C Program: String Hash function and Hash table

Last update on October 16 2025 11:02:56 (UTC/GMT +8 hours)

3. String Hash Function Challenges

Write a C program that creates a hash function specifically designed for strings and implements a hash table to store and retrieve string data.

Sample Solution:

C Code:

// Including necessary header files
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define TABLE_SIZE 10

// Structure for a key-value pair
struct KeyValuePair {
    char key[50];
    char value[50];
    struct KeyValuePair* next;
};

// Structure for the hash table
struct HashTable {
    int size;
    struct KeyValuePair** table;
};

// Hash function for strings (djb2 algorithm)
unsigned long hashFunction(const char* str) {
    unsigned long hash = 5381;
    int c;

    while ((c = *str++) != '\0') {
        hash = ((hash << 5) + hash) + c; // hash * 33 + c
    }

    return hash;
}

// Function to create a new key-value pair
struct KeyValuePair* createKeyValuePair(const char* key, const char* value) {
    struct KeyValuePair* newPair = (struct KeyValuePair*)malloc(sizeof(struct KeyValuePair));
    if (newPair != NULL) {
        strcpy(newPair->key, key);
        strcpy(newPair->value, value);
        newPair->next = NULL;
    }
    return newPair;
}

// Function to create a new hash table
struct HashTable* createHashTable(int size) {
    struct HashTable* newTable = (struct HashTable*)malloc(sizeof(struct HashTable));
    if (newTable != NULL) {
        newTable->size = size;
        newTable->table = (struct KeyValuePair**)calloc(size, sizeof(struct KeyValuePair*));
    }
    return newTable;
}

// Function to insert a key-value pair into the hash table
void insert(struct HashTable* hashTable, const char* key, const char* value) {
    unsigned long index = hashFunction(key) % hashTable->size;

    // Create a new key-value pair
    struct KeyValuePair* newPair = createKeyValuePair(key, value);

    // Insert the new pair at the beginning of the linked list
    newPair->next = hashTable->table[index];
    hashTable->table[index] = newPair;
}

// Function to retrieve the value associated with a key
const char* retrieve(struct HashTable* hashTable, const char* key) {
    unsigned long index = hashFunction(key) % hashTable->size;
    struct KeyValuePair* current = hashTable->table[index];

    // Traverse the linked list at the index
    while (current != NULL) {
        if (strcmp(current->key, key) == 0) {
            return current->value; // Key found, return the value
        }
        current = current->next;
    }
    return "Key not found"; // Key not found
}

// Function to display the contents of the hash table
void displayHashTable(struct HashTable* hashTable) {
    for (int i = 0; i < hashTable->size; i++) {
        printf("[%d] -> ", i);

        struct KeyValuePair* current = hashTable->table[i];
        while (current != NULL) {
            printf("(%s, %s) -> ", current->key, current->value);
            current = current->next;
        }

        printf("NULL\n");
    }
}

// Function to free the memory allocated for the hash table
void freeHashTable(struct HashTable* hashTable) {
    for (int i = 0; i < hashTable->size; i++) {
        struct KeyValuePair* current = hashTable->table[i];
        while (current != NULL) {
            struct KeyValuePair* temp = current;
            current = current->next;
            free(temp);
        }
    }

    free(hashTable->table);
    free(hashTable);
}

int main() {
    struct HashTable* hashTable = createHashTable(TABLE_SIZE);

    // Insert key-value pairs
    insert(hashTable, "Red", "#ff0000");
    insert(hashTable, "Green", "008000");
    insert(hashTable, "Blue", "#0000FF");
    insert(hashTable, "Yellow", "#FFFF00");
    insert(hashTable, "Orange", "#FFA500");

    printf("Initial Hash Table:\n");
    displayHashTable(hashTable);
    printf("\n");

    // Retrieve and print values for specific keys
    printf("Value for key 'Red': %s\n", retrieve(hashTable, "Red"));
    printf("Value for key 'Yellow': %s\n", retrieve(hashTable, "Yellow"));
    printf("Value for key 'White': %s\n", retrieve(hashTable, "White"));
    printf("\n");

    // Free allocated memory
    freeHashTable(hashTable);

    return 0;
}

Output:

Initial Hash Table:
[0] -> (Green, 008000) -> NULL
[1] -> (Orange, #FFA500) -> (Blue, #0000FF) -> NULL
[2] -> NULL
[3] -> NULL
[4] -> NULL
[5] -> NULL
[6] -> NULL
[7] -> NULL
[8] -> (Red, #ff0000) -> NULL
[9] -> (Yellow, #FFFF00) -> NULL

Value for key 'Red': #ff0000
Value for key 'Yellow': #FFFF00
Value for key 'White': Key not found

Explanation:

In the exercise above -

Structures:

KeyValuePair: Represents a key-value pair with strings for key and value, linked to handle collisions.
HashTable: Represents the hash table with a size and an array of pointers to key-value pairs.

Hash Function (hashFunction):

Implements the djb2 hash algorithm to convert a string key into a hash value.

Key-Value Pair Functions:

createKeyValuePair: Allocates memory for a new key-value pair and initializes it with the provided key and value.

Hash Table Functions:

createHashTable: Allocates memory for a new hash table and initializes its size and array of pointers.
insert: Inserts a key-value pair into the hash table using the hash function.
retrieve: Retrieves the value associated with a given key from the hash table.
displayHashTable: Displays the contents of the hash table, including linked lists for handling collisions.
freeHashTable: Frees the memory allocated for the hash table.

Main Function (main):

Creates a hash table of a specified size (TABLE_SIZE).
Inserts several key-value pairs into the hash table.
Displays the initial hash table.
Retrieves and prints values for specific keys.
Frees the memory allocated for the hash table.

Flowchart:

Flowchart: C Program: String Hash function and Hash table.

For more Practice: Solve these Related Problems:

Write a C program to implement a hash table for strings using a polynomial rolling hash function.
Write a C program to design a case-insensitive string hash function and integrate it into a hash table.
Write a C program to implement a hash table for long strings optimized with bitwise operations in the hash function.
Write a C program to implement a hash table that supports Unicode string keys by adjusting the hash function accordingly.

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