Queue and Stack behavior in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an item to the queue.

C Sharp (C#)

Queue<int> numbers = new Queue<int>();
numbers.[1](5);

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

APush

BInsert

CAdd

DEnqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Push instead of Enqueue for queues.

Trying to use Add or Insert which are not queue methods.

Use Enqueue to add an item to a queue in C#.

2fill in blank

medium

Complete the code to remove an item from the stack.

C Sharp (C#)

Stack<string> words = new Stack<string>();
words.Push("hello");
string top = words.[1]();

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

APop

BDequeue

CRemove

DPeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using Dequeue which is for queues.

Using Peek which does not remove the item.

Use Pop to remove and return the top item from a stack.

3fill in blank

hard

Fix the error in the code to get the next item from the queue without removing it.

C Sharp (C#)

Queue<char> letters = new Queue<char>();
letters.Enqueue('a');
char next = letters.[1]();

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

APop

BDequeue

CPeek

DTop

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop which is not a queue method.

Using Dequeue which removes the item.

Use Peek to look at the next item in the queue without removing it.

4fill in blank

hard

Fill both blanks to create a stack and add an item to it.

C Sharp (C#)

Stack<int> [1] = new [2]<int>();
[1].Push(10);

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

Anumbers

BQueue

CStack

Ditems

Attempts:

3 left

💡 Hint

Common Mistakes

Using Queue instead of Stack for the type.

Using inconsistent variable names.

Declare a stack variable named items and create a new Stack<int> instance.

5fill in blank

hard

Fill all three blanks to create a queue, add an item, and remove it.

C Sharp (C#)

Queue<string> [1] = new Queue<string>();
[1].[2]("apple");
string fruit = [1].[3]();

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

Afruits

BEnqueue

CDequeue

Ditems

Attempts:

3 left

💡 Hint

Common Mistakes

Using Push or Pop which are stack methods.

Mixing variable names inconsistently.

Create a queue named fruits, add "apple" with Enqueue, then remove it with Dequeue.

Practice

1. Which data structure removes elements in the order they were added, like a line at a grocery store?

easy

A. Array

B. Stack

C. Dictionary

D. Queue

Solution

1. Step 1: Understand FIFO behavior

   A queue removes elements in the order they were added, called First-In-First-Out (FIFO).

2. Step 2: Match behavior to real-life example

   A line at a grocery store is FIFO, so the queue matches this behavior.

3. Final Answer:

   Queue -> Option D

4. Quick Check:

   FIFO = Queue [OK]

Hint: FIFO means first in, first out like a queue line [OK]

Common Mistakes:

• Confusing stack with queue
• Thinking stack is FIFO
• Mixing array behavior with queue
• Assuming dictionary has order

2. Which of the following is the correct way to add an item to a Stack in C#?

easy

A. stack.Push(item);

B. stack.Enqueue(item);

C. stack.Add(item);

D. stack.Insert(item);

Solution

1. Step 1: Recall Stack method names

   In C#, Stack uses Push() to add items on top.

2. Step 2: Identify correct method

   Enqueue is for Queue, Add and Insert are not Stack methods.

3. Final Answer:

   stack.Push(item); -> Option A

4. Quick Check:

   Push adds to Stack [OK]

Hint: Use Push() to add to Stack, Enqueue() for Queue [OK]

Common Mistakes:

• Using Enqueue() on Stack
• Using Add() or Insert() which don't exist
• Confusing Stack and Queue methods
• Syntax errors with method calls

3. What is the output of this C# code?

var stack = new Stack<int>();
stack.Push(1);
stack.Push(2);
stack.Push(3);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());

medium

A. 3\n2

B. 1\n2

C. 2\n3

D. 3\n3

Solution

1. Step 1: Trace Push operations

   Stack after pushes: bottom=1, middle=2, top=3.

2. Step 2: Execute Pop and Peek

   Pop() removes and returns top (3). Peek() returns new top (2) without removing.

3. Final Answer:

   3\n2 -> Option A

4. Quick Check:

   Pop=3, Peek=2 [OK]

Hint: Pop removes top, Peek shows top without removing [OK]

Common Mistakes:

• Mixing Pop and Peek results
• Assuming FIFO order
• Confusing stack order
• Forgetting Pop removes item

4. Identify the error in this C# code using Queue:

var queue = new Queue<string>();
queue.Push("apple");
queue.Enqueue("banana");
Console.WriteLine(queue.Dequeue());

medium

A. Dequeue() returns last item added

B. Queue does not have Push() method

C. Enqueue() should be Dequeue()

D. Queue cannot store strings

Solution

1. Step 1: Check Queue methods

   Queue uses Enqueue() to add, not Push().

2. Step 2: Identify incorrect method usage

   Calling Push() on Queue causes a compile error.

3. Final Answer:

   Queue does not have Push() method -> Option B

4. Quick Check:

   Queue uses Enqueue, no Push [OK]

Hint: Queue uses Enqueue(), Stack uses Push() [OK]

Common Mistakes:

• Using Push() on Queue
• Confusing Enqueue and Dequeue
• Thinking Dequeue returns last item
• Assuming Queue can't hold strings

5. You want to reverse the order of words in a sentence using C#. Which data structure is best and why?

string sentence = "hello world from C#";

hard

A. Queue, because it keeps original order using FIFO

B. Dictionary, because it stores key-value pairs

C. Stack, because it reverses order using LIFO

D. List, because it sorts items automatically

Solution

1. Step 1: Understand the goal

   Reversing words means last word should come first, so order is reversed.

2. Step 2: Choose data structure behavior

   Stack uses Last-In-First-Out (LIFO), perfect for reversing order.

3. Step 3: Eliminate other options

   Queue keeps order (FIFO), Dictionary stores pairs unordered, List does not reverse automatically.

4. Final Answer:

   Stack, because it reverses order using LIFO -> Option C

5. Quick Check:

   Reverse order = Stack (LIFO) [OK]

Hint: Use Stack to reverse order with LIFO behavior [OK]

Common Mistakes:

• Choosing Queue for reversing
• Thinking List auto-sorts
• Using Dictionary for order
• Ignoring LIFO vs FIFO difference