Peek Top Element of Stack in DSA Python - Build from Scratch

Peek Top Element of Stack

📖 Scenario: You are managing a stack of books. You want to see which book is currently on the top without removing it.

🎯 Goal: Build a simple stack using a list and write code to peek (look at) the top element of the stack.

📋 What You'll Learn

Create a list called stack with exact elements 'Math', 'Science', 'History'

Create a variable called top_index to hold the index of the top element

Use top_index to get the top element from stack and store it in top_element

Print the value of top_element

💡 Why This Matters

🌍 Real World

Stacks are used in many places like undo features in apps, browser history, and managing tasks in order.

💼 Career

Understanding stacks helps in software development roles where managing data order and temporary storage is needed.

1

Create the stack

Create a list called stack with these exact elements in order: 'Math', 'Science', 'History'

DSA Python

# Create the stack list with the given books
# Your code here

Hint

Use square brackets [] to create a list and separate elements with commas.

2

Find the top index

Create a variable called top_index and set it to the index of the last element in stack using len(stack) - 1

DSA Python

stack = ['Math', 'Science', 'History']
# Create top_index to hold the last element's index
# Your code here

Hint

The last element's index is always the length of the list minus one.

3

Peek the top element

Create a variable called top_element and set it to the element at top_index in stack

DSA Python

stack = ['Math', 'Science', 'History']
top_index = len(stack) - 1
# Get the top element from stack using top_index
# Your code here

Hint

Use square brackets with the index to get an element from a list.

4

Print the top element

Print the value of top_element to show the book on top of the stack

DSA Python

stack = ['Math', 'Science', 'History']
top_index = len(stack) - 1
top_element = stack[top_index]
# Print the top_element
# Your code here

Hint

Use print(top_element) to display the top book.

Practice

1. What does the peek operation do in a stack?

easy

A. Returns the bottom element of the stack

B. Removes the top element from the stack

C. Adds a new element to the top of the stack

D. Returns the top element without removing it

Solution

1. Step 1: Understand stack operations

   A stack follows Last-In-First-Out (LIFO). The top element is the last added item.

2. Step 2: Define peek operation

   Peek returns the top element without removing it, unlike pop which removes it.

3. Final Answer:

   Returns the top element without removing it -> Option D

4. Quick Check:

   Peek = top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

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

2. Which of the following is the correct syntax to peek the top element from a stack implemented as a Python list named stack?

easy

A. stack[-1]

B. stack.peek()

C. stack.top()

D. stack.pop()

Solution

1. Step 1: Recall Python list indexing

   Python lists use negative indexing; stack[-1] accesses the last element.

2. Step 2: Differentiate peek from pop

   stack.pop() removes the element, so it is not peek. stack.peek() and stack.top() are not valid list methods.

3. Final Answer:

   stack[-1] -> Option A

4. Quick Check:

   Peek = stack[-1] [OK]

Hint: Use stack[-1] to peek top element in Python list [OK]

Common Mistakes:

• Using pop() which removes element
• Using non-existent peek() or top() methods
• Using positive index 0 instead of -1

3. What will be the output of this code?

stack = [10, 20, 30]
print(stack[-1])
stack.append(40)
print(stack[-1])

medium

A. 30\n40

B. 10\n40

C. 30\n30

D. 40\n40

Solution

1. Step 1: Check initial peek

   Initial stack is [10, 20, 30]. stack[-1] is 30, so first print outputs 30.

2. Step 2: Append and peek again

   After appending 40, stack becomes [10, 20, 30, 40]. stack[-1] is now 40, so second print outputs 40.

3. Final Answer:

   30 40 -> Option A

4. Quick Check:

   Peek before append = 30, after append = 40 [OK]

Hint: Peek shows last element; append adds new last element [OK]

Common Mistakes:

• Confusing peek with pop output
• Not updating peek after append
• Printing first element instead of last

4. Find the error in this code that tries to peek the top element of a stack implemented as a list:

stack = [5, 15, 25]
top = stack.pop[-1]
print(top)

medium

A. pop removes element, so peek is wrong

B. pop is used incorrectly with square brackets

C. stack is empty, so error occurs

D. print statement syntax is wrong

Solution

1. Step 1: Analyze pop usage

   pop is a method and should be called with parentheses, not square brackets. stack.pop[-1] is invalid syntax.

2. Step 2: Understand correct peek syntax

   To peek, use stack[-1]. To pop, use stack.pop(). Mixing them causes error.

3. Final Answer:

   pop is used incorrectly with square brackets -> Option B

4. Quick Check:

   pop() needs parentheses, not brackets [OK]

Hint: Use parentheses for pop(), brackets for indexing [OK]

Common Mistakes:

• Using pop with brackets instead of parentheses
• Confusing peek with pop
• Assuming pop[-1] is valid syntax

5. Given a stack implemented as a list, how can you safely peek the top element without causing an error if the stack is empty?

hard

A. Use stack.pop() and catch exceptions

B. Use stack[-1] directly without checks

C. Check if stack is not empty before peeking

D. Assign None to top element always

Solution

1. Step 1: Understand empty stack risk

   Accessing stack[-1] when stack is empty causes IndexError.

2. Step 2: Implement safe peek

   Check if stack length > 0 before peeking to avoid error.

3. Final Answer:

   Check if stack is not empty before peeking -> Option C

4. Quick Check:

   Peek safely by checking stack length first [OK]

Hint: Always check stack not empty before peek [OK]

Common Mistakes:

• Peeking without checking empty stack
• Using pop() which removes element
• Ignoring exceptions from empty stack access