Check if Stack is Empty or Full in DSA Python - Build from Scratch

Check if Stack is Empty or Full

📖 Scenario: Imagine you have a stack of books on a shelf. You want to check if the shelf is empty or if it is full and cannot hold more books.

🎯 Goal: You will create a simple stack using a list, set a maximum size, write functions to check if the stack is empty or full, and then print the results.

📋 What You'll Learn

Create a list called stack with these exact elements: ['book1', 'book2', 'book3']

Create a variable called max_size and set it to 5

Write a function called is_empty that returns True if stack is empty, otherwise False

Write a function called is_full that returns True if the length of stack is equal to max_size, otherwise False

Print the result of is_empty() and is_full() exactly as shown

💡 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 stack operations is important for software development, especially in areas like memory management and algorithm design.

1

Create the initial stack

Create a list called stack with these exact elements: 'book1', 'book2', 'book3'

DSA Python

# Create the stack list with 'book1', 'book2', 'book3'
# Your code here

Hint

Use square brackets to create a list and include the three book names as strings.

2

Set the maximum stack size

Create a variable called max_size and set it to 5

DSA Python

stack = ['book1', 'book2', 'book3']
# Create max_size variable and set it to 5
# Your code here

Hint

Just assign the number 5 to the variable max_size.

3

Write functions to check if stack is empty or full

Write a function called is_empty that returns True if stack is empty, otherwise False. Then write a function called is_full that returns True if the length of stack is equal to max_size, otherwise False

DSA Python

stack = ['book1', 'book2', 'book3']
max_size = 5

# Define is_empty function
# Your code here

# Define is_full function
# Your code here

Hint

Use len(stack) to check the number of items in the stack.

4

Print the results of the checks

Print the result of is_empty() and is_full() exactly as shown

DSA Python

stack = ['book1', 'book2', 'book3']
max_size = 5

def is_empty():
    return len(stack) == 0

def is_full():
    return len(stack) == max_size

# Print the results of is_empty() and is_full()
# Your code here

Hint

Use print("Is stack empty?", is_empty()) and similarly for is_full().

Practice

1. What does the isEmpty() function check in a stack?

easy

A. If the stack has exactly one element

B. If the stack is full

C. If the stack has more than one element

D. If the stack has no elements

Solution

1. Step 1: Understand the purpose of isEmpty()

   The isEmpty() function checks whether the stack contains any elements or not.

2. Step 2: Identify what empty means for a stack

   An empty stack means it has zero elements, so isEmpty() returns true if no elements are present.

3. Final Answer:

   If the stack has no elements -> Option D

4. Quick Check:

   Empty stack means no elements = true [OK]

Hint: Empty stack means zero elements inside [OK]

Common Mistakes:

• Confusing empty with full
• Thinking it checks for one element
• Assuming it checks for capacity

2. Which of the following is the correct syntax to check if a stack named stack is full in Python assuming max size is stored in max_size and current size in top?

easy

A. if top == max_size: print('Stack is full')

B. if top > max_size: print('Stack is full')

C. if top < max_size: print('Stack is full')

D. if top != max_size: print('Stack is full')

Solution

1. Step 1: Understand stack full condition

   A stack is full when the current top index equals the maximum allowed size.

2. Step 2: Match condition with syntax

   The correct check is top == max_size to confirm fullness.

3. Final Answer:

   if top == max_size: print('Stack is full') -> Option A

4. Quick Check:

   Full stack when top equals max size [OK]

Hint: Full stack means top equals max size [OK]

Common Mistakes:

• Using greater than instead of equal
• Using less than for full check
• Using not equal instead of equal

3. What will be the output of the following code?

class Stack:
    def __init__(self, max_size):
        self.stack = []
        self.max_size = max_size
    def isFull(self):
        return len(self.stack) == self.max_size
    def isEmpty(self):
        return len(self.stack) == 0

s = Stack(2)
s.stack.append(10)
s.stack.append(20)
print(s.isFull())
print(s.isEmpty())

medium

A. False\nTrue

B. True\nTrue

C. True\nFalse

D. False\nFalse

Solution

1. Step 1: Check stack contents after append

   Two elements (10, 20) are added, so length is 2 which equals max_size.

2. Step 2: Evaluate isFull and isEmpty

   isFull() returns True because length equals max_size; isEmpty() returns False because stack is not empty.

3. Final Answer:

   True False -> Option C

4. Quick Check:

   Stack full = True, empty = False [OK]

Hint: Full if length equals max size; empty if length zero [OK]

Common Mistakes:

• Confusing isFull and isEmpty return values
• Ignoring stack length after append
• Assuming stack is empty after append

4. Find the error in the following code that checks if a stack is empty:

class Stack:
    def __init__(self):
        self.stack = []
    def isEmpty(self):
        if self.stack == 0:
            return True
        else:
            return False

s = Stack()
print(s.isEmpty())

medium

A. Missing return statement in isEmpty

B. Comparing list to integer zero causes error

C. Stack initialization is incorrect

D. Print statement syntax is wrong

Solution

1. Step 1: Analyze the isEmpty condition

   The code compares self.stack == 0, but self.stack is a list, not a number.

2. Step 2: Identify correct way to check empty list

   To check if list is empty, use len(self.stack) == 0 or simply not self.stack.

3. Final Answer:

   Comparing list to integer zero causes error -> Option B

4. Quick Check:

   List cannot be compared to number 0 [OK]

Hint: Check list emptiness with length or not operator [OK]

Common Mistakes:

• Comparing list directly to 0
• Forgetting list length check
• Using wrong comparison operator

5. You have a stack with max size 3. After pushing 3 elements, you pop one element and then check if the stack is full or empty. What will be the correct output of isFull() and isEmpty() methods?

class Stack:
    def __init__(self, max_size):
        self.stack = []
        self.max_size = max_size
    def push(self, val):
        if len(self.stack) < self.max_size:
            self.stack.append(val)
    def pop(self):
        if not self.isEmpty():
            return self.stack.pop()
    def isFull(self):
        return len(self.stack) == self.max_size
    def isEmpty(self):
        return len(self.stack) == 0

s = Stack(3)
s.push(1)
s.push(2)
s.push(3)
s.pop()
print(s.isFull())
print(s.isEmpty())

hard

A. False\nFalse

B. True\nFalse

C. False\nTrue

D. True\nTrue

Solution

1. Step 1: Track stack size after operations

   Initially push 3 elements, stack size = 3 (full). After pop, size reduces to 2.

2. Step 2: Evaluate isFull and isEmpty

   Since size is 2 and max_size is 3, isFull() returns False. Stack is not empty, so isEmpty() returns False.

3. Final Answer:

   False False -> Option A

4. Quick Check:

   Stack size 2 < max size 3, not empty [OK]

Hint: Pop reduces size; full only if size equals max [OK]

Common Mistakes:

• Assuming stack is still full after pop
• Assuming stack is empty after pop
• Ignoring size changes after pop