Hash Table Concept and Hash Functions in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this hash function simulation?

Consider a simple hash function that sums the character codes of a string and then takes modulo 10. What is the output of the hash function for the key 'abc'?

DSA Javascript

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

console.log(simpleHash('abc'));

A 2

B 4

C 6

D 8

💡 Hint

Add the ASCII codes of 'a', 'b', and 'c' and then take modulo 10.

The ASCII codes are: 'a' = 97, 'b' = 98, 'c' = 99. Sum is 97 + 98 + 99 = 294. 294 % 10 = 4, which is option B.

❓ Predict Output

intermediate

What is the output of this hash table insertion simulation?

Given a hash table of size 5 and a hash function that returns key % 5, what is the final state of the table after inserting keys 1, 6, and 11 using linear probing?

DSA Javascript

const size = 5;
const table = new Array(size).fill(null);
function hash(key) {
  return key % size;
}
function insert(key) {
  let idx = hash(key);
  while (table[idx] !== null) {
    idx = (idx + 1) % size;
  }
  table[idx] = key;
}
insert(1);
insert(6);
insert(11);
console.log(table);

A [null, 1, 6, 11, null]

B [1, null, null, 6, 11]

C [1, null, null, null, 6]

D [1, 6, 11, null, null]

💡 Hint

Check the hash index for each key and apply linear probing if collision occurs.

Key 1 hashes to 1 % 5 = 1, placed at index 1. Key 6 hashes to 6 % 5 = 1, collision at index 1, linear probe to index 2. Key 11 hashes to 11 % 5 = 1, collision at 1 and 2, linear probe to index 3.

🧠 Conceptual

advanced

Which property is NOT desirable in a good hash function?

Select the property that is NOT desirable when designing a hash function for a hash table.

A High computational complexity to ensure security

B Deterministic output for the same input

C Uniform distribution of keys across the table

D Minimizing collisions between different keys

💡 Hint

Think about efficiency and speed in hash functions.

A good hash function should be fast and efficient, so high computational complexity is undesirable.

🔧 Debug

advanced

What error does this hash table insertion code produce?

Identify the error when inserting keys into a fixed-size hash table using the given code.

DSA Javascript

const size = 3;
const table = new Array(size).fill(null);
function hash(key) {
  return key % size;
}
function insert(key) {
  let idx = hash(key);
  let startIdx = idx;
  while (table[idx] !== null) {
    idx = (idx + 1) % size;
    if (idx === startIdx) {
      throw new Error('Hash table is full');
    }
  }
  table[idx] = key;
}
insert(1);
insert(4);
insert(7);
try {
  insert(10);
} catch (e) {
  console.error(e.message);
}
console.log(table);

A TypeError: Cannot read property of undefined

B No error, output: [1, 4, 7]

C Infinite loop error

D RangeError: Maximum call stack size exceeded

💡 Hint

Consider what happens when the table is full and you try to insert another key.

The table size is 3, but 4 keys are inserted. The insert function loops infinitely trying to find an empty slot.

🚀 Application

expert

How many unique keys can be stored without collision in a hash table of size 7 using this hash function?

Given a hash function h(key) = key % 7 and a hash table of size 7, what is the maximum number of unique integer keys that can be inserted without any collision?

A 6

B 1

C 14

D 7

💡 Hint

Think about the pigeonhole principle and the size of the table.

Since the table size is 7 and the hash function maps keys to 7 slots, at most 7 unique keys can be inserted without collision.

Practice

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

easy

A. To convert a key into an index for storing data

B. To sort the keys in alphabetical order

C. To encrypt the data stored in the table

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 that can be used as an index in the hash table.

2. Step 2: Identify the correct purpose

   Sorting keys or encrypting data is not the job of a hash function; it only maps keys to indexes.

