Check if Stack is Empty or Full in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean when a stack is empty?

A stack is empty when it has no elements inside it. This means the top pointer is at -1 or no items have been pushed yet.

beginner

How do you check if a stack is full in an array implementation?

You check if the top pointer is equal to the maximum size minus one. If yes, the stack is full and cannot hold more elements.

beginner

What is the role of the 'top' variable in a stack?

The 'top' variable keeps track of the index of the last added element in the stack. It helps to know where to push or pop elements.

beginner

Why is it important to check if a stack is empty before popping?

If you pop from an empty stack, it causes an error because there is no element to remove. Checking prevents this mistake.

beginner

What happens if you try to push an element into a full stack?

Trying to push into a full stack causes an overflow error because there is no space left to add new elements.

What condition checks if a stack is empty?

A top == maxSize

B top == -1

C top == 0

D top > maxSize

When top is -1, it means no elements are in the stack, so it is empty.

How do you know a stack is full in an array of size maxSize?

A top == maxSize - 1

B top == -1

C top == 0

D top > maxSize

If top equals maxSize - 1, the stack has reached its capacity.

What is the initial value of top in an empty stack?

A 1

B 0

C -1

D maxSize

Top starts at -1 to indicate the stack is empty.

What error occurs if you pop from an empty stack?

A Syntax error

B Overflow

C Segmentation fault

D Underflow

Popping from an empty stack causes underflow because there is nothing to remove.

What should you do before pushing an element onto the stack?

A Check if the stack is full

B Check if the stack is empty

C Reset top to -1

D Pop an element first

You must check if the stack is full to avoid overflow errors.

Explain how to check if a stack is empty or full using the 'top' variable.

Think about the position of 'top' relative to stack size.

Describe what happens if you try to pop from an empty stack or push into a full stack.

Consider what errors prevent safe stack operations.

Practice

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

easy

A. If the stack has exactly one element

B. If the stack is full

C. If the stack has no elements

D. If the stack has more than one element

Solution

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

   The isEmpty() function checks whether the stack contains any elements or not.

2. Step 2: Identify what 'empty' means for a stack

   A stack is empty when it has zero elements, meaning no data is stored inside.

3. Final Answer:

   If the stack has no elements -> Option C

4. Quick Check:

   Empty stack means zero elements = true [OK]

Hint: Empty stack means no elements inside [OK]

Common Mistakes:

• Confusing empty with full
• Thinking empty means one element
• Mixing up stack size with capacity

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

easy

A. if (top == MAX - 1)

B. if (top < MAX - 1)

C. if (top > MAX)

D. if (top == MAX)

Solution

1. Step 1: Understand stack indexing

   Stack indices start at 0, so the last valid index is MAX - 1.

2. Step 2: Check condition for full stack

   The stack is full when top points to the last index, which is MAX - 1.

3. Final Answer:

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

4. Quick Check:

   Top equals max index means full [OK]

Hint: Stack full when top equals MAX minus one [OK]

Common Mistakes:

• Using top == MAX (off by one error)
• Using top > MAX (invalid index)
• Using top < MAX - 1 (means not full)

3. Consider this C code snippet for a stack with 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 full
if (top == MAX - 1) printf("Full"); else printf("Not Full");

What will be printed?

medium

A. Not Full

B. Full

C. Runtime Error

D. Compilation Error

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

   Full means top == MAX - 1 which is 2. Here, top is 1, so stack is not full.

3. Final Answer:

   Not Full -> Option A

4. Quick Check:

   Top is 1, max index 2, so not full [OK]

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

Common Mistakes:

• Assuming two pushes fill the stack
• Confusing top index with count
• Ignoring initial top = -1

4. Find the error in this C function that checks if a stack is empty:

int isEmpty(int top) {
    if (top == 0)
        return 1;
    else
        return 0;
}

medium

A. It should check if top == MAX - 1

B. It should check if top == -1 instead of 0

C. It should return 0 when empty

D. No error, function is correct

Solution

1. Step 1: Understand stack empty condition

   In stack implementation, empty means top == -1, not 0.

2. Step 2: Identify the incorrect condition

   The function checks top == 0, which means stack has one element, so it's wrong.

3. Final Answer:

   It should check if top == -1 instead of 0 -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Checking top == 0 for empty
• Confusing full and empty conditions
• Returning wrong boolean values

5. You have a stack with capacity 5 and top = 3. You want to push two elements safely. Which sequence of checks and operations is correct to avoid overflow?

hard

A. No need to check; just increment top and push

B. Check if top == 5 before each push; if false, increment top and push

C. Check if top == 3 before each push; if false, increment top and push

D. Check if top == 4 before each push; if false, increment top and push

Solution

1. Step 1: Understand stack capacity and top indexing

   Stack capacity is 5, so max top index is 4 (0 to 4).

2. Step 2: Check condition before pushing

   Before each push, check if top == 4 (full). If not full, increment top and push.

3. Final Answer:

   Check if top == 4 before each push; if false, increment top and push -> Option D

4. Quick Check:

   Max index is capacity-1 = 4, check top == 4 [OK]

Hint: Check top equals capacity minus one before push [OK]

Common Mistakes:

• Checking top == capacity (off by one)
• Not checking before push causes overflow
• Assuming top == 3 means full