List generic collection in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a list of integers.

C Sharp (C#)

List<int> numbers = new List<[1]>();

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

Aint

Bstring

Cdouble

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type other than int when the list is meant for integers.

Forgetting to specify the generic type parameter.

The generic type parameter for the list should be int to create a list of integers.

2fill in blank

medium

Complete the code to add an element to the list.

C Sharp (C#)

numbers.[1](5);

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

AInsert

BAppend

CPush

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Append which is not a method of List in C#.

Using Push which is used in stacks, not lists.

The Add method is used to add an element to the end of a List in C#.

3fill in blank

hard

Fix the error in accessing the first element of the list.

C Sharp (C#)

int first = numbers[[1]];

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

A-1

B1

C0

Dnumbers.Count

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the index to get the first element.

Using -1 which is invalid index in C# lists.

List indexing in C# starts at 0, so the first element is at index 0.

4fill in blank

hard

Fill both blanks to create a list of strings and add a value.

C Sharp (C#)

List<[1]> names = new List<[2]>();
names.Add("Alice");

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

Astring

Bint

Cdouble

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types in the declaration and instantiation.

Using numeric types instead of string.

To create a list of strings, the generic type parameter must be string in both places.

5fill in blank

hard

Fill all three blanks to create a list, add an element, and get the count.

C Sharp (C#)

List<[1]> items = new List<[2]>();
items.[3](10);
int count = items.Count;

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

Aint

Bstring

CAdd

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove instead of Add to add elements.

Using string as the generic type when adding integers.

The list is of integers, so the generic type is int. To add an element, use the Add method.

Practice

1. What is the main feature of a List<T> in C#?

easy

A. It can only hold a fixed number of items.

B. It stores only unique items and does not allow duplicates.

C. It automatically sorts items when added.

D. It stores items in order and allows easy access by position.

Solution

1. Step 1: Understand List<T> behavior

   A List<T> stores items in the order they are added and allows access by index.

2. Step 2: Compare options with List<T> features

   Only It stores items in order and allows easy access by position. correctly describes this behavior; others describe different collection types or incorrect features.

3. Final Answer:

   It stores items in order and allows easy access by position. -> Option D

4. Quick Check:

   List<T> = ordered, indexed collection [OK]

Hint: Remember List<T> keeps order and supports indexing [OK]

Common Mistakes:

• Thinking List<T> enforces uniqueness
• Assuming List<T> auto-sorts items
• Believing List<T> has fixed size

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

easy

A. List<int> numbers = new List<int>();

B. List numbers = new List<int>();

C. List<int> numbers = List<int>();

D. List<int> numbers = new List();

Solution

1. Step 1: Recall correct List<T> syntax

   In C#, to declare a generic List, you must specify the type and use the new keyword with constructor.

2. Step 2: Check each option for syntax correctness

   List<int> numbers = new List<int>(); correctly declares and initializes a List of int. Others miss type, constructor, or use wrong syntax.

3. Final Answer:

   List<int> numbers = new List<int>(); -> Option A

4. Quick Check:

   Generic List declaration = new List<T>() [OK]

Hint: Use new List<T>() with type specified [OK]

Common Mistakes:

• Omitting new keyword
• Not specifying generic type in constructor
• Using non-generic List without type

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

var fruits = new List<string> { "apple", "banana", "cherry" };
fruits.RemoveAt(1);
Console.WriteLine(fruits[1]);

medium

A. banana

B. IndexOutOfRangeException

C. cherry

D. apple

Solution

1. Step 1: Understand RemoveAt effect on list

   RemoveAt(1) removes the item at index 1, which is "banana". The list becomes ["apple", "cherry"].

2. Step 2: Access the item at index 1 after removal

   After removal, fruits[1] is "cherry" because the list shifted left.

3. Final Answer:

   cherry -> Option C

4. Quick Check:

   RemoveAt shifts items left, fruits[1] = cherry [OK]

Hint: RemoveAt shifts list left; index 1 now points to next item [OK]

Common Mistakes:

• Assuming removed item still exists
• Expecting original index items unchanged
• Confusing RemoveAt with Remove

4. Identify the error in this C# code snippet using List<string>:

List<string> colors = new List<string>();
colors.Add("red");
colors[1] = "blue";
Console.WriteLine(colors[1]);

medium

A. IndexOutOfRangeException because index 1 does not exist yet.

B. Syntax error in Add method usage.

C. Cannot assign string to List<string> element.

D. No error; code runs and prints 'blue'.

Solution

1. Step 1: Analyze list content after Add

   After colors.Add("red"), list has one element at index 0 only.

2. Step 2: Check assignment to colors[1]

   colors[1] does not exist yet, so assigning to it causes IndexOutOfRangeException.

3. Final Answer:

   IndexOutOfRangeException because index 1 does not exist yet. -> Option A

4. Quick Check:

   Assigning to non-existing index throws exception [OK]

Hint: List index must exist before assignment; use Add to add items [OK]

Common Mistakes:

• Trying to assign to index without adding
• Confusing Add and index assignment
• Expecting automatic list expansion

5. Given a List<int> named numbers containing {1, 2, 3, 4, 5}, which code snippet correctly doubles each number in the list?

hard

A. numbers = numbers.Select(n => n * 2).ToList();

B. for (int i = 0; i < numbers.Count; i++) { numbers[i] = numbers[i] * 2; }

C. foreach (int n in numbers) { n = n * 2; }

D. numbers.ForEach(n => n = n * 2);

Solution

1. Step 1: Understand how to modify List elements

   Using a for loop with index allows modifying elements directly by assignment.

2. Step 2: Evaluate each option's effect

   for (int i = 0; i < numbers.Count; i++) { numbers[i] = numbers[i] * 2; } modifies elements in place. foreach (int n in numbers) { n = n * 2; } modifies copy of elements (no effect). numbers = numbers.Select(n => n * 2).ToList(); creates a new list but requires LINQ and ToList(). numbers.ForEach(n => n = n * 2); modifies copies in ForEach (no effect).

3. Final Answer:

   for (int i = 0; i < numbers.Count; i++) { numbers[i] = numbers[i] * 2; } -> Option B

4. Quick Check:

   Use for loop with index to update List elements [OK]

Hint: Use for loop with index to update List items directly [OK]

Common Mistakes:

• Using foreach expecting to modify list items
• Using ForEach with lambda that doesn't assign back
• Not creating new list when using LINQ Select