Push Operation on Stack in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after pushing elements on the stack?

Consider a stack implemented using an array with a maximum size of 5. Initially, the stack is empty. We perform the following push operations in order: push(10), push(20), push(30). What is the printed state of the stack after these operations?

DSA C

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

void push(int x) {
    if (top == 4) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = x;
}

void printStack() {
    for (int i = 0; i <= top; i++) {
        printf("%d -> ", stack[i]);
    }
    printf("null\n");
}

int main() {
    push(10);
    push(20);
    push(30);
    printStack();
    return 0;
}

A Stack Overflow

B 30 -> 20 -> 10 -> null

C null

D 10 -> 20 -> 30 -> null

💡 Hint

Remember that push adds elements on top, and printStack prints from bottom to top index.

The push function adds elements at the next top index. After pushing 10, 20, and 30, the stack array holds [10, 20, 30] from index 0 to 2. The printStack function prints from index 0 to top, so the output is 10 -> 20 -> 30 -> null.

❓ Predict Output

intermediate

What happens when pushing beyond stack capacity?

Given a stack with maximum size 3, initially empty, we push elements 5, 15, 25, and then try to push 35. What will be the output after these operations?

DSA C

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

void push(int x) {
    if (top == 2) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = x;
}

void printStack() {
    for (int i = 0; i <= top; i++) {
        printf("%d -> ", stack[i]);
    }
    printf("null\n");
}

int main() {
    push(5);
    push(15);
    push(25);
    push(35);
    printStack();
    return 0;
}

A

Stack Overflow
5 -> 15 -> 25 -> null

B 5 -> 15 -> 25 -> 35 -> null

C

Stack Overflow
null

D 5 -> 15 -> null

💡 Hint

Check the condition that prevents pushing when stack is full.

The stack size is 3, so pushing the fourth element triggers 'Stack Overflow' message. The stack remains with the first three elements. Printing shows '5 -> 15 -> 25 -> null'.

🧠 Conceptual

advanced

How does the push operation affect the 'top' pointer in a stack?

In a stack implemented using an array, what is the correct way the 'top' pointer changes when an element is pushed?

A Assign the new element at stack[top], then increment 'top' by 1

B Increment 'top' by 1, then assign the new element at stack[top]

C Decrement 'top' by 1, then assign the new element at stack[top]

D Assign the new element at stack[top], then decrement 'top' by 1

💡 Hint

Think about where the new element should be placed relative to the current top.

When pushing, we first move the 'top' pointer up by one to point to the new empty slot, then place the new element there. This ensures the top always points to the last inserted element.

🔧 Debug

advanced

Identify the error in this push function implementation

What error will occur when running this push function on a stack of size 5?

DSA C

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

void push(int x) {
    if (top == 4) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = x;
}

int main() {
    push(1);
    push(2);
    push(3);
    return 0;
}

A Compilation error due to syntax

B Stack Overflow message prints immediately

C Writes out of bounds and corrupts memory

D No error, works correctly

💡 Hint

Check how 'top' is used and updated in the push function.

The condition should be 'top == 4' to prevent overflow since array indices go from 0 to 4. Also, 'stack[top++] = x;' uses post-increment, so the first assignment is at stack[-1], which is invalid and causes memory corruption.

🚀 Application

expert

After a sequence of push and pop operations, what is the stack state?

A stack with max size 4 is initially empty. The following operations are performed: push(7), push(14), pop(), push(21), push(28), pop(), push(35). What is the printed stack state after these operations?

DSA C

int stack[4];
int top = -1;

void push(int x) {
    if (top == 3) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = x;
}

void pop() {
    if (top == -1) {
        printf("Stack Underflow\n");
        return;
    }
    top--;
}

void printStack() {
    for (int i = 0; i <= top; i++) {
        printf("%d -> ", stack[i]);
    }
    printf("null\n");
}

int main() {
    push(7);
    push(14);
    pop();
    push(21);
    push(28);
    pop();
    push(35);
    printStack();
    return 0;
}

A 7 -> 21 -> 35 -> null

B 7 -> 21 -> 28 -> 35 -> null

C 7 -> 14 -> 21 -> 35 -> null

D Stack Underflow

💡 Hint

Track the top index carefully after each push and pop.

