the output of the following Rust code?

let mut stack = vec![10, 20, 30];
let popped = stack.pop();
println!("{:?}", popped);
println!("{:?}", stack);

medium

A. Some(30)
[10, 20]

B. 30
[10, 20, 30]

C. None
[10, 20, 30]

D. Some(20)
[10, 30]

Solution

1. Step 1: Understand pop() return type

   pop() returns an Option containing the last element if present, here Some(30).

2. Step 2: Check stack after pop

   After popping 30, stack becomes [10, 20].

3. Final Answer:

   Some(30) [10, 20] -> Option A

4. Quick Check:

   pop returns Some(last element), stack shrinks [OK]

Hint: pop() returns Some(value) if not empty [OK]

Common Mistakes:

• Expecting pop() to return value directly, not Option
• Not updating stack after pop
• Confusing popped element with stack contents

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

let mut stack: Vec<i32> = Vec::new();
let x = stack.pop().unwrap();
println!("{}", x);

medium

A. pop() method does not exist on Vec

B. println! macro syntax is incorrect

C. stack is not mutable

D. Calling unwrap() on None causes panic

Solution

1. Step 1: Check pop() on empty Vec

   pop() returns None if the Vec is empty.

2. Step 2: Understand unwrap() behavior

   Calling unwrap() on None causes a runtime panic (crash).

3. Final Answer:

   Calling unwrap() on None causes panic -> Option D

4. Quick Check:

   unwrap on None = panic [OK]

Hint: Avoid unwrap() on pop() without checking Some [OK]

Common Mistakes:

• Assuming pop() always returns Some
• Ignoring mut keyword needed for pop
• Misreading println! syntax

5. Given a stack implemented as Vec<i32>, how can you safely pop all elements and collect them into a new vector in reverse order without panicking?

hard

A. Use a loop with while let Some(x) = stack.pop() and push x into new vector

B. Use stack.pop().unwrap() repeatedly until empty

C. Use stack.remove(0) repeatedly to pop elements

D. Use stack.pop_front() to remove elements safely

Solution

1. Step 1: Avoid unwrap() to prevent panic

   Using unwrap() on pop() can panic if stack is empty, so avoid it.

2. Step 2: Use while let to safely pop

   Using while let Some(x) = stack.pop() safely extracts elements until empty.

3. Step 3: Push popped elements into new vector

   Each popped element x is pushed into a new vector, reversing order.

4. Final Answer:

   Use a loop with while let Some(x) = stack.pop() and push x into new vector -> Option A

5. Quick Check:

   Safe pop loop = while let Some(x) = pop() [OK]

Hint: Use while let Some(x) = pop() to avoid panic [OK]

Common Mistakes:

• Using unwrap() causing panic on empty stack
• Using remove(0) which is inefficient
• Calling pop_front() which does not exist on Vec