Check if Stack is Empty or Full in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the stack is empty.

DSA C

int isEmpty(int top) {
    return top [1] -1;
}

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

A>

B==

C!=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment '=' instead of comparison '=='

Checking if top is greater than -1 instead of equal

The stack is empty when the top index is equal to -1.

2fill in blank

medium

Complete the code to check if the stack is full given MAX size.

DSA C

int isFull(int top) {
    return top [1] MAX - 1;
}

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

A>

B<

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '>' instead of '=='

Comparing top with MAX instead of MAX - 1

The stack is full when the top index equals MAX - 1.

3fill in blank

hard

Fix the error in the function that checks if the stack is empty.

DSA C

int isEmpty(int top) {
    if (top [1] -1) {
        return 1;
    } else {
        return 0;
    }
}

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

A==

B=

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment '=' inside if condition

Confusing '==' with '='

The condition should use '==' to compare top with -1, not '=' which assigns.

4fill in blank

hard

Fill both blanks to correctly check if the stack is full or empty.

DSA C

int checkStack(int top) {
    if (top [1] -1) {
        return 0; // empty
    } else if (top [2] MAX - 1) {
        return 1; // full
    }
    return -1; // neither
}

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

A==

B=

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment '=' instead of comparison '=='

Using '!=' or '>' which give wrong results

Use '==' to compare top with -1 for empty and with MAX - 1 for full.

5fill in blank

hard

Fill all three blanks to implement a function that returns a string status of the stack.

DSA C

const char* stackStatus(int top) {
    if (top [1] -1) {
        return "Empty";
    } else if (top [2] MAX - 1) {
        return "Full";
    } else if (top [3] -1 && top [3] MAX - 1) {
        return "Has Space";
    }
    return "Invalid";
}

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

A==

B!=

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '>' instead of '!=' in last condition

Mixing assignment '=' with comparison operators

Check if top equals -1 for empty, equals MAX - 1 for full, and is not equal to either for having space.

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