Dictionary methods and access patterns in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new key-value pair to the dictionary.

C Sharp (C#)

var dict = new Dictionary<string, int>();
dict.[1]("apple", 5);

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

AAdd

BInsert

CPut

DSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using Insert or Set which are not valid Dictionary methods.

Trying to assign directly without using a method.

The Add method is used to add a new key-value pair to a Dictionary in C#.

2fill in blank

medium

Complete the code to check if the dictionary contains the key "banana".

C Sharp (C#)

var dict = new Dictionary<string, int> { {"banana", 3} };
bool hasKey = dict.[1]("banana");

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

AContainsKey

BContainsValue

CHasKey

DExists

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsValue which checks for values, not keys.

Using non-existent methods like HasKey or Exists.

ContainsKey checks if a specific key exists in the dictionary.

3fill in blank

hard

Fix the error in the code to safely get the value for key "orange".

C Sharp (C#)

var dict = new Dictionary<string, int> { {"orange", 7} };
int value;
if (dict.[1]("orange", out value)) {
    Console.WriteLine(value);
}

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

AGetOrDefault

BGetValue

CContainsKey

DTryGetValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsKey which only checks existence but does not get the value.

Using non-existent methods like GetValue or GetOrDefault.

TryGetValue tries to get the value for a key and returns true if found.

4fill in blank

hard

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

C Sharp (C#)

var words = new List<string> { "cat", "house", "dog", "elephant" };
var lengths = words.Where(word => word.[1] > 3)
                   .ToDictionary(word => word, word => word.[2]);

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

ALength

BCount

CSize

DLength()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length() which is invalid because Length is a property, not a method.

Using Count or Size which do not exist for strings.

The Length property gives the number of characters in a string in C#.

5fill in blank

hard

Fill all three blanks to create a dictionary from a list of tuples filtering only those with value greater than 10.

C Sharp (C#)

var data = new List<(string, int)> { ("a", 5), ("b", 15), ("c", 20) };
var filtered = data.Where(item => item.[1] > 10)
                   .ToDictionary(item => item.[2], item => item.[3]);

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

AItem2

BItem1

DValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Value which is not a tuple property.

Mixing up Item1 and Item2 positions.

Tuples use Item1 for the first element and Item2 for the second element in C#.

Practice

1. What does the ContainsKey method do in a C# Dictionary?

easy

A. Checks if a specific key exists in the dictionary

B. Adds a new key-value pair to the dictionary

C. Removes a key and its value from the dictionary

D. Returns the number of items in the dictionary

Solution

1. Step 1: Understand the purpose of ContainsKey

   The ContainsKey method checks if a given key is present in the dictionary.

2. Step 2: Compare with other dictionary methods

   Add adds items, Remove deletes items, and Count returns the number of items, so these are different from ContainsKey.

3. Final Answer:

   Checks if a specific key exists in the dictionary -> Option A

4. Quick Check:

   ContainsKey checks key presence [OK]

Hint: ContainsKey checks if key exists before access [OK]

Common Mistakes:

• Confusing ContainsKey with Add method
• Thinking ContainsKey returns a value instead of a boolean
• Mixing ContainsKey with Count property

2. Which of the following is the correct way to add a key-value pair to a Dictionary<string, int> named ages?

easy

A. ages.Add("Alice", 30);

B. ages.Add["Alice"] = 30;

C. ages["Alice"].Add(30);

D. ages.Insert("Alice", 30);

Solution

1. Step 1: Recall the syntax for adding items to Dictionary

   The correct method to add a key-value pair is Add(key, value).

2. Step 2: Check each option's syntax

   ages.Add("Alice", 30); uses Add("Alice", 30) which is correct. ages.Add["Alice"] = 30; uses square brackets with Add which is invalid. ages["Alice"].Add(30); tries to call Add on the value, which is wrong. ages.Insert("Alice", 30); uses Insert which does not exist for Dictionary.

3. Final Answer:

   ages.Add("Alice", 30); -> Option A

4. Quick Check:

   Add(key, value) syntax [OK]

Hint: Use Add(key, value) to insert new pairs [OK]

Common Mistakes:

• Using square brackets with Add method
• Trying to call Add on a value instead of dictionary
• Using Insert method which does not exist

3. What will be the output of this code?

var dict = new Dictionary<string, int>();
dict.Add("x", 10);
dict["y"] = 20;
Console.WriteLine(dict["x"] + dict["y"]);

medium

A. 10 20

B. 30

C. x y

D. Runtime error

Solution

1. Step 1: Understand dictionary additions

   First, dict.Add("x", 10) adds key "x" with value 10. Then dict["y"] = 20 adds key "y" with value 20.

2. Step 2: Calculate the sum printed

   dict["x"] is 10 and dict["y"] is 20, so their sum is 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   10 + 20 = 30 [OK]

Hint: Sum values accessed by keys with dict[key] syntax [OK]

Common Mistakes:

• Expecting output as separate values instead of sum
• Confusing keys and values in output
• Thinking dict["y"] is invalid without Add

4. Identify the error in this code snippet:

var dict = new Dictionary<string, int>();
dict.Add("a", 1);
dict.Add("a", 2);
Console.WriteLine(dict["a"]);

medium

A. Key "a" will be removed automatically

B. Compilation error due to missing semicolon

C. Duplicate key exception on second Add call

D. Output will be 2 without error

Solution

1. Step 1: Understand Add method behavior with duplicate keys

   The Add method throws an exception if the key already exists.

2. Step 2: Analyze the code flow

   The first Add("a", 1) works fine. The second Add("a", 2) tries to add the same key again, causing an exception.

3. Final Answer:

   Duplicate key exception on second Add call -> Option C

4. Quick Check:

   Adding duplicate key throws exception [OK]

Hint: Add throws error if key exists; use indexer to overwrite [OK]

Common Mistakes:

• Assuming Add overwrites existing keys
• Expecting no error and value updated
• Confusing Add with indexer assignment

5. Given a dictionary scores with student names as keys and their scores as values, which code snippet safely retrieves the score for "John" without causing an error if the key is missing?

hard

A. int score = scores.ContainsKey("John");

B. int score = scores.GetValueOrDefault("John");

C. int score = scores["John"];

D. scores.TryGetValue("John", out int score);

Solution

1. Step 1: Understand safe retrieval methods

   Using TryGetValue safely tries to get the value and returns false if key is missing without error.

2. Step 2: Analyze each option

   int score = scores["John"]; throws an exception if "John" is missing. int score = scores.GetValueOrDefault("John"); is invalid in C# Dictionary (GetValueOrDefault is not standard). scores.TryGetValue("John", out int score); uses TryGetValue correctly. int score = scores.ContainsKey("John"); returns a boolean, not the score.

3. Final Answer:

   scores.TryGetValue("John", out int score); -> Option D

4. Quick Check:

   TryGetValue safely gets value [OK]

Hint: Use TryGetValue to avoid errors on missing keys [OK]

Common Mistakes:

• Using indexer without checking key existence
• Confusing ContainsKey with value retrieval
• Expecting GetValueOrDefault method on Dictionary