OrderBy and sorting in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the list of numbers in ascending order using LINQ.

C Sharp (C#)

var sortedNumbers = numbers.[1](n => n);

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

AOrderBy

BWhere

CSelect

DGroupBy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Where instead of OrderBy will filter elements, not sort them.

Select projects elements but does not sort.

GroupBy groups elements but does not sort.

The OrderBy method sorts elements in ascending order based on the key selector.

2fill in blank

medium

Complete the code to sort the list of strings by their length in descending order.

C Sharp (C#)

var sortedStrings = strings.OrderByDescending(s => s.[1]);

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

ACount

BCapacity

CSize

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count is for collections, not strings.

Size and Capacity are not valid string properties.

The Length property gives the number of characters in a string, which is used here to sort by string length.

3fill in blank

hard

Fix the error in the code to sort a list of objects by their Age property in ascending order.

C Sharp (C#)

var sortedPeople = people.[1](p => p.Age);

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

AOrderBy

BSort

COrder

DSortBy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Sort is a method on List but not in LINQ.

Order and SortBy are not valid LINQ methods.

The correct LINQ method to sort by a key is OrderBy. Methods like Sort or SortBy do not exist in LINQ.

4fill in blank

hard

Fill both blanks to create a dictionary that maps each word to its length, but only include words longer than 3 characters.

C Sharp (C#)

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

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

ALength

BCount

CSize

DCapacity

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count instead of Length for strings.

Using different properties in the two blanks.

The Length property is used to get the number of characters in a string. We use it both to filter words longer than 3 and to get their length for the dictionary values.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase words as keys and their lengths as values, including only words with length greater than 4.

C Sharp (C#)

var result = words.Where(w => w.[1] > 4).ToDictionary(w => w.[2](), w => w.[3]);

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

ACount

BLength

CToUpper

DToLower

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count instead of Length for strings.

Using ToLower instead of ToUpper for keys.

Mixing properties inconsistently.

We filter words by their Length property, convert them to uppercase with ToUpper() for keys, and use Length again for values.

Practice

1. What does the OrderBy method do in C#?

easy

A. Sorts a collection in ascending order based on a key

B. Deletes elements from a list

C. Reverses the order of elements in a list

D. Filters elements based on a condition

Solution

1. Step 1: Understand the purpose of OrderBy

   The OrderBy method sorts elements in a collection based on a key in ascending order.

2. Step 2: Compare with other options

   Options B, C, and D describe different operations (deletion, reversing, filtering) which OrderBy does not perform.

3. Final Answer:

   Sorts a collection in ascending order based on a key -> Option A

4. Quick Check:

   OrderBy = Sort ascending [OK]

Hint: OrderBy sorts ascending by key, not filtering or deleting [OK]

Common Mistakes:

• Confusing OrderBy with filtering methods like Where
• Thinking OrderBy modifies the original list
• Mixing up OrderBy with reversing or deleting

2. Which of the following is the correct syntax to sort a list of integers named numbers in ascending order using OrderBy?

easy

A. numbers.OrderBy();

B. numbers.OrderBy(n);

C. numbers.OrderBy(n => n);

D. numbers.OrderBy(n => );

Solution

1. Step 1: Check the correct lambda syntax

   OrderBy requires a key selector function like n => n to specify sorting key.

2. Step 2: Validate each option

   numbers.OrderBy(n => n); uses correct lambda syntax. numbers.OrderBy(); misses the key selector. numbers.OrderBy(n); passes a variable, not a lambda. numbers.OrderBy(n => ); has incomplete lambda syntax.

3. Final Answer:

   numbers.OrderBy(n => n); -> Option C

4. Quick Check:

   OrderBy needs a key selector lambda [OK]

Hint: OrderBy always needs a key selector lambda like n => n [OK]

Common Mistakes:

• Omitting the lambda expression inside OrderBy
• Passing a variable instead of a lambda
• Using incomplete or invalid lambda syntax

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

var fruits = new List<string> { "banana", "apple", "cherry" };
var sorted = fruits.OrderBy(f => f);
foreach(var fruit in sorted) {
    Console.Write(fruit + " ");
}

medium

A. banana apple cherry

B. apple banana cherry

C. cherry banana apple

D. apple cherry banana

Solution

1. Step 1: Understand the sorting key

   The code sorts the list of fruits alphabetically by their string value.

2. Step 2: Determine the sorted order

   Alphabetically, "apple" comes before "banana", which comes before "cherry".

3. Final Answer:

   apple banana cherry -> Option B

4. Quick Check:

   OrderBy sorts strings alphabetically [OK]

Hint: OrderBy sorts strings alphabetically ascending [OK]

Common Mistakes:

• Assuming original order is preserved
• Confusing OrderBy with OrderByDescending
• Not recognizing alphabetical order

4. The following code throws a compile-time error. What is the mistake?

var numbers = new List<int> { 3, 1, 2 };
var sorted = numbers.OrderBy();

medium

A. numbers must be an array, not a list

B. List<int> cannot be sorted

C. OrderBy should be OrderByDescending

D. OrderBy requires a key selector lambda expression

Solution

1. Step 1: Identify the method signature requirement

   OrderBy requires a key selector function to specify how to sort elements.

2. Step 2: Analyze the error cause

   Calling OrderBy() without any argument causes a compile error because the key selector is missing.

3. Final Answer:

   OrderBy requires a key selector lambda expression -> Option D

4. Quick Check:

   OrderBy needs a lambda key selector [OK]

Hint: Always provide a key selector lambda to OrderBy [OK]

Common Mistakes:

• Calling OrderBy without arguments
• Thinking OrderBy works without a key selector
• Confusing List and array types for sorting

5. Given a list of students with properties Name and Score, how do you sort the list first by Score descending, then by Name ascending using LINQ?

hard

A. students.OrderByDescending(s => s.Score).ThenBy(s => s.Name);

B. students.OrderBy(s => s.Score).OrderBy(s => s.Name);

C. students.OrderBy(s => s.Name).OrderByDescending(s => s.Score);

D. students.OrderByDescending(s => s.Name).ThenByDescending(s => s.Score);

Solution

1. Step 1: Understand multi-level sorting

   To sort by multiple keys, use OrderBy or OrderByDescending for the first key, then ThenBy or ThenByDescending for the next keys.

2. Step 2: Apply correct order and directions

   We want to sort by Score descending first, then by Name ascending, so use OrderByDescending(s => s.Score) followed by ThenBy(s => s.Name).

3. Final Answer:

   students.OrderByDescending(s => s.Score).ThenBy(s => s.Name); -> Option A

4. Quick Check:

   OrderByDescending + ThenBy for multi-level sort [OK]

Hint: Use ThenBy after OrderBy for secondary sorting [OK]

Common Mistakes:

• Using multiple OrderBy calls instead of ThenBy
• Mixing ascending and descending incorrectly
• Sorting by wrong property order