Hash Table Concept and Hash Functions in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty hash table using a JavaScript object.

DSA Typescript

const hashTable = [1];

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

A{}

B[]

Cnull

Dnew Map()

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty array [] instead of an object.

Using null which is not a data structure.

Using new Map() which is a different built-in structure.

In JavaScript/TypeScript, an empty object {} is commonly used to represent a hash table.

2fill in blank

medium

Complete the hash function to compute the index by taking the modulus of the key with the table size.

DSA Typescript

function hash(key: number, size: number): number {
  return key [1] size;
}

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

A+

B%

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or subtraction which can produce indexes outside the table size.

Using multiplication which can produce very large numbers.

The modulus operator % returns the remainder and is used to keep the index within the table size.

3fill in blank

hard

Fix the error in the hash function to correctly handle string keys by summing character codes before modulus.

DSA Typescript

function hash(key: string, size: number): number {
  let total = 0;
  for (let i = 0; i < key.length; i++) {
    total += key.charCodeAt(i);
  }
  return total [1] size;
}

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

A+

B*

C%

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or subtraction instead of modulus.

Multiplying which can cause overflow.

The modulus operator % ensures the hash value fits within the table size.

4fill in blank

hard

Fill both blanks to create a hash function that sums character codes and returns the modulus with size.

DSA Typescript

function hash(key: string, size: number): number {
  let sum = 0;
  for (let i = 0; i < key.length; i++) {
    sum [1]= key.charCodeAt(i);
  }
  return sum [2] size;
}

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

A+

B%

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition for sum.

Using addition instead of modulus for the final return.

The operator += adds each character code to sum, and % keeps the result within the table size.

5fill in blank

hard

Fill both blanks to create a hash function that converts a string key to a number index within the table size.

DSA Typescript

function hash(key: string, size: number): number {
  let hashValue = 0;
  for (let [1] = 0; [1] < key.length; [1]++) {
    hashValue += key.charCodeAt([1]);
  }
  return hashValue [2] size;
}

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

Ai

B%

Cj

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using addition instead of modulus for the final return.

Use i as the loop variable, % for modulus, and i to access characters.

Practice

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

easy

A. To sort the keys in ascending order

B. To count the number of elements in the table

C. To encrypt the stored data for security

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

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 D

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 (Map) in TypeScript?

easy

A. const map = new map();

B. const map = Map;

C. const map = new Map();

D. const map = new Map(); string, number;

Solution

1. Step 1: Recall TypeScript Map syntax

   In TypeScript, a Map is created with new Map().

2. Step 2: Check each option's syntax

   const map = new Map(); uses correct syntax: new Map(). Others have syntax errors or wrong usage.

3. Final Answer:

   const map = new Map(); -> Option C

4. Quick Check:

   Map declaration = new Map<Key, Value>() [OK]

Hint: Use new Map() to declare [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using lowercase 'map' instead of 'Map'
• Incorrect generic syntax

3. What will be the output of the following TypeScript code?

const map = new Map();
map.set('apple', 5);
map.set('banana', 3);
map.set('apple', 10);
console.log(map.get('apple'));

medium

A. 10

B. Error

C. undefined

D. 5

Solution

1. Step 1: Understand Map set behavior

   Setting a key that already exists updates its value.

2. Step 2: Trace the code

   First 'apple' is set to 5, then updated to 10. So map.get('apple') returns 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Updated value = 10 [OK]

Hint: Last set value for key is stored [OK]

Common Mistakes:

• Assuming first value stays
• Thinking get returns undefined if key repeated
• Confusing Map with object property behavior

4. Identify the error in the following TypeScript code using a hash table:

const map = new Map();
map.set(1, 'one');
map.set(2, 'two');
console.log(map.get('1'));

medium

A. Missing parentheses in map.get call

B. Using string '1' instead of number 1 as key

C. Map keys must be strings only

D. Cannot mix number and string values in Map

Solution

1. Step 1: Check Map key type

   Map is declared with number keys, so keys must be numbers.

2. Step 2: Analyze map.get argument

   map.get('1') uses string '1', which does not match number 1 key, so returns undefined.

3. Final Answer:

   Using string '1' instead of number 1 as key -> Option B

4. Quick Check:

   Key type mismatch causes undefined [OK]

Hint: Match key types exactly when using get() [OK]

Common Mistakes:

• Assuming '1' and 1 are same keys
• Thinking Map keys must be strings
• Ignoring type mismatch errors

5. You want to design a hash function for strings that minimizes collisions for English words. Which approach is best?

hard

A. Multiply each character code by a prime number and sum, then modulo table size

B. Use the length of the string as the hash value

C. Sum the ASCII codes of all characters and take modulo table size

D. Use the first character's ASCII code only

Solution

1. Step 1: Understand collision minimization

   Good hash functions spread keys evenly to reduce collisions.

2. Step 2: Evaluate each approach

   Sum the ASCII codes of all characters and take modulo table size sums codes but can cause many collisions (e.g., 'abc' and 'cab'). Use the length of the string as the hash value uses length only, very poor. Use the first character's ASCII code only uses first char only, also poor. Multiply each character code by a prime number and sum, then modulo table size uses weighted sum with primes, better distribution.

3. Final Answer:

   Multiply each character code by a prime number and sum, then modulo table size -> Option A

4. Quick Check:

   Weighted sum with primes = better hash [OK]

Hint: Use prime multipliers for better hash spread [OK]

Common Mistakes:

• Using simple sums causing collisions
• Ignoring character positions
• Using string length as hash