Dictionary methods and access patterns in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Dictionary methods and access patterns

Create Dictionary

↓

Add Key-Value Pairs

↓

Access Value by Key

↓

Key Exists?

No→Handle Missing Key

↓

Use Value

↓

Use Dictionary Methods

↓

Update or Remove Entries

↓

End

Start by creating a dictionary and adding pairs. Access values by keys, checking if keys exist. Use dictionary methods to update or remove entries.

Execution Sample

C Sharp (C#)

var dict = new Dictionary<string, int>();
dict["apple"] = 3;
int count = dict["apple"];
bool hasKey = dict.ContainsKey("banana");

Create a dictionary, add an entry, access a value, and check if a key exists.

Execution Table

Step	Action	Dictionary State	Result/Output
1	Create empty dictionary	{}	Dictionary created, empty
2	Add key "apple" with value 3	{"apple":3}	Entry added
3	Access value for key "apple"	{"apple":3}	3
4	Check if key "banana" exists	{"apple":3}	False
5	Try to access value for key "banana" (not shown in code, but common pattern)	{"apple":3}	KeyNotFoundException if accessed directly
6	Use ContainsKey before access to avoid exception	{"apple":3}	Safe check returns False
7	End of operations	{"apple":3}	Execution stops

💡 No more operations; dictionary remains with one entry

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
dict	{}	{"apple":3}	{"apple":3}	{"apple":3}	{"apple":3}
count	undefined	undefined	3	3	3
hasKey	undefined	undefined	undefined	False	False

Key Moments - 2 Insights

Why do we check ContainsKey before accessing a key?

Because accessing a key that does not exist causes an exception. Checking ContainsKey (see step 4 and 6) prevents this error by confirming the key is present.

What happens if we try to access a key that is not in the dictionary?

The program throws a KeyNotFoundException (step 5). To avoid this, use ContainsKey or TryGetValue methods.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'count' after step 3?

A 3

B undefined

C 0

D KeyNotFoundException

💡 Hint

Check the 'count' variable in variable_tracker after step 3

At which step does the dictionary contain the key "apple"?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the dictionary state column in execution_table at step 2

If we try to access dict["banana"] without checking ContainsKey, what happens?

A Returns 0

B Returns null

C Throws KeyNotFoundException

D Adds "banana" with default value

💡 Hint

See step 5 in execution_table for what happens when accessing a missing key

Concept Snapshot

Dictionary methods and access patterns in C#:
- Create with: var dict = new Dictionary<TKey, TValue>();
- Add/update: dict[key] = value;
- Access: dict[key] (throws if key missing)
- Check key: dict.ContainsKey(key)
- Use TryGetValue for safe access
- Remove with dict.Remove(key)

Full Transcript

This example shows how to create a dictionary in C#, add a key-value pair, access a value by key, and check if a key exists. We start with an empty dictionary, add "apple" with value 3, then read that value into a variable. We check if "banana" exists, which it does not. Accessing a missing key directly causes an exception, so we use ContainsKey to avoid errors. Variables track the dictionary state and values step-by-step.

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