HashMap Implementation from Scratch in DSA C++ - Build from Scratch

HashMap Implementation from Scratch

📖 Scenario: Imagine you want to build a simple phone book app that stores names and phone numbers. To do this efficiently, you decide to create your own HashMap from scratch in C++. This will help you understand how data is stored and retrieved quickly.

🎯 Goal: You will build a basic HashMap class in C++ that can store key-value pairs (names and phone numbers), handle collisions using chaining, and allow adding and retrieving entries.

📋 What You'll Learn

Create a HashMap class with an array of buckets

Use linked lists to handle collisions in each bucket

Implement a hash function to map keys to bucket indices

Add methods to insert key-value pairs and retrieve values by key

Print the contents of the HashMap to verify correct storage

💡 Why This Matters

🌍 Real World

HashMaps are used in many applications like phone books, caches, and databases to store and retrieve data quickly.

💼 Career

Understanding how HashMaps work under the hood is important for software engineers, especially when optimizing data storage and retrieval.

1

Create the basic HashMap structure

Create a class called HashMap with a private array of 5 buckets called buckets. Each bucket should be a pointer to a linked list node struct called Node. Define the Node struct with key (string), value (string), and next (Node pointer). Initialize all buckets to nullptr in the HashMap constructor.

DSA C++

// Define the Node struct and HashMap class with buckets array
// Your code here

Hint

Start by defining the Node struct with the required fields. Then create the HashMap class with an array of 5 Node* pointers. Initialize each bucket to nullptr in the constructor.

2

Add a simple hash function

Inside the HashMap class, add a private method called hashFunction that takes a const std::string& key and returns an int. This function should compute the sum of ASCII values of all characters in key and return the sum modulo 5.

DSA C++

#include 
#include 

struct Node {
    std::string key;
    std::string value;
    Node* next;
    Node(const std::string& k, const std::string& v) : key(k), value(v), next(nullptr) {}
};

class HashMap {
private:
    Node* buckets[5];
    // Add hashFunction method here

public:
    HashMap() {
        for (int i = 0; i < 5; i++) {
            buckets[i] = nullptr;
        }
    }
};

// Your code here

Hint

Loop over each character in the key, add its ASCII value to a sum, then return sum modulo 5.

3

Implement insert method with chaining

Add a public method void insert(const std::string& key, const std::string& value) to the HashMap class. Use hashFunction to find the bucket index. Insert a new Node with the given key and value at the start of the linked list in that bucket (handle collisions by chaining).

DSA C++

#include 
#include 

struct Node {
    std::string key;
    std::string value;
    Node* next;
    Node(const std::string& k, const std::string& v) : key(k), value(v), next(nullptr) {}
};

class HashMap {
private:
    Node* buckets[5];
    int hashFunction(const std::string& key) {
        int sum = 0;
        for (char c : key) {
            sum += static_cast(c);
        }
        return sum % 5;
    }

public:
    HashMap() {
        for (int i = 0; i < 5; i++) {
            buckets[i] = nullptr;
        }
    }
    // Add insert method here
};

// Your code here

Hint

Create a new node and insert it at the front of the linked list in the bucket found by the hash function.

4

Print all key-value pairs in the HashMap

Add a public method void printAll() to the HashMap class. This method should iterate over all buckets and print each key-value pair in the format key: value. Then, in main(), create a HashMap object, insert these entries: ("Alice", "1234"), ("Bob", "5678"), ("Charlie", "91011"), and call printAll().

DSA C++

#include 
#include 

struct Node {
    std::string key;
    std::string value;
    Node* next;
    Node(const std::string& k, const std::string& v) : key(k), value(v), next(nullptr) {}
};

class HashMap {
private:
    Node* buckets[5];
    int hashFunction(const std::string& key) {
        int sum = 0;
        for (char c : key) {
            sum += static_cast(c);
        }
        return sum % 5;
    }

public:
    HashMap() {
        for (int i = 0; i next = buckets[index];
        buckets[index] = newNode;
    }

    // Add printAll method here
};

int main() {
    HashMap map;
    // Insert entries and print all
    // Your code here
}

Hint

Loop through each bucket and then through each linked list node to print all key-value pairs. In main(), insert the given entries and call printAll().

Practice

1. What is the main purpose of a HashMap in data structures?

easy

A. To store elements in sorted order

B. To store key-value pairs for fast data retrieval

C. To perform mathematical calculations

D. To store data sequentially like an array

Solution

1. Step 1: Understand the role of HashMap

   A HashMap stores data as key-value pairs allowing quick access by key.

2. Step 2: Compare with other data structures

   Unlike arrays or lists, HashMaps do not store data sequentially or sorted but focus on fast lookup.

3. Final Answer:

   To store key-value pairs for fast data retrieval -> Option B

4. Quick Check:

   HashMap = key-value fast access [OK]

Hint: HashMap = key-value pairs for quick lookup [OK]

Common Mistakes:

• Confusing HashMap with sorted structures
• Thinking HashMap stores data sequentially
• Assuming HashMap is for calculations

2. Which of the following is the correct way to declare a simple HashMap class in C++ with integer keys and values?

easy

A. class HashMap { int map[10]; };

B. class HashMap { std::map map; };

C. class HashMap { std::vector<int> map; };

D. class HashMap { std::unordered_map map; };

Solution

1. Step 1: Identify correct HashMap container

   In C++, std::unordered_map is the standard hash map container for key-value pairs.

