Stack Concept and LIFO Principle in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA C++

stack.push([1]);

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

A10

Bpop()

Ctop()

Dempty()

Attempts:

3 left

💡 Hint

Common Mistakes

Using stack.pop() inside push, which removes elements instead of adding.

Using stack.top() which only accesses the top element without adding.

Using stack.empty() which checks if stack is empty, not adding elements.

The push function adds the given element to the top of the stack. Here, 10 is the element to add.

2fill in blank

medium

Complete the code to remove the top element from the stack.

DSA C++

stack.[1]();

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

Apush

Bpop

Cempty

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using push which adds elements instead of removing.

Using top which only accesses the top element.

Using empty which checks if stack is empty.

The pop method removes the top element from the stack, following the LIFO principle.

3fill in blank

hard

Fix the error in accessing the top element of the stack.

DSA C++

int value = stack.[1]();

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

Apop

Bpush

Ctop

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes the element and returns void.

Using push which adds elements.

Using empty which returns a boolean.

The top method returns the top element without removing it. Using pop would remove it and not return the value.

4fill in blank

hard

Fill both blanks to check if the stack is empty and print a message.

DSA C++

if (stack.[1]()) {
    std::cout << "Stack is [2]" << std::endl;
}

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

Aempty

Bfull

Cnot empty

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using full() which is not a standard stack method.

Printing "Stack is full" when checking if empty.

Using size() which returns a number, not a boolean.

The empty() method returns true if the stack has no elements. The message should say "Stack is empty" if the condition is true.

5fill in blank

hard

Fill all three blanks to create a stack, push an element, and print the top element.

DSA C++

#include <stack>
#include <iostream>

int main() {
    std::stack<int> [1];
    [1].[2](5);
    std::cout << [1].[3]() << std::endl;
    return 0;
}

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

AmyStack

Bpush

Ctop

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using pop instead of top to print the element.

Forgetting to push before accessing the top.

First, we create a stack named myStack. Then we push the value 5 onto it. Finally, we print the top element using top().

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the last element added is removed first

B. First In, First Out - the first element added is removed first

C. Elements are removed randomly

D. Elements are removed based on their value

Solution

1. Step 1: Understand LIFO principle

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is removed first -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking stack removes oldest element first
• Assuming random removal order

2. Which of the following is the correct way to push an integer 10 onto a stack named s in C++ STL?

easy

A. s.add(10);

B. s.insert(10);

C. push(s, 10);

D. s.push(10);

Solution

1. Step 1: Recall C++ STL stack push syntax

   In C++ STL, the method to add an element to stack is push() called on the stack object.

2. Step 2: Verify correct method usage

   Using s.push(10); correctly pushes 10 onto stack s.

3. Final Answer:

   s.push(10); -> Option D

4. Quick Check:

   Stack push method = push() [OK]

Hint: Use s.push(value) to add in C++ STL stack [OK]

Common Mistakes:

• Using non-existent methods like add or insert
• Calling push as a free function
• Confusing vector methods with stack

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

std::stack<int> s;
s.push(5);
s.push(10);
s.push(15);
int x = s.top();
s.pop();
int y = s.top();
std::cout << x << " " << y << std::endl;

medium

A. 15 10

B. 5 10

C. 10 5

D. 15 5

Solution

1. Step 1: Trace stack push operations

   Elements pushed in order: 5 (bottom), 10, 15 (top).

2. Step 2: Check top and pop operations

   First, x = s.top() gives 15. Then s.pop() removes 15. Next, y = s.top() gives 10.

3. Final Answer:

   15 10 -> Option A

4. Quick Check:

   Top after push = 15, after pop = 10 [OK]

Hint: Top shows last pushed; pop removes it [OK]

Common Mistakes:

• Confusing order of elements after pop
• Assuming pop returns value
• Mixing bottom and top elements

4. Identify the error in the following C++ code snippet using stack:

std::stack<int> s;
s.pop();
s.push(20);

medium

A. Syntax error in stack declaration

B. Cannot pop from an empty stack; causes runtime error

C. push should come before pop always

D. No error; code runs fine

Solution

1. Step 1: Analyze stack operations order

   The code calls pop() on an empty stack before pushing any element.

2. Step 2: Understand pop behavior on empty stack

   Calling pop() on empty stack is undefined and causes runtime error.

3. Final Answer:

   Cannot pop from an empty stack; causes runtime error -> Option B

4. Quick Check:

   Pop empty stack = runtime error [OK]

Hint: Never pop before pushing any element [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Assuming pop returns value
• Thinking push order fixes pop error

5. Given a stack with elements pushed in this order: 3, 7, 9, 2, 5. Which sequence of pop() operations will produce the output 5 2 9 when printing popped elements?

hard

A. Pop twice, push 1, then pop once

B. Pop once, push 4, then pop twice

C. Pop three times consecutively

D. Pop once, pop again, then push 6 and pop

Solution

1. Step 1: Understand stack order after pushes

   Stack top to bottom: 5 (top), 2, 9, 7, 3 (bottom).

2. Step 2: Pop three times consecutively

   First pop: 5, second pop: 2, third pop: 9. This matches output sequence.

3. Final Answer:

   Pop three times consecutively -> Option C

4. Quick Check:

   Pop order matches output 5 2 9 [OK]

Hint: Pop in order to get last pushed elements first [OK]

Common Mistakes:

• Adding extra pushes changes output
• Popping fewer times than needed
• Confusing push/pop order