Check if Stack is Empty or Full in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Stack Empty Check After Push and Pop

What is the output of the following C++ code that uses a stack to check if it is empty after some operations?

DSA C++

#include <iostream>
#include <stack>

int main() {
    std::stack<int> s;
    std::cout << (s.empty() ? "Empty" : "Not Empty") << std::endl;
    s.push(10);
    std::cout << (s.empty() ? "Empty" : "Not Empty") << std::endl;
    s.pop();
    std::cout << (s.empty() ? "Empty" : "Not Empty") << std::endl;
    return 0;
}

A

Not Empty
Empty
Not Empty

B

Empty
Not Empty
Empty

C

Empty
Empty
Empty

D

Not Empty
Not Empty
Not Empty

💡 Hint

Remember that the stack is empty initially, then has one element after push, and empty again after pop.

The stack starts empty, so s.empty() returns true and prints "Empty". After pushing 10, the stack is not empty, so it prints "Not Empty". After popping, the stack is empty again, so it prints "Empty".

❓ Predict Output

intermediate

Check if Stack is Full Using Array Implementation

Given the following C++ code for a stack implemented using an array, what will be the output after pushing 3 elements and checking if the stack is full?

DSA C++

#include <iostream>
#define MAX 3

class Stack {
    int arr[MAX];
    int top;
public:
    Stack() { top = -1; }
    bool isFull() { return top == MAX - 1; }
    void push(int x) {
        if (!isFull()) {
            arr[++top] = x;
        }
    }
    void printStatus() {
        std::cout << (isFull() ? "Full" : "Not Full") << std::endl;
    }
};

int main() {
    Stack s;
    s.push(1);
    s.push(2);
    s.push(3);
    s.printStatus();
    return 0;
}

A

Full

B

Not Full

C

Compilation Error

D

Runtime Error

💡 Hint

The stack size is 3 and after pushing 3 elements it should be full.

The stack has a maximum size of 3. After pushing 3 elements, the top index reaches 2 (MAX-1), so isFull() returns true and prints "Full".

🔧 Debug

advanced

Identify the Error in Stack Empty Check

What error will the following C++ code produce when checking if the stack is empty?

DSA C++

#include <iostream>
#include <stack>

int main() {
    std::stack<int> s;
    if (s.empty) {
        std::cout << "Stack is empty" << std::endl;
    } else {
        std::cout << "Stack is not empty" << std::endl;
    }
    return 0;
}

A Outputs: Stack is empty

B Runtime Error: Null pointer dereference

C Outputs: Stack is not empty

D Compilation Error: 'empty' is a function and needs parentheses

💡 Hint

Check how the empty() function is called in std::stack.

The empty method is a function and must be called with parentheses (). Using s.empty without parentheses causes a compilation error.

❓ Predict Output

advanced

Output of Stack Full Check After Partial Push

What will be the output of the following C++ code that checks if the stack is full after pushing two elements into a stack of max size 3?

DSA C++

#include <iostream>
#define MAX 3

class Stack {
    int arr[MAX];
    int top;
public:
    Stack() { top = -1; }
    bool isFull() { return top == MAX - 1; }
    void push(int x) {
        if (!isFull()) {
            arr[++top] = x;
        }
    }
    void printStatus() {
        std::cout << (isFull() ? "Full" : "Not Full") << std::endl;
    }
};

int main() {
    Stack s;
    s.push(5);
    s.push(10);
    s.printStatus();
    return 0;
}

A

Not Full

B

Full

C

Compilation Error

D

Runtime Error

💡 Hint

The stack has space for one more element after pushing two.

The stack size is 3. After pushing two elements, the top index is 1, which is less than MAX-1 (2), so isFull() returns false and prints "Not Full".

🧠 Conceptual

expert

Maximum Number of Items in Stack After Operations

Consider a stack with maximum size 4. The following operations are performed in order: push(1), push(2), pop(), push(3), push(4), push(5), pop(). How many items are in the stack at the end?

A 2

B 4

C 3

D 1

💡 Hint

Track the stack size step-by-step after each operation.

Step-by-step:
push(1) → size 1
push(2) → size 2
pop() → size 1
push(3) → size 2
push(4) → size 3
push(5) → size 4 (max size reached)
pop() → size 3

Final size is 3.

Practice

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

easy

A. If the stack has no elements

B. If the stack is full

C. If the stack has exactly one element

D. If the stack is sorted

Solution

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

   The isEmpty() function is designed to check whether the stack contains any elements or not.

