Lambda expression syntax in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a lambda that adds two integers.

C Sharp (C#)

Func<int, int, int> add = (x, y) => x [1] y;

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

A+

B/

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using - instead of +

Using * or / which changes the operation

The lambda expression uses the + operator to add x and y.

2fill in blank

medium

Complete the code to declare a lambda that checks if a number is even.

C Sharp (C#)

Func<int, bool> isEven = n => n [1] 2 == 0;

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

A+

B%

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using + or - instead of %

Using * which multiplies instead of checking remainder

The % operator gives the remainder. If remainder is 0 when divided by 2, the number is even.

3fill in blank

hard

Fix the error in the lambda that returns the square of a number.

C Sharp (C#)

Func<int, int> square = x => x [1] 2;

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

A*

B**

C^

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using ** which is invalid in C#

Using ^ which is bitwise XOR, not power

In C#, the multiplication operator * is used to square a number. The ** operator is not valid.

4fill in blank

hard

Fill the blank to create a lambda that filters numbers greater than 10.

C Sharp (C#)

Func<int, bool> isGreaterThanTen = n => n [1] 10;

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

A==

B<

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of >

Using != which checks inequality

The lambda checks if n is greater than 10 using the > operator.

5fill in blank

hard

Fill all three blanks to create a lambda that returns a dictionary of words and their lengths for words longer than 3 characters.

C Sharp (C#)

var lengths = words.Where(w => w.Length [1] 3).ToDictionary([2] => [3].ToLower(), [2] => [2].Length);

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

A>

Bword

Cw

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the filter

Using inconsistent variable names in the lambdas

The lambda filters words with length greater than 3 using >. Then ToDictionary uses w as the key selector and w.Length as the value selector. The key is converted to lowercase with w.ToLower().

Practice

1. What does the lambda expression x => x * 2 represent in C#?

easy

A. A variable declaration

B. A function that doubles the input value

C. A loop that runs twice

D. A conditional statement

Solution

1. Step 1: Understand lambda syntax

   The expression x => x * 2 means a function taking input x and returning x * 2.

2. Step 2: Interpret the operation

   It doubles the input value by multiplying it by 2.

3. Final Answer:

   A function that doubles the input value -> Option B

4. Quick Check:

   Lambda x => x * 2 doubles input [OK]

Hint: Lambda uses => to define short functions quickly [OK]

Common Mistakes:

• Thinking lambda is a variable
• Confusing lambda with loops
• Assuming lambda is a conditional

2. Which of the following is the correct syntax for a lambda expression that takes two integers and returns their sum?

easy

A. (a, b) => a + b

B. a, b => a + b

C. (a, b) => { a + b }

D. (a, b) => a + b;

Solution

1. Step 1: Check parameter syntax

   Parameters must be enclosed in parentheses when there is more than one, so (a, b) is correct.

2. Step 2: Check expression body

   The expression a + b without braces is valid for single expressions. (a, b) => a + b matches this.

3. Final Answer:

   (a, b) => a + b -> Option A

4. Quick Check:

   Two parameters need parentheses and expression body [OK]

Hint: Use parentheses for multiple parameters in lambda [OK]

Common Mistakes:

• Omitting parentheses for multiple parameters
• Adding semicolon inside lambda expression
• Using braces without return keyword incorrectly

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

Func<int, int> square = x => x * x;
Console.WriteLine(square(4));

medium

A. Compilation error

B. 8

C. 4

D. 16

Solution

1. Step 1: Understand the lambda function

   The lambda x => x * x returns the square of the input number.

2. Step 2: Calculate the output for input 4

   4 * 4 = 16, so square(4) returns 16.

3. Final Answer:

   16 -> Option D

4. Quick Check:

   Square of 4 is 16 [OK]

Hint: Lambda returns expression result directly if no braces [OK]

Common Mistakes:

• Confusing multiplication with addition
• Expecting lambda to print instead of return
• Thinking lambda causes syntax error

4. Identify the error in this lambda expression:

Func<int, int> add = (x, y) => x + y;

medium

A. Parentheses around parameters are not allowed

B. Missing return keyword inside lambda

C. Lambda has too many parameters for Func<int, int>

D. Lambda expression syntax is correct

Solution

1. Step 1: Check Func delegate signature

   Func<int, int> expects one input parameter and one output (int).

2. Step 2: Compare lambda parameters

   The lambda has two parameters (x, y), which does not match the delegate expecting one.

3. Final Answer:

   Lambda has too many parameters for Func<int, int> -> Option C

4. Quick Check:

   Func<int, int> means one input, one output [OK]

Hint: Match lambda parameters count with delegate type [OK]

Common Mistakes:

• Assuming multiple parameters allowed for Func
• Forgetting delegate parameter count rules
• Thinking return keyword is always needed

5. Given a list of integers, which lambda expression correctly filters out all even numbers using List<int>.Where()?

var numbers = new List<int> {1, 2, 3, 4, 5, 6};
var odds = numbers.Where( /* which lambda? */ );

hard

A. x => x % 2 == 1

B. x => x % 2 = 1

C. x => { return x % 2 == 0; }

D. x => x / 2 == 1

Solution

1. Step 1: Understand filtering condition

   We want to keep odd numbers, so condition is x % 2 == 1.

2. Step 2: Check lambda syntax and logic

   x => x % 2 == 1 uses correct syntax and logic to filter odds. x => x % 2 = 1 uses assignment (=) which is invalid. x => { return x % 2 == 0; } filters evens, not odds. x => x / 2 == 1 checks division, not oddness.

3. Final Answer:

   x => x % 2 == 1 -> Option A

4. Quick Check:

   Filter odds with modulo == 1 [OK]

Hint: Use == for comparison, % 2 == 1 filters odd numbers [OK]

Common Mistakes:

• Using = instead of == in lambda
• Filtering evens instead of odds
• Confusing division with modulo