Hash Table Concept and Hash Functions in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a hash table?

A hash table is a data structure that stores data in key-value pairs. It uses a hash function to convert keys into an index where values are stored, allowing fast data lookup.

beginner

What does a hash function do in a hash table?

A hash function takes a key and converts it into an integer index. This index tells where to store or find the value in the hash table.

intermediate

Why do hash tables sometimes have collisions?

Collisions happen when two different keys produce the same index from the hash function. The hash table must handle collisions to store both values correctly.

intermediate

Name two common ways to handle collisions in hash tables.

1. Chaining: Store multiple values in a list at the same index.
2. Open addressing: Find another empty spot in the table using a probing method.

intermediate

What qualities make a good hash function?

A good hash function distributes keys evenly across the table, is fast to compute, and minimizes collisions.

What does a hash table store?

A Only values

B Only keys

C Key-value pairs

D Sorted list of values

Hash tables store data as key-value pairs for fast lookup.

What is the main purpose of a hash function?

A Sort the keys

B Remove duplicates

C Store values directly

D Convert keys to indexes

Hash functions convert keys into indexes to find where values are stored.

What is a collision in a hash table?

A Two keys map to the same index

B A key is missing

C Value is duplicated

D Table is empty

Collision happens when different keys produce the same index.

Which method stores multiple values at the same index to handle collisions?

A Hashing

B Chaining

C Sorting

D Open addressing

Chaining uses a list at each index to store multiple values.

Which is NOT a quality of a good hash function?

A Always returns zero

B Fast computation

C Minimizes collisions

D Even distribution

Always returning zero causes all keys to collide, which is bad.

Explain how a hash table uses a hash function to store and retrieve data.

Think about how a key becomes a place to find the value.

Describe what a collision is in a hash table and two ways to handle it.

Imagine two friends trying to sit in the same chair.

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