Hash map vs hash set in Data Structures Theory - Hands-On Comparison

Understanding Hash Map vs Hash Set

📖 Scenario: You are organizing a collection of items for a small library. You want to keep track of books and also maintain a list of unique genres available.

🎯 Goal: Build a simple data structure example to understand the difference between a hash map and a hash set by storing book titles with authors and a set of unique genres.

📋 What You'll Learn

Create a dictionary to store book titles as keys and authors as values.

Create a set to store unique genres.

Add at least three books with their authors to the dictionary.

Add at least three genres to the set, including duplicates to show uniqueness.

Demonstrate the difference between storing key-value pairs and unique items.

💡 Why This Matters

🌍 Real World

Libraries, inventory systems, and many applications use hash maps to store paired data like names and details, and hash sets to keep track of unique categories or tags.

💼 Career

Understanding hash maps and hash sets is fundamental for software developers, data analysts, and anyone working with data structures to efficiently store and retrieve information.

1

Create a dictionary for books

Create a dictionary called books with these exact entries: '1984': 'George Orwell', 'To Kill a Mockingbird': 'Harper Lee', and 'The Great Gatsby': 'F. Scott Fitzgerald'.

Data Structures Theory

# Create the books dictionary with titles and authors
# Your code here

Hint

Use curly braces {} to create a dictionary with keys as book titles and values as authors.

2

Create a set for genres

Create a set called genres with these exact values: 'Fiction', 'Classic', and 'Fiction' (include the duplicate to show uniqueness).

Data Structures Theory

books = {
    '1984': 'George Orwell',
    'To Kill a Mockingbird': 'Harper Lee',
    'The Great Gatsby': 'F. Scott Fitzgerald'
}
# Create the genres set with duplicates
# Your code here

Hint

Use curly braces {} to create a set. Sets automatically remove duplicates.

3

Add a new book and genre

Add a new entry to the books dictionary with key 'Brave New World' and value 'Aldous Huxley'. Then add the genre 'Dystopian' to the genres set.

Data Structures Theory

books = {
    '1984': 'George Orwell',
    'To Kill a Mockingbird': 'Harper Lee',
    'The Great Gatsby': 'F. Scott Fitzgerald'
}
genres = {'Fiction', 'Classic', 'Fiction'}
# Add a new book and a new genre
# Your code here

Hint

Use square brackets [] to add a key-value pair to the dictionary. Use the add() method to add an item to the set.

4

Explain the difference

Add a comment explaining that books is a hash map storing key-value pairs, while genres is a hash set storing unique items without duplicates.

Data Structures Theory

books = {
    '1984': 'George Orwell',
    'To Kill a Mockingbird': 'Harper Lee',
    'The Great Gatsby': 'F. Scott Fitzgerald'
}
genres = {'Fiction', 'Classic', 'Fiction'}
books['Brave New World'] = 'Aldous Huxley'
genres.add('Dystopian')
# Add a comment explaining the difference between books and genres
# Your code here

Hint

Write a clear comment starting with # describing the difference between the dictionary and the set.

Practice

1. Which data structure is best suited for storing unique items without any associated values?

easy

A. Hash set

B. Linked list

C. Array

D. Hash map

Solution

1. Step 1: Understand the purpose of a hash set

   A hash set stores only unique values and does not associate keys with values.

2. Step 2: Compare with hash map

   A hash map stores key-value pairs, so it is not just for unique values but for mapping keys to values.

3. Final Answer:

   Hash set -> Option A

4. Quick Check:

   Unique values only = Hash set [OK]

Hint: Unique items without keys? Think hash set [OK]

Common Mistakes:

• Confusing hash map with hash set
• Thinking hash map stores unique values only
• Assuming arrays enforce uniqueness

2. Which of the following is the correct way to declare a hash map in Python?

easy

A. my_map = ["key1", "value1", "key2", "value2"]

B. my_map = ("key1", "value1", "key2", "value2")

C. my_map = {"key1": "value1", "key2": "value2"}

D. my_map = set("key1", "key2")

Solution

1. Step 1: Recognize Python dictionary syntax

   Python hash maps are dictionaries declared with curly braces and key-value pairs separated by colons.

2. Step 2: Check each option

   my_map = {"key1": "value1", "key2": "value2"} uses curly braces and key-value pairs correctly. Options A and C use lists or tuples, not dictionaries. my_map = set("key1", "key2") creates a set, not a map.

3. Final Answer:

   my_map = {"key1": "value1", "key2": "value2"} -> Option C

4. Quick Check:

   Curly braces with key:value = dict [OK]

Hint: Curly braces with colons mean hash map in Python [OK]

Common Mistakes:

• Using lists or tuples instead of dictionaries
• Confusing sets with maps
• Missing colons between keys and values

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

my_set = {1, 2, 3}
my_set.add(2)
print(len(my_set))

medium

A. 4

B. 3

C. 2

D. Error

Solution

1. Step 1: Understand set behavior on adding duplicates

   Sets store unique values. Adding 2 again does not increase size.

2. Step 2: Calculate length after adding duplicate

   Initial set has 3 elements. Adding 2 again keeps it at 3.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Set ignores duplicates = length unchanged [OK]

Hint: Adding duplicate to set doesn't increase size [OK]

Common Mistakes:

• Assuming duplicates increase set size
• Confusing set with list behavior
• Expecting an error on duplicate add

4. Identify the error in this JavaScript code snippet that tries to use a hash set:

const mySet = new Set();
mySet.add('apple');
mySet.add('banana');
mySet.add('apple');
console.log(mySet.length);

medium

A. Using 'length' property instead of 'size' for Set

B. Adding duplicate values to Set causes error

C. Set cannot store strings

D. Missing parentheses in Set constructor

Solution

1. Step 1: Check Set properties in JavaScript

   JavaScript Sets use 'size' property to get number of elements, not 'length'.

2. Step 2: Analyze the code

   Code uses 'mySet.length' which is undefined, causing incorrect output.

3. Final Answer:

   Using 'length' property instead of 'size' for Set -> Option A

4. Quick Check:

   Set size = size property, not length [OK]

Hint: Use .size, not .length, to get Set size in JavaScript [OK]

Common Mistakes:

• Using length instead of size for Sets
• Thinking duplicates cause errors
• Believing Sets can't store strings

5. You have a list of student names with possible duplicates. You want to count how many unique students there are and also store their grades. Which data structures should you use together?

hard

A. Use two hash sets: one for names and one for grades

B. Use only a hash map with names as keys and grades as values

C. Use a hash set for grades and a hash map for names

D. Use a hash set for unique names and a hash map for name-grade pairs

Solution

1. Step 1: Identify need for uniqueness and mapping

   Counting unique students requires a hash set to store unique names. Storing grades requires a hash map to map each name to a grade.

2. Step 2: Evaluate options

   Use a hash set for unique names and a hash map for name-grade pairs uses both structures correctly. Use only a hash map with names as keys and grades as values does not separate uniqueness check explicitly. Options C and D misuse sets and maps.

3. Final Answer:

   Use a hash set for unique names and a hash map for name-grade pairs -> Option D

4. Quick Check:

   Uniqueness + mapping = set + map [OK]

Hint: Unique names = set; name to grade = map [OK]

Common Mistakes:

• Using only one data structure for both tasks
• Confusing sets for storing grades
• Not separating uniqueness from mapping