HashMap Implementation from Scratch in DSA C

Mental Model

A HashMap stores key-value pairs using a special function to find where to keep each pair quickly.

Analogy: Imagine a library where each book has a special code that tells you exactly which shelf and spot to find it, so you don't have to search all shelves.

Buckets array:
[0] -> null
[1] -> null
[2] -> null
[3] -> null
[4] -> null
Each bucket can hold a linked list of key-value pairs.

Dry Run Walkthrough

Input: Insert keys 1, 6, and 11 with values 'A', 'B', 'C' into a HashMap with 5 buckets.

Goal: Store key-value pairs so that keys 1, 6, and 11 are placed in correct buckets using hash function key % 5, handling collisions by chaining.

Step 1: Insert key=1, value='A'. Compute hash index = 1 % 5 = 1.

[0] -> null
[1] -> (1, 'A') -> null
[2] -> null
[3] -> null
[4] -> null

Why: We place the pair in bucket 1 because hash function gives index 1.

Step 2: Insert key=6, value='B'. Compute hash index = 6 % 5 = 1. Collision at bucket 1.

[0] -> null
[1] -> (6, 'B') -> (1, 'A') -> null
[2] -> null
[3] -> null
[4] -> null

Why: We add new pair at the head of linked list in bucket 1 to handle collision.

Step 3: Insert key=11, value='C'. Compute hash index = 11 % 5 = 1. Collision at bucket 1.

[0] -> null
[1] -> (11, 'C') -> (6, 'B') -> (1, 'A') -> null
[2] -> null
[3] -> null
[4] -> null

Why: We add new pair at the head of linked list in bucket 1 to handle collision.

Result:

[0] -> null
[1] -> (11, 'C') -> (6, 'B') -> (1, 'A') -> null
[2] -> null
[3] -> null
[4] -> null

Annotated Code

DSA C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUCKET_COUNT 5

// Node for linked list in each bucket
typedef struct Node {
    int key;
    char value[10];
    struct Node* next;
} Node;

// HashMap structure
typedef struct HashMap {
    Node* buckets[BUCKET_COUNT];
} HashMap;

// Create a new node
Node* create_node(int key, const char* value) {
    Node* new_node = (Node*)malloc(sizeof(Node));
    new_node->key = key;
    strcpy(new_node->value, value);
    new_node->next = NULL;
    return new_node;
}

// Initialize HashMap
void init_hashmap(HashMap* map) {
    for (int i = 0; i < BUCKET_COUNT; i++) {
        map->buckets[i] = NULL;
    }
}

// Hash function: key modulo bucket count
int hash_function(int key) {
    return key % BUCKET_COUNT;
}

// Insert key-value pair into HashMap
void hashmap_insert(HashMap* map, int key, const char* value) {
    int index = hash_function(key); // compute bucket index
    Node* head = map->buckets[index];

    // Check if key exists, update value if found
    Node* curr = head;
    while (curr != NULL) {
        if (curr->key == key) {
            strcpy(curr->value, value); // update existing value
            return;
        }
        curr = curr->next;
    }

    // Insert new node at head for collision chaining
    Node* new_node = create_node(key, value);
    new_node->next = head;
    map->buckets[index] = new_node;
}

// Print HashMap contents
void print_hashmap(HashMap* map) {
    for (int i = 0; i < BUCKET_COUNT; i++) {
        printf("[%d] -> ", i);
        Node* curr = map->buckets[i];
        while (curr != NULL) {
            printf("(%d, '%s') -> ", curr->key, curr->value);
            curr = curr->next;
        }
        printf("null\n");
    }
}

int main() {
    HashMap map;
    init_hashmap(&map);

    hashmap_insert(&map, 1, "A");
    hashmap_insert(&map, 6, "B");
    hashmap_insert(&map, 11, "C");

    print_hashmap(&map);
    return 0;
}

int index = hash_function(key); // compute bucket index

Calculate bucket index using hash function

while (curr != NULL) { if (curr->key == key) { strcpy(curr->value, value); return; } curr = curr->next; }

Check if key exists to update value instead of inserting duplicate

new_node->next = head; map->buckets[index] = new_node;

Insert new node at head of linked list to handle collisions

Output

[0] -> null

[1] -> (11, 'C') -> (6, 'B') -> (1, 'A') -> null

[2] -> null

[3] -> null

[4] -> null

Complexity Analysis

Time: O(n/k) on average, where n is number of keys and k is bucket count, because each bucket holds about n/k nodes; worst case O(n) if all keys collide.

Space: O(n) because we store each key-value pair in a node.

vs Alternative: Compared to a simple array search O(n), HashMap insertion and lookup are faster on average due to direct bucket access and chaining.

Edge Cases

Inserting duplicate key

Value is updated in existing node instead of adding new node.

DSA C

if (curr->key == key) { strcpy(curr->value, value); return; }

Empty HashMap

Buckets are initialized to NULL, no nodes present.

DSA C

for (int i = 0; i < BUCKET_COUNT; i++) { map->buckets[i] = NULL; }

All keys collide to same bucket

All nodes form a linked list in one bucket, performance degrades to O(n).

DSA C

new_node->next = head; map->buckets[index] = new_node;

When to Use This Pattern

When you need fast key-value storage with quick insert and lookup, and see problems mentioning hashing or collisions, reach for HashMap implementation with chaining.

Common Mistakes

Mistake: Not handling collisions, overwriting existing bucket pointer losing previous nodes.
Fix: Insert new nodes at head of linked list to preserve existing nodes.

Mistake: Not checking for existing key before inserting, causing duplicates.
Fix: Traverse bucket list to update value if key exists before inserting new node.

Summary

Stores key-value pairs using a hash function and linked lists to handle collisions.

Use when you want fast insert and lookup by key with average constant time.

The key insight is using a hash function to find a bucket and chaining to handle collisions.

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