Check if Stack is Empty or Full in DSA C++ - 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 completely full before adding or removing books.

🎯 Goal: You will create a simple stack using an array, set its size, and write code to check if the stack is empty or full.

📋 What You'll Learn

Create an integer array called stack with size 5

Create an integer variable called top and set it to -1

Create an integer variable called max_size and set it to 5

Write a function called isEmpty that returns true if top is -1

Write a function called isFull that returns true if top is equal to max_size - 1

Print the results of isEmpty() and isFull()

💡 Why This Matters

🌍 Real World

Stacks are used in many places like undo features in text editors, browser history, and expression evaluation.

💼 Career

Understanding stack operations is essential for software development roles, especially those involving algorithms and system design.

1

Create the stack array and top variable

Create an integer array called stack with size 5 and an integer variable called top set to -1.

DSA C++

// Your code here

Hint

Use int stack[5]; to create the array and int top = -1; to mark the stack as empty.

2

Create max_size variable

Create an integer variable called max_size and set it to 5.

DSA C++

int stack[5];
int top = -1;
// Your code here

Hint

This variable will store the maximum size of the stack.

3

Write isEmpty and isFull functions

Write a function called isEmpty that returns true if top is -1, and a function called isFull that returns true if top is equal to max_size - 1.

DSA C++

int stack[5];
int top = -1;
int max_size = 5;

// Your code here

Hint

Use simple return statements to check the conditions.

4

Print the results of isEmpty and isFull

Print the results of isEmpty() and isFull() using std::cout.

DSA C++

int stack[5];
int top = -1;
int max_size = 5;

bool isEmpty() {
    return top == -1;
}

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

// Your code here

Hint

Use std::cout and the ternary operator to print true or false.

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