Operations: - push(7): stack=[7], top=0 - push(14): stack=[7,14], top=1 - pop(): removes 14, top=0 - push(21): stack=[7,21], top=1 - push(28): stack=[7,21,28], top=2 - pop(): removes 28, top=1 - push(35): stack=[7,21,35], top=2 Printing from 0 to 2 gives '7 -> 21 -> 35 -> null'.

Practice

1. What does the push operation do in a stack?

easy

A. Checks if the stack is empty

B. Adds an element to the top of the stack

C. Removes the top element from the stack

D. Returns the bottom element of the stack

Solution

1. Step 1: Understand stack operations

   A stack is a data structure where elements are added or removed from the top only.

2. Step 2: Define push operation

   The push operation adds a new element on top of the existing elements in the stack.

3. Final Answer:

   Adds an element to the top of the stack -> Option B

4. Quick Check:

   Push means add on top = C [OK]

Hint: Push always adds on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Mixing stack with queue operations

2. Which of the following is the correct syntax to push an element 5 onto a stack named s in C?

easy

A. s.push = 5;

B. s.push(5);

C. push(5, s);

D. push(s, 5);

Solution

1. Step 1: Understand typical C stack push syntax

   In C, stack operations are usually functions where the stack is passed as the first argument.

2. Step 2: Match syntax with parameters

   The correct function call to push 5 onto stack s is push(s, 5);.

3. Final Answer:

   push(s, 5); -> Option D

4. Quick Check:

   Function push(stack, value) = A [OK]

Hint: In C, function push(stack, value) is standard [OK]

Common Mistakes:

• Using object.method syntax (not C style)
• Swapping argument order
• Assigning value instead of calling function

3. What will be the content of the stack after executing the following code?

int stack[5];
int top = -1;
// Push function
void push(int val) {
  if(top < 4) {
    top++;
    stack[top] = val;
  }
}

push(10);
push(20);
push(30);

medium

A. [10, 20, 30]

B. [30, 20, 10]

C. [10, 20]

D. Empty stack

Solution

1. Step 1: Trace each push call

   Initially, top = -1. After push(10), top=0, stack[0]=10. After push(20), top=1, stack[1]=20. After push(30), top=2, stack[2]=30.

2. Step 2: List stack content from bottom to top

   Stack elements from index 0 to top (2) are [10, 20, 30].

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   Push adds at top increasing index = B [OK]

Hint: Stack stores pushed elements in order at increasing top index [OK]

Common Mistakes:

• Reversing order of elements
• Confusing top index with element values
• Assuming stack is empty after pushes

4. Identify the error in the following push function for a stack of size 3:

int stack[3];
int top = 0;
void push(int val) {
  if(top <= 3) {
    stack[top] = val;
    top++;
  }
}

medium

A. Condition should be top < 3, not top <= 3

B. top should be incremented before assignment

C. stack array size should be 4

D. top should start at -1, not 0

Solution

1. Step 1: Analyze top and condition

   top starts at 0, which is valid for empty stack at index 0. The condition allows top to be 3, which is out of bounds for stack[3] (indices 0 to 2).

2. Step 2: Correct the condition

   Change condition to top < 3 to prevent overflow and invalid access.

3. Final Answer:

   Condition should be top < 3, not top <= 3 -> Option A

4. Quick Check:

   Stack max index 2 means top < 3 [OK]

Hint: Check array bounds: top must be less than size [OK]

Common Mistakes:

• Allowing top to equal array size causing overflow
• Starting top at -1 but not adjusting code
• Incrementing top before assignment causing skip

5. Given a stack implemented with an array of size 4 and top = 2 currently, what will happen if we try to push two more elements without popping?
Choose the correct behavior.

hard

A. Both pushes fail immediately due to full stack

B. Both pushes succeed, stack size automatically increases

C. First push succeeds, second push causes stack overflow

D. First push causes overflow, second push is not attempted

Solution

1. Step 1: Understand current stack state

   Array size is 4, so valid indices are 0 to 3. top=2 means top element is at index 2, so one slot remains at index 3.

2. Step 2: Push two elements

   First push increments top to 3 and stores element at index 3 (valid). Second push tries to increment top to 4, which is out of bounds, causing overflow.

3. Final Answer:

   First push succeeds, second push causes stack overflow -> Option C

4. Quick Check:

   Array size 4, top max index 3, overflow after that [OK]

Hint: Stack size fixed; overflow after last index [OK]

Common Mistakes:

• Assuming stack auto-expands
• Thinking both pushes fail immediately
• Confusing top index with count