# How To Implement a Sample Hash Table in C/C++

Hash tables are an essential data structure in computer science that facilitates the retrieval and storage of data efficiently. A hash table utilizes a hash function to map identifiers to values, making it an ideal solution for applications requiring quick lookups.

## Understanding Hash Tables

Prior to implementing a hash table, it is essential to comprehend how they operate. Each container in a hash table can contain one or more key-value pairs. Prior to inserting data into the hash table, the key is hashed using a hash function, which returns an array index. The value is then stored in the container associated with that index.

When retrieving data from a hash table, the key is re-hashed and the resulting index is used to locate the value in the container. A collision occurs when multiple values are stored in the same container. The hash table employs a collision resolution strategy to manage the collision in this instance.

## Implementing a Hash Table in C/C++

The stages involved in implementing a hash table in C/C++ are as follows:

### Step 1: Define a Hash Function

The hash function is used to translate array keys to indexes. A decent hash function should evenly distribute keys across an array to reduce collisions. There are numerous implementation methods for hash functions, including:

- Division Method
- Multiplication Method
- Universal Hashing

Here is an example of a basic hash function that employs the division algorithm:

unsigned int hash(const char* key, int size) { unsigned int hashVal = 0; for (int i = 0; key[i] != '\0'; i++) { hashVal = (hashVal * 31 + key[i]) % size; } return hashVal; }

This hash function computes a hash value using the ASCII values of the characters in the key. The modulo operator is then used to map the hash value to an index in the array.

### Step 2: Define a Bucket

A container is a data structure that contains at least one key-value pair. In C/C++, a container can be defined using a struct:

struct bucket { const char* key; int value; };

This structure defines a container for key-value pairs. The value is an integer, while the key is a string.

### Step 3: Define the Hash Table

Once the hash function and container have been defined, the hash table can be defined. A hash table is a collection of buckets:

const int TABLE_SIZE = 128; struct hashTable { bucket table[TABLE_SIZE]; };

This code defines a 128-bucket hash table.

### Step 4: Implement Insertion and Retrieval

Implementing the insertion and retrieval functions is the final phase. The insertion function inserts a key-value pair into the hash table given a key-value pair.

void insert(hashTable* ht, const char* key, int value) { int index = hash(key, TABLE_SIZE); bucket* b = &ht->table[index]; while (b->key != NULL && strcmp(b->key, key) != 0) { index = ( index + 1) % TABLE_SIZE; b = &ht->table[index]; } b->key = key; b->value = value; }

This code hashes the key before obtaining its index in the array. Using linear probing, it then seeks for an empty bucket or a bucket with the same key. If the bucket is vacant, the key-value pair is inserted. If a container with the same key is found, the value is updated.

The retrieval function accepts a key and returns its associated value:

int retrieve(hashTable* ht, const char* key) {

int index = hash(key, TABLE_SIZE); bucket* b = &ht->table[index]; while (b->key != NULL && strcmp(b->key, key) != 0) { index = (index + 1) % TABLE_SIZE; b = &ht->table[index]; } return b->value; }

This code hashes the key before obtaining its index in the array. Using linear probing, it then searches for the container with the same key. If the container is present, the value is returned. If the bucket is vacant, it indicates that the key does not exist in the hash table.

## Conclusion

A hash table is implemented in C/C++ by defining a hash function, a container, and a hash table. Implementing insertion and retrieval functions utilizing collision resolution strategies such as linear probing is also required. This article explains how to implement a hash table in C/C++ for efficient data retrieval and storage.

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# How To Implement a Sample Hash Table in C/C++

This guide demonstrates how to implement a prototype hash table in C/C++. Hash tables are an essential data structure in computer science that facilitates the retrieval and storage of data efficiently. Create your own hash table and manage collisions using linear probing by following these steps.

## Understanding Hash Tables

Before implementing a hash table in C/C++, it is essential to comprehend how they operate. Hash tables map keys to values using a hash function, enabling for quick lookups. You will be able to construct your own hash function and implement a hash table in C/C++ using this guide.

## Implementing a Hash Table in C/C++

Four stages are required to create a hash table in C/C++: defining a hash function, defining a bucket, defining the hash table itself, and implementing insertion and retrieval. This guide will walk you through each step and provide code samples as needed.

### Step 1: Define a Hash Function

The initial stage involves defining a hash function. Methods for implementing hash functions include the division method, the multiplication method, and universal hashing. In this example, we will use a straightforward hash function based on the division algorithm.

unsigned int hash(const char* key, int size) { unsigned int hashVal = 0; for (int i = 0; key[i] != '\0'; i++) { hashVal = (hashVal * 31 + key[i]) % size; } return hashVal; }

### Step 2: Define a Bucket

Next, we must define a container for key-value pairs:

struct bucket { const char* key; int value; };

### Step 3: Define the Hash Table

Once the hash function and container have been defined, we can define the actual hash table:

const int TABLE_SIZE = 128; struct hashTable { bucket table[TABLE_SIZE]; };

### Step 4: Implement Insertion and Retrieval

Finally, insertion and retrieval functions can be implemented using linear probing to manage collisions:

void insert(hashTable* ht, const char* key, int value) { int index = hash(key, TABLE_SIZE); bucket* b = &ht->table[index]; while (b->key != NULL && strcmp(b->key, key) != 0) { index = (index + 1) % TABLE_SIZE; b = &ht->table[index]; } b->key = key;