Hash Table Concept and Hash Functions in DSA Typescript - Build from Scratch

Hash Table Concept and Hash Functions

📖 Scenario: Imagine you are organizing a small library. You want to quickly find books by their titles. To do this, you decide to use a simple hash table where each book title is linked to its shelf number.

🎯 Goal: You will create a hash table using a TypeScript object, write a simple hash function to convert book titles into shelf numbers, and then store and retrieve books using this hash function.

📋 What You'll Learn

Create a hash table as a TypeScript object with no initial entries.

Create a hash function called simpleHash that takes a string and returns a number.

Use the simpleHash function to assign shelf numbers to book titles.

Store book titles and their shelf numbers in the hash table.

Print the hash table to show the stored books and their shelf numbers.

💡 Why This Matters

🌍 Real World

Hash tables are used in libraries, databases, and many apps to quickly find information by keys like book titles or user names.

💼 Career

Understanding hash tables and hash functions is important for software developers to build fast and efficient data storage and retrieval systems.

1

Create an empty hash table

Create an empty object called hashTable to store book titles and their shelf numbers.

DSA Typescript

# Your code here
// Create an empty object called hashTable

Hint

Use const hashTable: { [key: string]: number } = {}; to create an empty object.

2

Write a simple hash function

Create a function called simpleHash that takes a string key and returns a number. The function should add the Unicode values of all characters in key and return the sum modulo 10.

DSA Typescript

const hashTable: { [key: string]: number } = {};
// Write the simpleHash function below
// It sums char codes of key and returns sum % 10
# Your code here

Hint

Use a for loop to add character codes and return the sum modulo 10.

3

Store books using the hash function

Use the simpleHash function to assign shelf numbers to these book titles: 'Math', 'Science', and 'History'. Store them in hashTable with the shelf number as the value and the book title as the key.

DSA Typescript

const hashTable: { [key: string]: number } = {};

function simpleHash(key: string): number {
    let sum = 0;
    for (let i = 0; i < key.length; i++) {
        sum += key.charCodeAt(i);
    }
    return sum % 10;
}

// Store 'Math', 'Science', and 'History' in hashTable using simpleHash
# Your code here

Hint

Call simpleHash with each book title and assign the result to hashTable[title].

4

Print the hash table

Print the hashTable object to show the stored book titles and their shelf numbers.

DSA Typescript

const hashTable: { [key: string]: number } = {};

function simpleHash(key: string): number {
    let sum = 0;
    for (let i = 0; i < key.length; i++) {
        sum += key.charCodeAt(i);
    }
    return sum % 10;
}

hashTable['Math'] = simpleHash('Math');
hashTable['Science'] = simpleHash('Science');
hashTable['History'] = simpleHash('History');

// Print the hashTable below
# Your code here

Hint

Use console.log(hashTable); to print the object.

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