3. Final Answer:

   To convert a key into an index for storing data -> Option A

4. Quick Check:

   Hash function = key to index [OK]

Hint: Hash functions map keys to indexes, not sort or encrypt [OK]

Common Mistakes:

• Thinking hash functions sort keys
• Confusing hash functions with encryption
• Believing hash functions count elements

2. Which of the following is the correct way to create an empty hash table (object) in JavaScript?

easy

A. const table = [];

B. const table = {};

C. const table = new Map();

D. const table = new HashTable();

Solution

1. Step 1: Recall JavaScript object syntax

   In JavaScript, an empty object is created using curly braces {} which can act as a hash table.

2. Step 2: Evaluate options

   const table = []; creates an array, not a hash table. const table = new Map(); creates a Map, which is a different structure. const table = new HashTable(); is invalid as HashTable is not a built-in class.

3. Final Answer:

   const table = {}; -> Option B

4. Quick Check:

   Empty hash table = {} [OK]

Hint: Use curly braces {} to create an empty object hash table [OK]

Common Mistakes:

• Using [] which creates an array, not a hash table
• Using Map() which is different from object
• Assuming HashTable is a built-in JavaScript class

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

const hashTable = {};
hashTable['apple'] = 5;
hashTable['banana'] = 3;
hashTable['apple'] = 10;
console.log(hashTable['apple']);

medium

A. 10

B. 3

C. 5

D. undefined

Solution

1. Step 1: Understand key assignment in hash tables

   Assigning a value to an existing key updates the value stored at that key.

2. Step 2: Trace the code

   Initially, 'apple' is set to 5, then updated to 10. So, hashTable['apple'] will be 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Last assignment to 'apple' = 10 [OK]

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

Common Mistakes:

• Thinking the first assigned value remains
• Confusing keys and values
• Expecting undefined for existing keys

4. Identify the error in the following code snippet that tries to use a hash function:

function hash(key) {
  return key.length % 10;
}
const table = {};
table[hash('cat')] = 'animal';
table[hash('dog')] = 'animal';
console.log(table['cat']);

medium

A. Using key.length causes runtime error

B. Cannot use numbers as keys in JavaScript objects

C. Accessing table['cat'] is incorrect; should use table[hash('cat')]

D. Hash function should return a string, not a number

Solution

1. Step 1: Understand how keys are stored and accessed

   The hash function returns a number used as a key in the table object.

2. Step 2: Check how the table is accessed

   The code tries to access table['cat'], but the key stored is a number from hash('cat'), so it should access table[hash('cat')].

3. Final Answer:

   Accessing table['cat'] is incorrect; should use table[hash('cat')] -> Option C

4. Quick Check:

   Access key must match stored key type [OK]

Hint: Access hash table with hashed key, not original key [OK]

Common Mistakes:

• Accessing with original key instead of hashed key
• Thinking keys must be strings only
• Assuming hash function causes error due to key.length

5. Given the hash function below, what is the best way to handle collisions when two keys produce the same hash?

function simpleHash(key) {
  let sum = 0;
  for (let char of key) {
    sum += char.charCodeAt(0);
  }
  return sum % 5;
}

Choose the best collision handling method.

hard

A. Increase the hash size to avoid collisions

B. Overwrite the existing value with the new one

C. Ignore the new key and keep the old value

D. Use chaining by storing multiple values in a list at the same index

Solution

1. Step 1: Understand collision in hash tables

   When two keys produce the same hash index, a collision occurs and must be handled to store both values.

2. Step 2: Evaluate collision handling methods

   Overwriting loses data, ignoring new keys is incorrect, increasing hash size alone doesn't solve collisions. Chaining stores multiple values in a list at the same index, preserving all data.

3. Final Answer:

   Use chaining by storing multiple values in a list at the same index -> Option D

4. Quick Check:

   Chaining handles collisions by lists [OK]

Hint: Use lists (chaining) to store multiple values at one index [OK]

Common Mistakes:

• Overwriting values causing data loss
• Ignoring new keys on collision
• Thinking increasing hash size removes all collisions