HashMap Implementation from Scratch in DSA C - Build from Scratch

HashMap Implementation from Scratch

📖 Scenario: Imagine you are building a simple phone book application. You want to store names and their phone numbers so you can quickly find a number by a name. To do this efficiently, you will create your own HashMap from scratch in C.

🎯 Goal: Build a basic HashMap in C that can store key-value pairs (names and phone numbers), handle collisions using chaining, and allow insertion and lookup operations.

📋 What You'll Learn

Create a struct for HashMap entries with key and value

Create a HashMap struct with an array of pointers for buckets

Implement a simple hash function for strings

Implement insert and search functions

Handle collisions using linked lists (chaining)

Print the phone number for a given name

💡 Why This Matters

🌍 Real World

HashMaps are used in many applications like phone books, caches, and databases to quickly find data by keys.

💼 Career

Understanding how HashMaps work internally helps in optimizing software and solving problems efficiently in software development roles.

1

Create the basic data structures

Create a struct called Entry with char key[50], char value[15], and a pointer next to another Entry. Then create a struct called HashMap with an array buckets of size 10 holding pointers to Entry.

DSA C

// Your code here for struct Entry and struct HashMap

Hint

Think of Entry as a node in a linked list holding a name and phone number. The HashMap holds an array of these lists.

2

Add a hash function and initialize the HashMap

Add a function int hash(char* key) that returns the sum of ASCII values of characters in key modulo 10. Also, write a function void initHashMap(HashMap* map) that sets all buckets pointers to NULL.

DSA C

// Your code here for hash function and initHashMap function

Hint

The hash function converts a string to an index between 0 and 9. Initializing sets all bucket pointers to NULL.

3

Implement insert and search functions

Write a function void insert(HashMap* map, char* key, char* value) that adds a new Entry to the correct bucket using chaining. Also write char* search(HashMap* map, char* key) that returns the value for the given key or NULL if not found.

DSA C

// Your code here for insert and search functions

Hint

Insert adds new entries at the start of the linked list for the bucket. Search walks through the list to find the key.

4

Test the HashMap by inserting and searching

In main(), create a HashMap variable called phoneBook. Initialize it using initHashMap. Insert these entries: ("Alice", "12345"), ("Bob", "67890"), ("Charlie", "54321"). Then print the phone number for "Bob" using search.

DSA C

int main() {
    // Your code here to create, initialize, insert, and print
    return 0;
}

Hint

Use printf to display the phone number found by search. Remember to check if the result is NULL.

Practice

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

easy

A. To sort the keys in ascending order

B. To store values in a linked list

C. To convert a key into an index for storing the value

D. To delete keys from the HashMap

Solution

1. Step 1: Understand the role of a hash function

   A hash function takes a key and converts it into a number (index) that tells where to store the value in the HashMap's internal array.

2. Step 2: Compare other options

   Sorting keys or deleting keys are not the job of the hash function. Storing values in a linked list is a collision handling method, not hashing.

3. Final Answer:

   To convert a key into an index for storing the value -> Option C

4. Quick Check:

   Hash function = key to index [OK]

Hint: Hash function maps keys to array indexes fast [OK]

Common Mistakes:

• Thinking hash function sorts keys
• Confusing hash function with collision handling
• Believing hash function deletes keys

2. Which of the following is the correct way to declare a simple HashMap structure in C that stores integer keys and values?

easy

A. typedef struct { int key; int value; } HashNode;

B. typedef struct { char* key; int value; } HashMap;

C. typedef struct { int key; int value; } HashMap;

D. typedef struct { int key; int value; HashNode* next; } HashMap;

Solution

1. Step 1: Identify the basic node structure

   A HashMap stores key-value pairs. A node with int key and int value is a basic unit, so HashNode struct is correct.

2. Step 2: Check other options

   typedef struct { char* key; int value; } HashMap; uses char* key which is not integer keys. typedef struct { int key; int value; } HashMap; names the struct HashMap but it's actually a node. typedef struct { int key; int value; HashNode* next; } HashMap; adds a pointer for chaining but names it HashMap, which is confusing.

3. Final Answer:

   typedef struct { int key; int value; } HashNode; -> Option A

4. Quick Check:

   Node struct with int key and value = typedef struct { int key; int value; } HashNode; [OK]

Hint: Node struct holds key and value pairs [OK]

Common Mistakes:

• Confusing node and map struct names
• Using wrong key types
• Missing pointer for chaining in basic struct

3. Given the following code snippet for inserting keys into a HashMap with size 5, what will be the index for key 12?

int hash(int key) {
    return key % 5;
}

int index = hash(12);
printf("%d", index);

medium

A. 4

B. 2

C. 3

D. 1

Solution

1. Step 1: Apply the hash function to key 12

   The hash function returns key % 5. For key 12, 12 % 5 = 2.

2. Step 2: Confirm the printed index

   The code prints the index, which is 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   12 % 5 = 2 [OK]

Hint: Modulo operator gives index in hash function [OK]

Common Mistakes:

• Calculating 12 % 5 incorrectly
• Confusing modulo with division
• Ignoring hash function logic

4. Consider this buggy code snippet for inserting a key-value pair into a HashMap with chaining:

void insert(HashNode** table, int size, int key, int value) {
    int index = key % size;
    HashNode* newNode = malloc(sizeof(HashNode));
    newNode->key = key;
    newNode->value = value;
    newNode->next = NULL;

    if (table[index] == NULL) {
        table[index] = newNode;
    } else {
        table[index]->next = newNode;
    }
}

What is the main problem in this code?

medium

A. It overwrites the existing chain instead of appending correctly

B. It does not handle memory allocation failure

C. It uses wrong hash function

D. It does not set the key and value fields

Solution

1. Step 1: Analyze insertion logic for chaining

   If the bucket is empty, it sets the new node. Otherwise, it sets table[index]->next = newNode, which overwrites the existing next pointer, losing the chain.

2. Step 2: Identify correct chaining method

   To append, it should traverse to the last node in the chain and then add the new node, not overwrite the first node's next pointer.

3. Final Answer:

   It overwrites the existing chain instead of appending correctly -> Option A

4. Quick Check:

   Chaining requires appending, not overwriting [OK]

Hint: Traverse chain to append new node, don't overwrite next [OK]

Common Mistakes:

• Ignoring chain traversal
• Assuming single node per bucket
• Not checking for NULL before insertion

5. You want to implement a HashMap from scratch in C that supports integer keys and values, handles collisions using chaining, and dynamically resizes when load factor exceeds 0.7. Which of the following steps is NOT necessary for this implementation?

hard

A. Resize the internal array and rehash all keys when load factor is high

B. Create linked lists at each bucket to handle collisions

C. Implement a hash function to convert keys to indexes

D. Sort the keys in each bucket to speed up search

Solution

1. Step 1: Understand necessary components of HashMap

   A hash function, chaining with linked lists, and resizing with rehashing are all essential for correct and efficient HashMap behavior.

2. Step 2: Evaluate sorting keys in buckets

   Sorting keys in buckets is not required because chaining uses linked lists and search is done by traversing nodes. Sorting adds unnecessary overhead.

3. Final Answer:

   Sort the keys in each bucket to speed up search -> Option D

4. Quick Check:

   Sorting buckets is unnecessary in chaining [OK]

Hint: Chaining uses linked lists; sorting buckets is extra work [OK]

Common Mistakes:

• Thinking sorting improves chaining search
• Ignoring rehashing on resize
• Skipping collision handling