Peek Top Element of Stack in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to peek the top element of the stack.

DSA C#

int top = stack.[1]();

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

APeek

BClear

CPush

DPop

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop instead of Peek removes the element.

Using Push or Clear does not return the top element.

The Peek method returns the top element of the stack without removing it.

2fill in blank

medium

Complete the code to check if the stack is empty before peeking.

DSA C#

if (stack.[1] > 0) {
    int top = stack.Peek();
}

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

ACapacity

BLength

CSize

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length or Size which are not properties of Stack.

Using Capacity which is not related to current element count.

The Count property gives the number of elements in the stack.

3fill in blank

hard

Fix the error in the code to peek the top element safely.

DSA C#

bool success = stack.[1](out int top); // Safely peeks without throwing an exception if empty

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

APop

BTryPeek

CPeek

DTop

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek or Pop without checking if stack is empty causes exceptions.

Using Top which is not a Stack method.

TryPeek safely tries to get the top element without throwing an exception if the stack is empty.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths if length is greater than 3.

DSA C#

var result = words.ToDictionary(word => word[1] , word => word.Length).Where(kv => kv.Value [2] 3).ToDictionary(kv => kv.Key, kv => kv.Value);

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

A.ToUpper()

B>

C<

D.ToLower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > for filtering lengths.

Not converting words to uppercase as keys.

The first blank converts words to uppercase keys. The second blank filters lengths greater than 3.

5fill in blank

hard

Fill both blanks to create a dictionary of uppercase keys and values filtered by value > 0.

DSA C#

var result = data.Where(kv => kv.Value [1] 0).ToDictionary(kv => kv.Key[2], kv => kv.Value);

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

A>

C.ToUpper()

D.ToString()

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > for filtering.

Trying to convert values to string unnecessarily.

The first blank filters values greater than 0. The second blank converts keys to uppercase. The third blank leaves values unchanged.

Practice

1. What does the Peek() method do in a stack?

easy

A. Returns the top element without removing it

B. Removes and returns the top element

C. Adds a new element to the top

D. Checks if the stack is empty

Solution

1. Step 1: Understand the Peek method purpose

   The Peek method is designed to look at the top element of the stack without changing the stack itself.

2. Step 2: Differentiate from Pop method

   Unlike Pop, Peek does not remove the element; it only returns it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   Peek() returns top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing Peek with Pop
• Thinking Peek adds elements
• Assuming Peek empties the stack

2. Which of the following is the correct syntax to peek the top element of a stack named myStack in C#?

easy

A. var top = myStack.Pop();

B. var top = myStack.Top();

C. var top = myStack.Peek();

D. var top = myStack.GetTop();

Solution

1. Step 1: Recall Stack method names in C#

   The Stack class in C# uses Peek() to look at the top element without removing it.

2. Step 2: Verify method correctness

   Top(), GetTop() are not valid Stack methods; Pop() removes the element.

3. Final Answer:

   var top = myStack.Peek(); -> Option C

4. Quick Check:

   Peek() is correct syntax for top element [OK]

Hint: Use Peek() method to get top element [OK]

Common Mistakes:

• Using Pop() instead of Peek()
• Using non-existent methods like Top()
• Missing parentheses after Peek

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

Stack<int> numbers = new Stack<int>();
numbers.Push(10);
numbers.Push(20);
numbers.Push(30);
Console.WriteLine(numbers.Peek());
Console.WriteLine(numbers.Count);

medium

A. 10
3

B. 30
3

C. 20
2

D. 30
2

Solution

1. Step 1: Trace the Push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is 30.

2. Step 2: Evaluate Peek and Count

   Peek returns top element 30 without removing it. Count remains 3 as no Pop occurred.

3. Final Answer:

   30 3 -> Option B

4. Quick Check:

   Peek = 30, Count = 3 [OK]

Hint: Peek shows last pushed; Count unchanged by Peek [OK]

Common Mistakes:

• Assuming Peek removes element
• Confusing Count after Peek
• Mixing order of pushed elements

4. Identify the error in this code snippet that tries to peek the top element of a stack:

Stack<string> stack = new Stack<string>();
string top = stack.Peek;
Console.WriteLine(top);

medium

A. Peek removes the element, so top is null

B. Stack is not initialized properly

C. Console.WriteLine cannot print stack elements

D. Peek is used without parentheses, causing a syntax error

Solution

1. Step 1: Check method call syntax

   Peek is a method and must be called with parentheses: Peek()

2. Step 2: Identify error type

   Using Peek without parentheses causes a compile-time syntax error.

3. Final Answer:

   Peek is used without parentheses, causing a syntax error -> Option D

4. Quick Check:

   Method calls need () [OK]

Hint: Always use parentheses when calling methods [OK]

Common Mistakes:

• Forgetting parentheses on Peek
• Assuming Peek removes element
• Thinking Console.WriteLine can't print strings

5. Given a stack Stack<char> letters with elements 'a', 'b', 'c' pushed in that order, write the output after this code:

char top1 = letters.Peek();
letters.Pop();
char top2 = letters.Peek();
Console.WriteLine(top1);
Console.WriteLine(top2);
Console.WriteLine(letters.Count);

hard

A. c
b
2

B. a
b
3

C. c
c
3

D. b
a
1

Solution

1. Step 1: Trace initial stack and first Peek

   Stack after pushes: bottom 'a', then 'b', top 'c'. Peek() returns 'c' as top1.

2. Step 2: Pop removes top element 'c'

   After Pop(), top 'c' is removed. Stack top is now 'b'.

3. Step 3: Second Peek returns 'b' as top2

   Peek() now returns 'b'. Count is 2 because one element was removed.

4. Final Answer:

   c b 2 -> Option A

5. Quick Check:

   Peek then Pop then Peek updates top correctly [OK]

Hint: Peek shows current top; Pop removes it; next Peek shows new top [OK]

Common Mistakes:

• Assuming Peek removes element
• Confusing order of elements after Pop
• Miscounting stack size after Pop