Why Hash Map Exists and What Problem It Solves in DSA C++ - See It Work

Why Hash Map Exists and What Problem It Solves

📖 Scenario: Imagine you run a small library. You want to quickly find the author of a book by its title. You have a list of books and authors, but searching one by one is slow when the list grows.

🎯 Goal: You will create a simple program that stores book titles and authors, then quickly finds the author for a given book title using a hash map concept.

📋 What You'll Learn

Create a dictionary-like data structure with book titles as keys and authors as values

Add a variable to hold the book title to search

Use the hash map concept to find the author by the book title

Print the author found or a message if the book is not in the collection

💡 Why This Matters

🌍 Real World

Hash maps are used in many apps to quickly find information, like looking up a phone number by name or a product price by ID.

💼 Career

Understanding hash maps is essential for software developers to write efficient code for searching and storing data.

1

Create the book collection

Create a std::unordered_map<std::string, std::string> called books with these exact entries: "The Hobbit": "J.R.R. Tolkien", "1984": "George Orwell", "To Kill a Mockingbird": "Harper Lee".

DSA C++

# Your code here

Hint

Use std::unordered_map<std::string, std::string> to create a map from book titles to authors.

2

Set the book title to search

Add a std::string variable called search_title and set it to "1984".

DSA C++

#include 
#include 
#include 

int main() {
    std::unordered_map books = {
        {"The Hobbit", "J.R.R. Tolkien"},
        {"1984", "George Orwell"},
        {"To Kill a Mockingbird", "Harper Lee"}
    };

    // Your code here
}

Hint

Declare a string variable search_title and assign it the value "1984".

3

Find the author using the hash map

Use books.find(search_title) to check if the book exists. If found, store the author in a variable called author. If not found, set author to "Not found".

DSA C++

#include 
#include 
#include 

int main() {
    std::unordered_map books = {
        {"The Hobbit", "J.R.R. Tolkien"},
        {"1984", "George Orwell"},
        {"To Kill a Mockingbird", "Harper Lee"}
    };

    std::string search_title = "1984";

    // Your code here

    return 0;
}

Hint

Use auto it = books.find(search_title); and check if it != books.end() to find the author.

4

Print the author or not found message

Write std::cout << author << std::endl; to print the author or "Not found" message.

DSA C++

#include 
#include 
#include 

int main() {
    std::unordered_map books = {
        {"The Hobbit", "J.R.R. Tolkien"},
        {"1984", "George Orwell"},
        {"To Kill a Mockingbird", "Harper Lee"}
    };

    std::string search_title = "1984";

    std::string author;
    auto it = books.find(search_title);
    if (it != books.end()) {
        author = it->second;
    } else {
        author = "Not found";
    }

    // Your code here

    return 0;
}

Hint

Use std::cout << author << std::endl; to print the result.

Practice

1. Why do we use a hash map instead of a simple array for storing key-value pairs?

easy

A. Because arrays do not allow storing multiple values

B. Because arrays can only store numbers, not keys and values

C. Because hash maps use less memory than arrays

D. Because hash maps provide faster search, insert, and delete operations on average

Solution

1. Step 1: Understand array limitations for key-value storage

   Arrays store elements by index, so searching by key requires scanning all elements, which is slow (O(n)).

2. Step 2: Recognize hash map advantages

   Hash maps use a hash function to find the index quickly, allowing average O(1) time for search, insert, and delete.

3. Final Answer:

   Because hash maps provide faster search, insert, and delete operations on average -> Option D

4. Quick Check:

   Hash map speed advantage = D [OK]

Hint: Hash maps speed up key-based access compared to arrays [OK]

Common Mistakes:

• Thinking arrays can't store key-value pairs
• Assuming hash maps always use less memory
• Confusing hash maps with linked lists

2. Which of the following is the correct way to declare a hash map in C++ that maps strings to integers?

easy

A. std::unordered_map map;

B. std::unordered_map map;

C. std::map map;

D. std::unordered_map map;

Solution

1. Step 1: Identify correct template parameters for unordered_map

   The first template parameter is the key type, the second is the value type. Here, key is string, value is int.

2. Step 2: Check syntax correctness

   std::unordered_map map; uses std::unordered_map map; which is correct syntax and types.

3. Final Answer:

   std::unordered_map map; -> Option A

4. Quick Check:

   Correct unordered_map declaration = A [OK]

Hint: Key type first, value type second in unordered_map template [OK]

Common Mistakes:

• Swapping key and value types
• Missing std:: before unordered_map
• Incorrect template syntax

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

std::unordered_map map;
map["apple"] = 5;
map["banana"] = 3;
map["apple"] = 7;
std::cout << map["apple"] << std::endl;

medium

A. 5

B. 7

C. 3

D. Compilation error

Solution

1. Step 1: Understand insertion and update in unordered_map

   map["apple"] = 5 inserts key "apple" with value 5. Then map["apple"] = 7 updates the value to 7.

2. Step 2: Output the value for key "apple"

   After update, map["apple"] holds 7, so printing it outputs 7.

3. Final Answer:

   7 -> Option B

4. Quick Check:

   Updated value for "apple" = 7 [OK]

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

Common Mistakes:

• Assuming values accumulate instead of overwrite
• Confusing keys and values
• Expecting compilation error due to syntax

4. Identify the error in the following code snippet that uses a hash map in C++:

std::unordered_map map;
map.insert(1, "one");
std::cout << map[1] << std::endl;

medium

A. insert() requires a pair, not two separate arguments

B. map[1] cannot be used to access elements

C. unordered_map keys must be strings, not integers

D. std::cout syntax is incorrect

Solution

1. Step 1: Check usage of insert() method in unordered_map

   insert() expects a std::pair, not two separate arguments.

2. Step 2: Identify correct way to insert key-value pair

   Use map.insert({1, "one"}); or map[1] = "one"; to insert correctly.

3. Final Answer:

   insert() requires a pair, not two separate arguments -> Option A

4. Quick Check:

   insert() needs pair argument [OK]

Hint: insert() needs pair, not separate key and value [OK]

Common Mistakes:

• Passing key and value as separate arguments to insert()
• Thinking map[1] cannot access elements
• Believing keys must be strings

5. You want to count the frequency of each word in a large text using a hash map in C++. Which problem does using a hash map solve best compared to other data structures?

hard

A. It sorts the words alphabetically automatically

B. It uses less memory than arrays for storing counts

C. It allows counting frequencies in O(n) time by direct key access

D. It prevents duplicate words from being stored

Solution

1. Step 1: Understand frequency counting requirements

   Counting frequencies requires fast lookup and update of counts by word keys.

2. Step 2: Recognize hash map advantage for frequency counting

   Hash maps provide average O(1) access to keys, enabling counting in O(n) time over all words.

3. Step 3: Eliminate incorrect options

   Hash maps do not sort keys automatically, may use more memory than arrays, and allow duplicates as keys with updated counts.

4. Final Answer:

   It allows counting frequencies in O(n) time by direct key access -> Option C

5. Quick Check:

   Fast key access enables O(n) frequency count [OK]

Hint: Hash maps give fast key lookup for counting [OK]

Common Mistakes:

• Thinking hash maps sort keys automatically
• Assuming hash maps use less memory than arrays
• Believing hash maps prevent duplicate keys