Hash Table Concept and Hash Functions in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a hash table using C# Dictionary.

DSA C#

Dictionary<int, string> hashTable = new Dictionary<int, [1]>();

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

Astring

Bbool

Cint

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using int as value type instead of string.

Using bool or double which are unrelated to the stored data.

The Dictionary key is int and the value type is string, so we use Dictionary.

2fill in blank

medium

Complete the code to add a key-value pair to the hash table.

DSA C#

hashTable.[1](5, "apple");

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AAdd

BRemove

CClear

DContainsKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove instead of Add.

Using ContainsKey which only checks for keys.

To add a new key-value pair, use the Add method.

3fill in blank

hard

Fix the error in the code to retrieve a value by key safely.

DSA C#

if (hashTable.[1](3, out string value)) {
    Console.WriteLine(value);
} else {
    Console.WriteLine("Key not found");
}

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

AContainsKey

BTryGetValue

CRemove

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using ContainsKey which does not output the value.

Using Add or Remove which modify the dictionary.

TryGetValue tries to get the value for a key safely without throwing an error if the key is missing.

4fill in blank

hard

Fill both blanks to compute a simple hash index for a key in an array of size 10.

DSA C#

int hashIndex = key [1] 10 [2] 0;

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A%

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication or subtraction instead of modulo.

Omitting the addition operator.

The modulo operator (%) gives the remainder to fit the key into array size. Adding zero (+) keeps the value unchanged.

5fill in blank

hard

Fill all three blanks to create a hash function that sums character codes and uses modulo 16.

DSA C#

int HashFunction(string key) {
    int sum = 0;
    foreach (char c in key) {
        sum [1]= (int)c;
    }
    return sum [2] [3];
}

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

A+

B%

C16

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' to add character codes.

Using wrong modulo value or operator.

We add each character code to sum, then use modulo 16 to keep the hash within 0-15.

Practice

1. What is the main purpose of a hash function in a hash table?

easy

A. To encrypt the data stored in the table

B. To sort the keys in ascending order

C. To convert a key into an index for storing values

D. To count the number of elements in the table

Solution

1. Step 1: Understand the role of a hash function

   A hash function takes a key and converts it into a number called an index.

2. Step 2: Connect index to storage in hash table

   This index tells where to store or find the value in the hash table.

3. Final Answer:

   To convert a key into an index for storing values -> Option C

4. Quick Check:

   Hash function = key to index [OK]

Hint: Hash function maps keys to storage spots [OK]

Common Mistakes:

• Thinking hash function sorts keys
• Confusing hash function with encryption
• Believing hash function counts elements

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

easy

A. var table = new Hashtable();

B. var table = new HashTable();

C. var table = new hash_table();

D. var table = new hashTable();

Solution

1. Step 1: Recall correct class name in C#

   The correct class name is 'Hashtable' with capital H and T.

2. Step 2: Check syntax for creating instance

   Using 'new Hashtable()' with correct casing creates a hash table object.

3. Final Answer:

   var table = new Hashtable(); -> Option A

4. Quick Check:

   Correct class name and syntax [OK]

Hint: Remember C# is case-sensitive; use Hashtable exactly [OK]

Common Mistakes:

• Using wrong capitalization
• Adding underscores in class name
• Using lowercase letters incorrectly

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

var table = new Hashtable();
table["apple"] = 5;
table["banana"] = 3;
table["apple"] = 7;
Console.WriteLine(table["apple"]);

medium

A. 7

B. 3

C. Error

D. 5

Solution

1. Step 1: Understand key-value assignment in Hashtable

   Assigning table["apple"] = 5 stores 5 at key "apple".

2. Step 2: Overwriting value for the same key

   Later, table["apple"] = 7 replaces the old value 5 with 7.

3. Step 3: Print the current value for key "apple"

   Console.WriteLine outputs the latest value 7.

4. Final Answer:

   7 -> Option A

5. Quick Check:

   Last assigned value for key = output [OK]

Hint: Last value assigned to key is stored [OK]

Common Mistakes:

• Thinking values add up
• Expecting first value to print
• Assuming error on duplicate key

4. Identify the error in this C# code snippet using Hashtable:

var table = new Hashtable();
table.Add("key1", 10);
table.Add("key1", 20);
Console.WriteLine(table["key1"]);

medium

A. No error; output is 20

B. Runtime error due to duplicate key

C. Compile-time error due to Add method

D. Output is 10

Solution

1. Step 1: Understand Add method behavior

   Add throws an error if the key already exists in the Hashtable.

2. Step 2: Check duplicate key usage

   Adding "key1" twice causes a runtime exception on second Add.

3. Final Answer:

   Runtime error due to duplicate key -> Option B

4. Quick Check:

   Add fails on duplicate keys [OK]

Hint: Add method errors on duplicate keys; use indexer to overwrite [OK]

Common Mistakes:

• Assuming Add overwrites existing keys
• Expecting compile-time error
• Thinking output prints without error

5. Given a hash table with a simple hash function: hash(key) = key % 5, which of the following keys will cause a collision if inserted in order: 12, 8, 9, 22?

hard

A. 12 and 8

B. 8 and 9

C. 9 and 22

D. 12 and 22

Solution

1. Step 1: Calculate hash for each key

   12 % 5 = 2, 8 % 5 = 3, 9 % 5 = 4, 22 % 5 = 2.

2. Step 2: Identify keys with same hash index

   12 and 22 both hash to index 2. Others have unique indices.

3. Step 3: Check insertion order

   Insert 12 (index 2), 8 (3) no collision, 9 (4) no collision, 22 (2) causes collision with 12.

4. Final Answer:

   12 and 22 -> Option D

5. Quick Check:

   Keys with same hash cause collision [OK]

Hint: Calculate key % 5 to find collisions [OK]

Common Mistakes:

• Calculating wrong modulo
• Ignoring insertion order
• Assuming different hashes for close numbers