Why Hash Table Concept and Hash Functions in DSA Typescript?

The Big Idea

What if you could find anything instantly without searching through everything?

The Scenario

Imagine you have a huge phone book with thousands of names and numbers. You want to find a friend's number quickly, but you have to flip through every page one by one.

The Problem

Looking up a name manually takes a lot of time and effort. You might lose your place, make mistakes, or get tired flipping pages. It's slow and frustrating.

The Solution

A hash table uses a special math trick called a hash function to jump directly to the spot where your friend's number is stored. No flipping through pages, just instant access!

Before vs After

✗ Before

function findNumber(name: string, phoneBook: Array<{name: string, number: string}>) {
  for (let i = 0; i < phoneBook.length; i++) {
    if (phoneBook[i].name === name) return phoneBook[i].number;
  }
  return null;
}

✓ After

class HashTable {
  private table: {[key: string]: string} = {};
  set(key: string, value: string) { this.table[key] = value; }
  get(key: string) { return this.table[key] ?? null; }
}

What It Enables

Hash tables let you find, add, or remove data instantly, even in huge collections.

Real Life Example

When you search for a contact on your phone, it uses a hash table behind the scenes to find the number instantly.

Key Takeaways

Manual search is slow and error-prone.

Hash functions map keys to exact spots quickly.

Hash tables make data access fast and efficient.

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