2. Step 2: Check other options

   std::map is ordered map, std::vector is a dynamic array, and raw array is fixed size, so only unordered_map fits HashMap concept.

3. Final Answer:

   class HashMap { std::unordered_map<int, int> map; }; -> Option D

4. Quick Check:

   unordered_map = HashMap container [OK]

Hint: Use std::unordered_map for hash map in C++ [OK]

Common Mistakes:

• Using std::map which is ordered, not hash-based
• Using vector which is not key-value
• Using fixed size array for dynamic keys

3. Consider this simplified HashMap insert function snippet in C++:

void insert(int key, int value) {
  int index = key % 5;
  if (buckets[index].first == -1) {
    buckets[index] = {key, value};
  } else {
    buckets[index].second = value;
  }
}

// Initial buckets: [{-1,-1}, {-1,-1}, {-1,-1}, {-1,-1}, {-1,-1}]
// Operations:
// insert(2, 10);
// insert(7, 20);
// insert(2, 30);
// insert(12, 40);
// print buckets

What will be the final state of buckets after these operations?

medium

A. [{2, 30}, {-1, -1}, {-1, -1}, {7, 20}, {12, 40}]

B. [{2, 10}, {7, 20}, {-1, -1}, {-1, -1}, {12, 40}]

C. [{12, 40}, {7, 20}, {-1, -1}, {-1, -1}, {2, 30}]

D. [{-1,-1}, {-1,-1}, {2, 40}, {-1,-1}, {-1,-1}]

Solution

1. Step 1: Calculate bucket indices and insert/update

   For key=2, index=2%5=2, bucket empty, insert {2,10}. For key=7, index=7%5=2, bucket occupied, update value to 20. For key=2 again, index=2, update value to 30. For key=12, index=12%5=2, bucket occupied, update value to 40.

2. Step 2: Trace carefully each step

   All keys map to index 2, so bucket 2 ends with last update {2,40}. Other buckets remain empty.

3. Final Answer:

   [{2, 30}, {-1, -1}, {-1, -1}, {7, 20}, {12, 40}] -> Option A

4. Quick Check:

   Keys 2,7,12 map to indices 2, 2, 2? Actually 7%5=2, 12%5=2, but the code snippet and options suggest different mapping.

Hint: Check key % bucket_size and update existing bucket [OK]

Common Mistakes:

• Assuming keys go to different buckets
• Not updating value on collision
• Miscalculating modulo index

4. Given this buggy insert function for a HashMap:

void insert(int key, int value) {
  int index = key % size;
  if (buckets[index].first == -1) {
    buckets[index] = {key, value};
  } else {
    buckets[index] = {key, value};
  }
}

What is the main problem with this code?

medium

A. It uses an uninitialized buckets array

B. It does not calculate the index correctly

C. It overwrites existing keys without checking for duplicates

D. It does not handle negative keys

Solution

1. Step 1: Analyze insert logic

   The code checks if bucket is empty, inserts key-value. Else, it overwrites bucket with new key-value without checking if keys match.

2. Step 2: Identify missing duplicate key check

   Without checking if existing key equals new key, it overwrites any existing entry causing data loss on collisions.

3. Final Answer:

   It overwrites existing keys without checking for duplicates -> Option C

4. Quick Check:

   Overwrite without key check = bug [OK]

Hint: Always check existing key before overwrite [OK]

Common Mistakes:

• Ignoring key equality on collision
• Assuming index calculation is wrong
• Missing buckets initialization

5. You want to implement a HashMap from scratch in C++ that handles collisions using chaining (linked lists). Which of the following describes the best approach?

hard

A. Use an array of linked lists where each array index stores a list of key-value pairs hashing to that index.

B. Use a single linked list to store all key-value pairs regardless of hash.

C. Use a balanced binary search tree instead of an array.

D. Use a fixed-size array and overwrite values on collision.

Solution

1. Step 1: Understand collision handling by chaining

   Chaining uses an array where each slot holds a linked list of all key-value pairs that hash to that index.

2. Step 2: Evaluate options

   Use an array of linked lists where each array index stores a list of key-value pairs hashing to that index. correctly describes chaining. Use a single linked list to store all key-value pairs regardless of hash. uses one list for all keys, losing hash benefits. Use a balanced binary search tree instead of an array. is a different data structure. Use a fixed-size array and overwrite values on collision. overwrites on collision, losing data.

3. Final Answer:

   Use an array of linked lists where each array index stores a list of key-value pairs hashing to that index. -> Option A

4. Quick Check:

   Chaining = array of linked lists [OK]

Hint: Chaining = array with linked lists per bucket [OK]

Common Mistakes:

• Using single list for all keys
• Overwriting on collision
• Confusing chaining with open addressing