2. Step 2: Interpret the meaning of empty 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 A

4. Quick Check:

   isEmpty() = stack empty check [OK]

Hint: Empty means zero elements in stack [OK]

Common Mistakes:

• Confusing isEmpty() with isFull()
• Thinking isEmpty() checks for one element
• Assuming isEmpty() checks stack order

2. Which of the following is the correct syntax to check if a stack named s is full in C++ assuming MAX is the stack capacity and top is the index of the top element?

easy

A. if (top == MAX - 1) {}

B. if (top == MAX) {}

C. if (top > MAX) {}

D. if (top >= MAX + 1) {}

Solution

1. Step 1: Understand stack indexing

   Stack top starts at -1 when empty and increments by 1 for each push. The last valid index is MAX - 1.

2. Step 2: Check condition for full stack

   The stack is full when top equals MAX - 1, meaning all positions are filled.

3. Final Answer:

   if (top == MAX - 1) {} -> Option A

4. Quick Check:

   Full stack = top == MAX - 1 [OK]

Hint: Full means top equals max index (MAX-1) [OK]

Common Mistakes:

• Using top == MAX instead of MAX - 1
• Checking top > MAX which is invalid
• Using >= MAX + 1 which is out of range

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

const int MAX = 3;
int stack[MAX];
int top = -1;

// Push elements
if (top < MAX - 1) top++;
stack[top] = 10;
if (top < MAX - 1) top++;
stack[top] = 20;

// Check if stack is full
if (top == MAX - 1) std::cout << "Full";
else std::cout << "Not Full";

medium

A. Full

B. Compilation Error

C. Runtime Error

D. Not Full

Solution

1. Step 1: Trace the push operations

   Initially, top = -1. First push increments top to 0 and stores 10. Second push increments top to 1 and stores 20.

2. Step 2: Check if stack is full

   Stack is full if top == MAX - 1 which is 2. Currently, top is 1, so stack is not full.

3. Final Answer:

   Not Full -> Option D

4. Quick Check:

   top = 1, MAX - 1 = 2, so Not Full [OK]

Hint: Count pushes; full if top equals MAX-1 [OK]

Common Mistakes:

• Assuming stack is full after 2 pushes when MAX is 3
• Confusing top index with number of elements
• Ignoring initial top = -1

4. Identify the error in the following C++ code that checks if a stack is empty:

bool isEmpty(int top) {
    if (top == 0)
        return true;
    else
        return false;
}

medium

A. There is no error in the function

B. The function should check if top == -1 for empty stack

C. The function should return false when top == 0

D. The function should check if top == MAX for empty stack

Solution

1. Step 1: Understand stack top initialization

   In stack implementation, top is usually -1 when stack is empty, not 0.

2. Step 2: Analyze the condition in code

   The code checks top == 0 which means stack has one element, so it incorrectly returns true for empty.

3. Final Answer:

   The function should check if top == -1 for empty stack -> Option B

4. Quick Check:

   Empty stack top = -1, not 0 [OK]

Hint: Empty stack top is -1, not 0 [OK]

Common Mistakes:

• Checking top == 0 instead of -1
• Confusing empty with one element
• Returning wrong boolean for empty condition

5. You have a stack with capacity 5 and top initialized to -1. After pushing 5 elements, you pop 2 elements. Which condition correctly checks if the stack is full now?

hard

A. if (top == 3) {}

B. if (top == 5) {}

C. if (top == 4) {}

D. if (top == -1) {}

Solution

1. Step 1: Calculate top after operations

   Initially, top = -1. After pushing 5 elements, top = 4 (0-based index). After popping 2 elements, top decreases by 2 to 2.

2. Step 2: Determine full stack condition now

   Stack is full only when top = 4. After popping, top is 2, so stack is not full. To check if full now, top must be 4.

3. Step 3: Understand the question's intent

   The question asks which condition correctly checks if stack is full now. Since top is 2, stack is not full. The correct condition to check full is top == 4.

4. Final Answer:

   if (top == 4) {} -> Option C

5. Quick Check:

   Full stack top = capacity - 1 = 4 [OK]

Hint: Full means top equals capacity - 1 (4 here) [OK]

Common Mistakes:

• Using top == 5 which is out of range
• Confusing popped elements with full condition
• Checking top == -1 which means empty