Stack Implementation Using Array in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the top of the stack.

DSA Python

def push(stack, element):
    stack.[1](element)

Drag options to blanks, or click blank then click option'

Ainsert

Bremove

Cappend

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of append removes an element instead of adding one.

Using remove tries to delete a specific element, not add one.

The append method adds an element to the end of the list, which represents the top of the stack.

2fill in blank

medium

Complete the code to remove and return the top element from the stack.

DSA Python

def pop(stack):
    if len(stack) == 0:
        return None
    return stack.[1]()

Drag options to blanks, or click blank then click option'

Apop

Bappend

Cremove

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using append adds an element instead of removing one.

Using remove requires a value and does not return the removed element.

The pop method removes and returns the last element of the list, which is the top of the stack.

3fill in blank

hard

Fix the error in the code to check if the stack is empty.

DSA Python

def is_empty(stack):
    return len(stack) [1] 0

Drag options to blanks, or click blank then click option'

A!=

B==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' checks if stack is not empty.

Using '>' or '<' does not correctly check for emptiness.

To check if the stack is empty, we compare the length to zero using ==.

4fill in blank

hard

Fill both blanks to peek at the top element without removing it.

DSA Python

def peek(stack):
    if len(stack) == 0:
        return None
    return stack[1][2]

Drag options to blanks, or click blank then click option'

A[0]

B[-2]

C()

D[-1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses tries to call the list as a function.

Using index 0 accesses the bottom, not the top.

Using stack[-1] accesses the last element, which is the top of the stack, without removing it.

5fill in blank

hard

Fill all three blanks to implement a function that returns the size of the stack and clears it.

DSA Python

def clear_and_size(stack):
    size = len(stack)
    stack[1]()
    return [2], [3]

Drag options to blanks, or click blank then click option'

Aclear

Bsize

Clen(stack)

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop removes only one element, not all.

Returning variables in wrong order or forgetting to save size.

The clear() method empties the list. We return the saved size and the current length (which is 0) using len(stack).

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