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

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

const hashTable = [1];

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

A[]

Bnull

C{}

Dnew Map()

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty array [] instead of an object.

Using null which is not a hash table.

Using new Map() which is a different data structure.

In JavaScript, an empty hash table can be created using an empty object literal {}.

2fill in blank

medium

Complete the code to add a key-value pair to the hash table.

DSA Javascript

hashTable[[1]] = 'apple';

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

Afruit

B'fruit'

C'apple'

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable without quotes which will be treated as a variable name.

Using the value 'apple' as the key.

Using an undefined variable like key.

Keys in JavaScript objects must be strings or symbols. Here, the key is the string 'fruit'.

3fill in blank

hard

Fix the error in the hash function to return a valid index within the array size.

DSA Javascript

function hash(key, size) {
  let total = 0;
  for (let char of key) {
    total += char.charCodeAt(0);
  }
  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 division (/) which returns a float, not an index.

Using multiplication (*) which can increase the number.

Using subtraction (-) which can produce negative numbers.

The modulo operator % ensures the hash value fits within the array size by returning the remainder.

4fill in blank

hard

Fill both blanks to complete the function that checks if a key exists in the hash table.

DSA Javascript

function hasKey(hashTable, key) {
  return [1] in hashTable && hashTable[[2]] !== undefined;
}

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

Akey

Bvalue

ChashTable

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names that are not defined.

Checking for value instead of key presence.

We check if the key exists in the hashTable and its value is not undefined.

5fill in blank

hard

Fill all three blanks to complete the function that deletes a key from the hash table if it exists.

DSA Javascript

function deleteKey(hashTable, key) {
  if ([1] in [2]) {
    [3] hashTable[key];
    return true;
  }
  return false;
}

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

Akey

BhashTable

Cdelete

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' instead of 'delete' which is not a JavaScript operator.

Mixing up variable names in the if condition.

Not using the delete operator to remove the key.

The function checks if the key exists in the hashTable, then uses the delete operator to remove it.

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