Why Stack Exists and What Problems It Solves in DSA C - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA C

void push(int stack[], int *top, int value) {
    stack[++[1]] = value;
}

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

Avalue

Btop

C*top

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' instead of '*top' causes incorrect pointer usage.

Assigning value before incrementing top.

The top pointer is incremented first, then the value is assigned at that position.

2fill in blank

medium

Complete the code to pop an element from the stack.

DSA C

int pop(int stack[], int *top) {
    if (*top == -1) return -1; // stack empty
    return stack[[1]--];
}

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

A*top

Bstack

Ctop

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using prefix decrement causes wrong element to be returned.

Using 'top' instead of '*top'.

Return the current top element, then decrement the top pointer.

3fill in blank

hard

Fix the error in the stack overflow check condition.

DSA C

int isFull(int top, int maxSize) {
    return [1] >= maxSize - 1;
}

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

A*top

Btop

Cstack

DmaxSize

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*top' causes compilation error.

Comparing with maxSize instead of maxSize - 1.

The top index is compared to maxSize - 1 to check if stack is full.

4fill in blank

hard

Fill both blanks to check if the stack is empty and return the top element safely.

DSA C

int safePop(int stack[], int *top) {
    if (*top [1] -1) return -1;
    return stack[[2]--];
}

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

A<=

B==

C*top

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' instead of '==' causes wrong empty check.

Using 'top' instead of '*top'.

Check if top equals -1 to detect empty stack, then return element at *top and decrement.

5fill in blank

hard

Fill all three blanks to implement a function that returns the current top element without popping it.

DSA C

int peek(int stack[], int *top) {
    if (*top [1] -1) return -1;
    return stack[[2]];
    // top remains [3]
}

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

A==

B>

C*top

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '==' causes wrong empty check.

Modifying top in peek causes stack corruption.

Check if stack is empty by comparing *top to -1, then return element at *top without changing top.

Practice

1. Why do we use a stack data structure in programming?

easy

A. To store data in random order

B. To keep track of function calls and return addresses

C. To sort numbers quickly

D. To store large amounts of data permanently

Solution

1. Step 1: Understand the purpose of a stack

   A stack stores data in a last-in, first-out order, which is perfect for tracking function calls and returns.

2. Step 2: Identify common uses of stack

   Stacks are mainly used to keep track of function calls, especially in recursion, where the last called function must finish first.

3. Final Answer:

   To keep track of function calls and return addresses -> Option B

4. Quick Check:

   Stack = function call tracking [OK]

Hint: Stack = last-in, first-out for function calls [OK]

Common Mistakes:

• Thinking stack stores data randomly
• Confusing stack with queue
• Assuming stack sorts data

2. Which of the following is the correct way to push an element onto a stack in C?

easy

A. stack[++top] = value;

B. stack[top] = value++;

C. stack[top++] = value;

D. stack[top--] = value;

Solution

1. Step 1: Understand stack push operation

   To push, we first increase the top index, then assign the value at that position.

2. Step 2: Check each option

   stack[++top] = value; increments top before assignment, which is correct. stack[top++] = value; assigns then increments, which may overwrite. stack[top] = value++; increments value, not top. stack[top--] = value; decrements top, which is wrong for push.

3. Final Answer:

   stack[++top] = value; -> Option A

4. Quick Check:

   Push = increment top then assign [OK]

Hint: Push means increment top first, then assign [OK]

Common Mistakes:

• Incrementing after assignment causes overwrite
• Decrementing top on push
• Incrementing value instead of top

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

int stack[3];
int top = -1;
stack[++top] = 10;
stack[++top] = 20;
stack[++top] = 30;
printf("%d", stack[top]);

medium

A. 10

B. 20

C. 30

D. Runtime error

Solution

1. Step 1: Trace stack push operations

   Initially, top = -1. After first push, top=0, stack[0]=10. Second push: top=1, stack[1]=20. Third push: top=2, stack[2]=30.

2. Step 2: Check printed value

   printf prints stack[top], which is stack[2] = 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Last pushed value = 30 [OK]

Hint: Last pushed value is at stack[top] [OK]

Common Mistakes:

• Printing stack[0] instead of stack[top]
• Confusing top index after pushes
• Assuming runtime error without overflow

4. Identify the error in this stack pop function in C:

int pop(int stack[], int *top) {
    if (*top == -1) {
        printf("Stack Underflow\n");
        return -1;
    }
    return stack[*top--];
}

medium

A. Incorrect decrement of top pointer

B. Missing check for stack overflow

C. Returning wrong stack element

D. No error, function is correct

Solution

1. Step 1: Understand pointer decrement in return statement

   The expression stack[*top--] uses post-decrement on *top, but *top-- decrements the pointer, not the value it points to.

2. Step 2: Correct way to decrement top value

   It should be stack[(*top)--] to decrement the value pointed by top after accessing stack.

3. Final Answer:

   Incorrect decrement of top pointer -> Option A

4. Quick Check:

   Use (*top)-- to decrement value, not pointer [OK]

Hint: Use (*top)-- to decrement value, not pointer [OK]

Common Mistakes:

• Using *top-- instead of (*top)--
• Ignoring pointer vs value decrement
• Not checking stack underflow

5. You want to reverse a string using a stack in C. Which approach correctly uses the stack to solve this problem?

hard

A. Use a queue instead of a stack to reverse the string

B. Push characters only if they are vowels, then pop to reverse

C. Pop characters first, then push them back to reverse

D. Push each character of the string onto the stack, then pop all characters to form the reversed string

Solution

1. Step 1: Understand stack reversal logic

   Pushing all characters onto the stack stores them in order. Popping them reverses the order due to last-in, first-out behavior.

2. Step 2: Evaluate options

   Push each character of the string onto the stack, then pop all characters to form the reversed string correctly pushes all characters then pops to reverse. Push characters only if they are vowels, then pop to reverse only pushes vowels, losing other characters. Pop characters first, then push them back to reverse pops before pushing, which is invalid. Use a queue instead of a stack to reverse the string suggests queue, which reverses order differently.

3. Final Answer:

   Push each character of the string onto the stack, then pop all characters to form the reversed string -> Option D

4. Quick Check:

   Stack reversal = push all, then pop all [OK]

Hint: Push all chars, then pop all to reverse [OK]

Common Mistakes:

• Pushing only some characters
• Popping before pushing
• Using queue instead of stack for reversal