Why Hash Map Exists and What Problem It Solves in DSA C# - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

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

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

AContainsKey

BRemove

CClear

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove instead of Add will delete items.

Clear empties the whole dictionary.

ContainsKey only checks if a key exists.

The Add method inserts a new key-value pair into the dictionary.

2fill in blank

medium

Complete the code to check if a key exists in the dictionary.

DSA C#

if (map.[1]("apple")) {
    Console.WriteLine("Found apple!");
}

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

AContainsKey

BClear

CRemove

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add will cause an error if the key exists.

Remove deletes keys, not checks them.

Clear empties the dictionary.

ContainsKey checks if the dictionary has the given key.

3fill in blank

hard

Fix the error in accessing a value by key.

DSA C#

int value = map[[1]];

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

A"apple"

B5

Cmap

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number instead of a string key.

Using the dictionary variable itself as a key.

Using null as a key.

Keys in the dictionary are strings, so use the string "apple" to access the value.

4fill in blank

hard

Fill both blanks to create a dictionary with initial values and check a key.

DSA C#

var map[1] new Dictionary<string, int> { {"apple", 5}, {"banana", 3} };
bool hasBanana = map.[2]("banana");

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

A=

BContainsKey

CAdd

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Add' instead of 'ContainsKey' for checking keys.

Using '==' instead of '=' for assignment.

Confusing method names.

Use = to assign the dictionary with initial values, and ContainsKey to check if "banana" exists.

5fill in blank

hard

Fill all three blanks to create a dictionary, add a new item, and retrieve a value.

DSA C#

var map = new Dictionary<string, int>();
map.[1]("orange", 7);
int orangeCount = map[[2]];
Console.WriteLine(orangeCount [3] 0);

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

AAdd

B"orange"

C>

DContainsKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsKey instead of Add to insert.

Accessing with wrong key type.

Using wrong comparison operator.

Use Add to insert "orange" with value 7, access it by key "orange", and check if the count is greater than 0.

Practice

1. Why do we use a hash map in programming?
HashMap<K, V> stores key-value pairs. What problem does it solve?

easy

A. It stores data in a fixed-size array only.

B. It sorts data automatically in ascending order.

C. It compresses data to save memory.

D. It allows fast lookup of values by keys, avoiding slow searches.

Solution

1. Step 1: Understand the problem hash maps solve

   Hash maps provide a way to find values quickly using keys without searching all data.

2. Step 2: Compare with other data structures

   Unlike lists or arrays, hash maps avoid slow linear searches by using a hash function to jump directly to data.

3. Final Answer:

   It allows fast lookup of values by keys, avoiding slow searches. -> Option D

4. Quick Check:

   Fast key-based lookup = C [OK]

Hint: Hash maps speed up key-based searches [OK]

Common Mistakes:

• Thinking hash maps sort data automatically
• Confusing hash maps with arrays
• Believing hash maps compress data

2. Which of the following is the correct way to declare a hash map in C# that maps strings to integers?

easy

A. HashMap map = new HashMap();

B. Dictionary map = new Dictionary();

C. Map map = new Map();

D. Hashtable map = new Hashtable();

Solution

1. Step 1: Identify the correct C# collection type for hash maps

   In C#, the standard hash map is called Dictionary<K, V>.

2. Step 2: Check syntax correctness

   Dictionary map = new Dictionary(); uses Dictionary with correct generic syntax and constructor call.

3. Final Answer:

   Dictionary map = new Dictionary(); -> Option B

4. Quick Check:

   C# hash map = Dictionary [OK]

Hint: Use Dictionary for hash maps in C# [OK]

Common Mistakes:

• Using Java's HashMap class name in C#
• Using Map which is not a C# type
• Trying to use Hashtable with generics

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

var map = new Dictionary<string, int>();
map["apple"] = 5;
map["banana"] = 3;
map["apple"] = 7;
Console.WriteLine(map["apple"]);

medium

A. 7

B. 5

C. 3

D. KeyNotFoundException

Solution

1. Step 1: Understand dictionary key assignment behavior

   Assigning a value to an existing key updates the value, it does not add a new entry.

2. Step 2: Trace the code assignments

   "apple" is first set to 5, then updated to 7. So map["apple"] is 7 at print time.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   Updated value for "apple" = 7 [OK]

Hint: Last assignment to a key wins [OK]

Common Mistakes:

• Thinking dictionary stores multiple values per key
• Expecting original value after update
• Confusing key existence with error

4. The following code throws a runtime error. What is the problem?

var map = new Dictionary<string, int>();
Console.WriteLine(map["key"]);

medium

A. The dictionary is empty; accessing a missing key causes an exception.

B. The dictionary keys must be integers, not strings.

C. The dictionary must be initialized with values before use.

D. The syntax for dictionary declaration is incorrect.

Solution

1. Step 1: Identify the cause of runtime error

   Accessing a key that does not exist in a Dictionary throws a KeyNotFoundException.

2. Step 2: Check dictionary state before access

   The dictionary is empty, so "key" is missing, causing the error.

3. Final Answer:

   The dictionary is empty; accessing a missing key causes an exception. -> Option A

4. Quick Check:

   Missing key access throws error [OK]

Hint: Check key exists before accessing dictionary [OK]

Common Mistakes:

• Assuming dictionary auto-creates missing keys
• Confusing key type requirements
• Thinking initialization fixes missing keys

5. You want to count how many times each word appears in a list of strings using a hash map in C#. Which approach correctly solves this problem?

hard

A. Use a Dictionary with word lengths as keys.

B. Use a List<string> and call Contains to count occurrences.

C. Use a Dictionary, loop over words, increment count for each word key.

D. Use an array of strings and sort it to count duplicates.

Solution

1. Step 1: Understand the counting problem

   We need to track how many times each unique word appears, so keys are words and values are counts.

2. Step 2: Choose data structure and method

   A Dictionary allows fast lookup and update of counts per word during iteration.

3. Final Answer:

   Use a Dictionary, loop over words, increment count for each word key. -> Option C

4. Quick Check:

   Word count needs Dictionary with word keys [OK]

Hint: Use Dictionary to count frequencies by keys [OK]

Common Mistakes:

• Using list Contains is slow for counting
• Using word length as key loses word info
• Sorting array is inefficient for counting