C Program To Implement Dictionary Using Hashing Algorithms < RECENT – OVERVIEW >

To implement a robust dictionary in C using hashing, you should focus on three core components: a reliable hash function collision resolution strategy dynamic resizing to maintain performance. 1. Robust Hash Function (FNV-1a) For strings, the FNV-1a algorithm

is highly recommended due to its speed and low collision rate. It works by performing a series of XOR and multiply operations on each byte of the key. FNV_OFFSET 14695981039346656037ULL 1099511628211ULL hash = FNV_OFFSET; * p = key; *p; p++) hash ^= ( )(*p); hash *= FNV_PRIME; Use code with caution. Copied to clipboard

2. Collision Resolution: Separate Chaining vs. Open Addressing

Since multiple keys can produce the same hash index, you must choose a resolution method: Separate Chaining : Each slot in the hash table points to a linked list

of entries. This is simple to implement and the table never truly "fills up," though lookups degrade to if lists become long. Open Addressing (Linear Probing) : If a collision occurs, the program searches for the next available slot

in the array. This is more cache-friendly but suffers from "clustering," where occupied slots group together and slow down operations. GeeksforGeeks 3. Dynamic Resizing (Load Factor Management) A hash table's performance is tied to its load factor

(number of items / total slots). As the table fills, collisions increase and speed drops. : Generally, you should resize when the table is : Allocate a new array (typically double the size

), rehash every existing item into the new array, and free the old memory. Feature Summary Table Recommendation Hash Function FNV-1a or djb2 Fast with excellent distribution for string keys. Collision Handling Separate Chaining Easier to manage deletions and avoids clustering. Resizing Rule Load Factor > 0.75 Balances memory usage with average time complexity. Growth Strategy Double Table Size Amortizes the cost of rehashing. For a complete reference, you can look at the K & R C implementation which uses a fixed-size table with chaining for simplicity. Stack Overflow code example combining these features into a single C file? How to implement a hash table (in C) - Ben Hoyt c program to implement dictionary using hashing algorithms

The Challenge: Collisions

Ideally, two different keys would never map to the same index. In reality, because the set of possible keys is usually larger than the size of the array, collisions are inevitable.

There are two primary ways to handle collisions:

1. Open Addressing (Probing): If a slot is taken, find the next empty slot.
2. Separate Chaining: Use an array of linked lists. If two keys hash to the same index, add them to the list at that index.

We will implement Separate Chaining because it is robust, maintains performance even as the table fills up, and simplifies the deletion logic.

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Conclusion

Implementing a dictionary using hashing algorithms in C is a quintessential exercise in data structure design, balancing theory with systems-level pragmatism. The choice of hash function influences distribution and speed, while collision resolution (chaining vs. open addressing) affects memory layout and performance under load. Dynamic resizing ensures scalability, and careful memory management is mandatory in C’s manual environment. The resulting structure provides efficient, average constant-time operations, making hashing-based dictionaries indispensable in areas like compiler symbol tables, caching systems, and network routers. Through such an implementation, a programmer gains deep insight into algorithmic trade-offs and the power of transforming keys into direct memory indices—a cornerstone of modern computing.

3.2 Choosing a Hash Algorithm

For strings, one of the most effective and simple algorithms is the DJB2 hash function, created by Daniel J. Bernstein. It provides excellent distribution and is easy to compute.

// DJB2 hash function for strings
unsigned long hash_djb2(const char *str) 
    unsigned long hash = 5381;
    int c;
while ((c = *str++)) 
    hash = ((hash << 5) + hash) + c;  // hash * 33 + c
return hash;

We'll also implement the SDBM hash as an alternative for comparison.

// SDBM hash function
unsigned long hash_sdbm(const char *str) 
    unsigned long hash = 0;
    int c;
while ((c = *str++)) 
    hash = c + (hash << 6) + (hash << 16) - hash;
return hash;

Index calculation: int index = hash_function(key) % table->size;

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3.3 Table Size

A prime number is chosen to reduce collisions. The initial size is 101. Dynamic resizing (rehashing) can be added but is omitted for simplicity in this base version.

Chapter 5: Complete Example Program

Here is a working example that ties everything together:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// [Include all the functions defined above: hash_djb2, create_hash_table,
//  insert, search, delete_key, display, destroy_hash_table] To implement a robust dictionary in C using
int main() 
// Create a dictionary with 10007 buckets
HashTable *dict = create_hash_table(TABLE_SIZE);
if (!dict) 
printf("Failed to create dictionary\n");
return 1;

printf("=== Dictionary Implementation using Hashing in C ===\n\n");
// Insert some key-value pairs
printf("Inserting entries...\n");
insert(dict, "apple", 10);
insert(dict, "banana", 20);
insert(dict, "orange", 30);
insert(dict, "grape", 40);
insert(dict, "apple", 99);  // Update existing key
display(dict);
// Search for keys
printf("\nSearching for keys:\n");
int found;
int value = search(dict, "banana", &found);
if (found) printf("banana -> %d\n", value);
else printf("banana not found\n");
value = search(dict, "kiwi", &found);
if (found) printf("kiwi -> %d\n", value);
else printf("kiwi not found\n");
// Delete a key
printf("\nDeleting 'orange'...\n");
if (delete_key(dict, "orange")) 
    printf("'orange' deleted successfully.\n");
 else 
    printf("'orange' not found.\n");
display(dict);
// Check dictionary size
printf("\nTotal number of key-value pairs: %d\n", dict->count);
// Cleanup
destroy_hash_table(dict);
return 0;

Expected Output:

=== Dictionary Implementation using Hashing in C ===
Inserting entries...
3.3 Search Function
char* search(Dictionary *dict, const char *key) 
    unsigned long hash = dict->hash_func(key);
    unsigned long index = hash % dict->size;
Entry *curr = dict->buckets[index];
while (curr) 
    if (strcmp(curr->key, key) == 0) 
        return curr->value;
curr = curr->next;
return NULL; // Not found



4.5 Display the Dictionary
// Display all key-value pairs (for debugging)
void display(HashTable *table) 
    if (!table) return;
printf("\n=== Dictionary Contents (Total: %d entries) ===\n", table->count);
for (int i = 0; i < table->size; i++) 
    if (table->buckets[i]) 
        printf("Bucket[%d]: ", i);
        KeyValuePair *current = table->buckets[i];
        while (current) 
            printf("(%s -> %d) ", current->key, current->value);
            current = current->next;
printf("\n");
printf("==========================================\n");