Stack vs Array Direct Use Why We Need Stack Abstraction in DSA C++ - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack using array indexing.

DSA C++

int stack[5];
int top = -1;
// Push 10 onto stack
stack[++[1]] = 10;

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

Asize

Bindex

Ccapacity

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable that does not track the stack top.

Forgetting to increment the top before assignment.

We use top to track the current top index of the stack. Incrementing top and assigning the value pushes the element.

2fill in blank

medium

Complete the code to check if the stack is empty before popping.

DSA C++

if ([1] == -1) {
    // Stack is empty
    std::cout << "Stack Underflow" << std::endl;
}

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

Asize

Bindex

Ctop

Dcapacity

Attempts:

3 left

💡 Hint

Common Mistakes

Checking wrong variable for emptiness.

Using size or capacity incorrectly.

The top variable is -1 when the stack is empty, so we check top == -1.

3fill in blank

hard

Fix the error in the pop operation to correctly return the top element and update the stack.

DSA C++

int pop() {
    if (top == -1) {
        std::cout << "Stack Underflow" << std::endl;
        return -1;
    }
    return stack[[1]--];
}

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

Atop

Bsize

Ccapacity

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable for indexing.

Pre-decrement instead of post-decrement causing wrong element return.

We return the element at stack[top] and then decrement top to remove it from the stack.

4fill in blank

hard

Fill the blank to implement a function that checks if the stack is full.

DSA C++

bool isFull() {
    return top == [1] - 1;
}

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

Asize

Bcapacity

Ctop

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using size or index instead of capacity.

Incorrect comparison operator.

The stack is full when top equals capacity - 1. The capacity variable stores the maximum size.

5fill in blank

hard

Fill all three blanks to create a function that pushes an element if the stack is not full.

DSA C++

void push(int value) {
    if (top == [1] - 1) {
        std::cout << "Stack Overflow" << std::endl;
        return;
    }
    stack[++[2]] = [3];
}

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

Acapacity

Btop

Cvalue

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable for capacity check.

Assigning wrong variable to stack.

Not incrementing top before assignment.

We check if top == capacity - 1 to detect overflow. Then increment top and assign the value to the stack.

Practice

1. Why do we prefer using a stack abstraction over directly using an array for stack operations?

easy

A. Because stack abstraction hides implementation details and enforces LIFO order

B. Because arrays cannot store multiple elements

C. Because stack abstraction uses less memory than arrays

D. Because arrays automatically resize when full

Solution

1. Step 1: Understand array limitations

   Arrays allow direct access to any element and do not enforce any order on insertion or deletion.

2. Step 2: Understand stack abstraction benefits

   Stack abstraction restricts operations to push and pop, enforcing Last-In-First-Out (LIFO) order and hiding internal details.

3. Final Answer:

   Because stack abstraction hides implementation details and enforces LIFO order -> Option A

4. Quick Check:

   Stack abstraction = hides details + LIFO [OK]

Hint: Stack abstraction enforces LIFO and hides array details [OK]

Common Mistakes:

• Thinking arrays cannot store multiple elements
• Believing stack uses less memory than arrays
• Assuming arrays auto-resize like stacks

2. Which of the following is the correct syntax to push an element '5' onto a stack named stk in C++ STL?

easy

A. stk.insert(5);

B. stk.add(5);

C. stk.push(5);

D. stk.append(5);

Solution

1. Step 1: Recall C++ STL stack method for adding elements

   The standard method to add an element to a stack is push().

2. Step 2: Verify other options

   add(), insert(), and append() are not valid stack methods in C++ STL.

3. Final Answer:

   stk.push(5); -> Option C

4. Quick Check:

   Push element syntax = stk.push(value) [OK]

Hint: Use push() to add elements to C++ STL stack [OK]

Common Mistakes:

• Using add() or append() which are not stack methods
• Confusing vector methods with stack methods
• Trying to use insert() on stack

3. What will be the output of the following C++ code snippet?

std::stack<int> stk;
stk.push(10);
stk.push(20);
stk.push(30);
int top1 = stk.top();
stk.pop();
int top2 = stk.top();
std::cout << top1 << "," << top2 << std::endl;

medium

A. 30,10

B. 10,20

C. 20,10

D. 30,20

Solution

1. Step 1: Trace stack push operations

   Elements pushed in order: 10 (bottom), 20, 30 (top).

2. Step 2: Check top and pop operations

   First top() returns 30, then pop removes 30, next top() returns 20.

3. Final Answer:

   30,20 -> Option D

4. Quick Check:

   Stack top after push = 30, after pop = 20 [OK]

Hint: Top shows last pushed; pop removes it; next top shows previous [OK]

Common Mistakes:

• Confusing order of elements in stack
• Assuming pop returns the popped value
• Mixing up top values before and after pop

4. Identify the error in the following code snippet that tries to use an array as a stack:

int arr[3];
int top = -1;
arr[++top] = 5;
arr[++top] = 10;
arr[++top] = 15;
arr[++top] = 20; // Error here
std::cout << arr[top] << std::endl;

medium

A. Array index out of bounds due to exceeding size

B. Incorrect initialization of top variable

C. Using pre-increment instead of post-increment

D. Missing stack pop operation before push

Solution

1. Step 1: Analyze array size and push operations

   Array size is 3, but code tries to push 4 elements, causing overflow.

2. Step 2: Identify error at last push

   Incrementing top to 3 exceeds array bounds (0 to 2), causing out-of-bounds error.

3. Final Answer:

   Array index out of bounds due to exceeding size -> Option A

4. Quick Check:

   Array size 3 but 4 pushes cause overflow [OK]

Hint: Check array size before pushing elements [OK]

Common Mistakes:

• Ignoring array size limits
• Confusing pre-increment with post-increment
• Assuming pop is mandatory before push

5. You want to implement a stack using an array but also want to prevent errors like overflow and underflow. Which approach best solves this problem?

hard

A. Use a queue instead of a stack

B. Use stack abstraction with push/pop methods that check boundaries

C. Use direct array access without any checks

D. Use a larger array and ignore boundary checks

Solution

1. Step 1: Understand risks of direct array use

   Direct array use can cause overflow (too many pushes) or underflow (pop from empty) without checks.

2. Step 2: Benefits of stack abstraction

   Stack abstraction provides push/pop methods that include boundary checks to prevent errors.

3. Step 3: Evaluate other options

   Larger arrays waste memory, ignoring checks causes errors, and queue is a different data structure.

4. Final Answer:

   Use stack abstraction with push/pop methods that check boundaries -> Option B

5. Quick Check:

   Stack abstraction = safe push/pop with checks [OK]

Hint: Use stack methods with checks to avoid overflow/underflow [OK]

Common Mistakes:

• Ignoring boundary checks leads to errors
• Using queue instead of stack for LIFO
• Assuming bigger array solves overflow