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++

stack.[1](10);

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

Apop

Bpush

Ctop

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' instead of 'push' removes an element instead of adding.

The push function adds an element to the top of the stack.

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'

Apop

Bpush

Ctop

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push' instead of 'pop' adds an element instead of removing.

The pop function removes the top element from the stack.

3fill in blank

hard

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

DSA C++

int topElement = stack.[1]();

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

Atop

Bpop

Cpush

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' removes the element instead of just accessing it.

The top function returns the top element without removing it.

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

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'empty' function with 'top' or 'pop'.

The empty() function checks if the stack has no elements. If true, print 'Stack is empty'.

5fill in blank

hard

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

DSA C++

#include <stack>
#include <iostream>

int main() {
    std::stack<int> [1];
    [1].[2](5);
    [1].[2](10);
    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 'pop' instead of 'top' to access the element.

This code creates a stack named myStack, pushes 5 and 10 onto it, then prints the top element, which is 10.

Practice

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

easy

A. To store data in random order

B. To sort elements automatically

C. To keep track of function calls and return addresses

D. To store data permanently on disk

Solution

1. Step 1: Understand stack usage in function calls

   Stacks help keep track of which function called which, storing return addresses and local variables.

2. Step 2: Identify why other options don't fit

   Stacks do not store data randomly, sort automatically, or store data permanently on disk.

3. Final Answer:

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

4. Quick Check:

   Stack = function call tracking [OK]

Hint: Stack remembers last task first, like a pile of plates [OK]

Common Mistakes:

• Thinking stack sorts data automatically
• Confusing stack with permanent storage
• Assuming stack stores data randomly

2. Which of the following is the correct way to declare a stack of integers in C++?

easy

A. stack<int> s[];

B. stack s<int>;

C. stack<int> s()

D. stack<int> s;

Solution

1. Step 1: Recall C++ stack declaration syntax

   The correct syntax is stack<Type> variableName; so stack<int> s; is correct.

2. Step 2: Check other options for syntax errors

   A declares an array which is invalid here, B has wrong order stack s<int>, C uses parentheses incorrectly.

3. Final Answer:

   stack<int> s; -> Option D

4. Quick Check:

   Correct stack declaration = stack<int> s; [OK]

Hint: Use template syntax: stack<Type> name; [OK]

Common Mistakes:

• Placing template brackets incorrectly
• Using parentheses instead of semicolon
• Declaring stack as an array mistakenly

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

stack<int> s;
s.push(10);
s.push(20);
s.push(30);
int x = s.top();
s.pop();
int y = s.top();
cout << x << "," << y << endl;

medium

A. 30,20

B. 10,20

C. 20,10

D. 30,10

Solution

1. Step 1: Trace stack push operations

   Stack after pushes: bottom->10, 20, top->30.

2. Step 2: Trace top and pop operations

   x = top() = 30; pop removes 30; now top() = 20; y = 20.

3. Final Answer:

   30,20 -> Option A

4. Quick Check:

   Last in first out = 30, then 20 [OK]

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

Common Mistakes:

• Confusing order of elements after pop
• Mixing up top() values before and after pop
• Assuming stack behaves like queue

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

stack<int> s;
s.push(5);
s.pop();
s.pop();
cout << s.top() << endl;

medium

A. Accessing top of empty stack causes runtime error

B. Syntax error in stack declaration

C. Cannot push integer 5 into stack<int>

D. pop() does not remove elements

Solution

1. Step 1: Analyze stack operations

   Push 5 adds one element, first pop removes it, second pop tries to remove from empty stack.

2. Step 2: Check consequences of extra pop and top()

   Second pop on empty stack is undefined; then top() on empty stack causes runtime error.

3. Final Answer:

   Accessing top of empty stack causes runtime error -> Option A

4. Quick Check:

   Empty stack top() = error [OK]

Hint: Never pop or top from empty stack [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Assuming pop() always safe
• Thinking push fails for int type

5. You want to reverse a string using a stack in C++. Which approach correctly uses a stack to reverse the string "hello"?

hard

A. Push all characters, then print stack without popping

B. Push each character, then pop all to build reversed string

C. Pop characters first, then push to stack

D. Use stack to sort characters alphabetically

Solution

1. Step 1: Understand stack reversal logic

   Pushing characters stores them in order; popping reverses order due to LIFO.

2. Step 2: Evaluate options for correctness

   Push each character, then pop all to build reversed string correctly pushes then pops to reverse; B prints original order; C pops before push invalid; D sorts, not reverses.

3. Final Answer:

   Push each character, then pop all to build reversed string -> Option B

4. Quick Check:

   Stack reverses order by pop after push [OK]

Hint: Push then pop to reverse order [OK]

Common Mistakes:

• Printing stack without popping reverses nothing
• Trying to pop before pushing
• Confusing reversal with sorting