Stack Implementation Using Array in DSA Python - Build from Scratch

Stack Implementation Using Array

📖 Scenario: Imagine you are managing a stack of books on a table. You can only add or remove the top book. This is like a stack data structure where the last item added is the first one removed.

🎯 Goal: You will build a simple stack using a Python list (array). You will add items (push), remove the top item (pop), and see the current stack.

📋 What You'll Learn

Create an empty list called stack to hold stack items.

Create a variable called max_size and set it to 5 to limit stack size.

Write a function push(item) to add item to stack only if it is not full.

Write a function pop() to remove and return the top item from stack if it is not empty.

Print the final state of stack after some push and pop operations.

💡 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 implementation helps in software development, debugging, and preparing for coding interviews.

1

Create an empty stack

Create an empty list called stack to hold the stack items.

DSA Python

# Create an empty list called stack
# Your code here

Hint

Use [] to create an empty list in Python.

2

Set the maximum stack size

Create a variable called max_size and set it to 5 to limit the stack size.

DSA Python

stack = []
# 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 push and pop functions

Write a function called push(item) that adds item to stack only if the length of stack is less than max_size. Also write a function called pop() that removes and returns the last item from stack if it is not empty; otherwise, return None.

DSA Python

stack = []
max_size = 5

# Write push(item) function
# Write pop() function
# Your code here

Hint

Use len(stack) to check size. Use stack.append(item) to add. Use stack.pop() to remove last item.

4

Test stack operations and print final stack

Use push to add 10, 20, 30 to stack. Then use pop() once. Finally, print the stack.

DSA Python

stack = []
max_size = 5

def push(item):
    if len(stack)  0:
        return stack.pop()
    else:
        return None

# Push 10, 20, 30
# Pop once
# Print the stack
# Your code here

Hint

After pushing 10, 20, 30 and popping once, the stack should have 10 and 20 left.

Practice

1. What is the main purpose of using an array to implement a stack?

easy

A. To allow random access to elements

B. To store elements in a first-in, first-out (FIFO) order

C. To sort elements automatically

D. To store elements in a last-in, first-out (LIFO) order

Solution

1. Step 1: Understand stack behavior

   A stack stores elements so that the last element added is the first one removed, called LIFO.

2. Step 2: Role of array in stack

   An array helps store elements in order and access the last added element easily for push/pop operations.

3. Final Answer:

   To store elements in a last-in, first-out (LIFO) order -> Option D

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack means last-in, first-out order [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking array sorts elements automatically
• Assuming random access is main stack feature

2. Which of the following is the correct way to initialize an empty stack using an array in Python?

easy

A. stack = []

B. stack = {}

C. stack = ()

D. stack = null

Solution

1. Step 1: Identify Python data types for stack

   An array-like structure for stack is a list, initialized with square brackets [].

2. Step 2: Check each option

   stack = [] uses [], which is a list. stack = {} is a dictionary, () is a tuple, null (no data).

3. Final Answer:

   stack = [] -> Option A

4. Quick Check:

   Empty list = empty stack [OK]

Hint: Use [] to create empty list for stack [OK]

Common Mistakes:

• Using {} which creates a dictionary
• Using () which creates a tuple
• Assigning null instead of a list

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

stack = []
stack.append(10)
stack.append(20)
stack.pop()
print(stack)

medium

A. [10, 20]

B. [20]

C. [10]

D. []

Solution

1. Step 1: Trace push operations

   stack.append(10) adds 10, stack becomes [10]. Then stack.append(20) adds 20, stack becomes [10, 20].

2. Step 2: Trace pop operation

   stack.pop() removes last element 20, stack becomes [10].

3. Final Answer:

   [10] -> Option C

4. Quick Check:

   Pop removes last item, stack left with [10] [OK]

Hint: Pop removes last pushed element [OK]

Common Mistakes:

• Thinking pop removes first element
• Confusing append with insert
• Expecting stack to be empty after pop

4. Identify the error in the following stack implementation code:

stack = []
stack.push(5)
stack.append(10)
print(stack)

medium

A. 'push' method does not exist for list

B. Missing initialization of stack

C. Cannot append to list

D. print statement syntax error

Solution

1. Step 1: Check list methods

   Python lists use append() to add elements, not push().

2. Step 2: Identify error

   stack.push(5) causes AttributeError because 'push' is not a list method.

3. Final Answer:

   'push' method does not exist for list -> Option A

4. Quick Check:

   Use append(), not push() for list [OK]

Hint: Use append() to add elements to list [OK]

Common Mistakes:

• Using push() instead of append()
• Assuming push() is valid for Python lists
• Ignoring error messages

5. Given a stack implemented using an array, how can you check if the stack is empty before popping an element to avoid errors?

hard

A. Check if stack is null

B. Check if stack's length is zero using len(stack) == 0

C. Try popping and catch exceptions if stack is empty

D. Use stack[0] == null to check emptiness

Solution

1. Step 1: Understand stack emptiness check

   To avoid errors, check if stack has no elements by verifying its length is zero.

2. Step 2: Evaluate options

   Check if stack's length is zero using len(stack) == 0 correctly checks length with len(stack) == 0. Try popping and catch exceptions if stack is empty is valid but less efficient. Check if stack is null checks null which is incorrect. Use stack[0] == null to check emptiness checks first element which may cause error if empty.

3. Final Answer:

   Check if stack's length is zero using len(stack) == 0 -> Option B

4. Quick Check:

   Empty stack means length zero [OK]

Hint: Use len(stack) == 0 to check empty stack [OK]

Common Mistakes:

• Checking stack is null instead of empty
• Accessing stack[0] without checking length
• Relying only on try-except without condition