How to Approach and Solve Open Hash Table-Based Assignments in C

Breaking Down the Requirements

The assignment specifies that:

• An Open Hash Table must be used.
• The user provides:
• The size of the hash table.
• The file name containing words to process.
• The program should:
• Convert all words to lowercase.
• Ignore special characters and numbers.
• Count the total words and unique words.
• Print statistics about the hash table.

Key Constraints and Assumptions

To simplify the implementation, the following assumptions are given:

• The input size is always positive.
• The file always exists.
• Words and file names are at most 254 characters long.

Why Use an Open Hash Table?

An Open Hash Table is an efficient way to store and retrieve data quickly. Unlike closed hashing, which requires a rehashing strategy for collisions, open hashing uses linked lists at each index to handle multiple entries effectively.

Designing the Solution

Defining the Open Hash Table Structure

An Open Hash Table is implemented using an array where each index contains a linked list to handle collisions.

Data Structures Required

• Node Structure:
• Stores a word (string).
• Points to the next node in case of collisions.
• Hash Table Structure:
• An array of linked lists.
• A function to create the table dynamically.

Function Prototypes to Implement

• OpenHash* newOpenHash(int size); - Initializes the hash table.
• int hash(char* word, int n); - Computes the hash index.
• bool member(char* word, OpenHash* table); - Checks if a word exists in the table.
• void insert(char* word, OpenHash* table); - Inserts a word into the hash table if not already present.

How Hash Tables Improve Efficiency

A well-implemented hash table allows for nearly constant-time complexity, O(1), for insertions and lookups. This makes it ideal for handling large datasets efficiently.

Implementing the Solution

Setting Up the Hash Function

A crucial part of the implementation is the hash function, which maps words to indices in the hash table.

int hash(char* word, int n) { int total = 0; for (int i = 0; word[i] != 0; i++) { total = (total + (int)word[i]) * 101 % n; } return total; }

This function ensures that the word is mapped consistently within the given table size.

Creating and Initializing the Hash Table

OpenHash* newOpenHash(int size) { OpenHash* table = (OpenHash*)malloc(sizeof(OpenHash)); table->size = size; table->array = (Node**)malloc(size * sizeof(Node*)); for (int i = 0; i < size; i++) { table->array[i] = NULL; } return table; }

Checking for Word Existence

bool member(char* word, OpenHash* table) { int index = hash(word, table->size); Node* temp = table->array[index]; while (temp) { if (strcmp(temp->word, word) == 0) { return true; } temp = temp->next; } return false; }

Inserting a Word into the Hash Table

void insert(char* word, OpenHash* table) { if (!member(word, table)) { int index = hash(word, table->size); Node* newNode = (Node*)malloc(sizeof(Node)); strcpy(newNode->word, word); newNode->next = table->array[index]; table->array[index] = newNode; } }

Handling Word Processing

Cleaning and Normalizing Words

A function to convert words to lowercase and remove special characters:

void cleanWord(char* word) { int j = 0; for (int i = 0; word[i] != 0; i++) { if (isalpha(word[i])) { word[j++] = tolower(word[i]); } } word[j] = '\0'; }

Reading Words from the File

void processFile(char* fileName, OpenHash* table) { FILE* file = fopen(fileName, "r"); char word[255]; int totalWords = 0, uniqueWords = 0; while (fscanf(file, "%s", word) != EOF) { cleanWord(word); if (strlen(word) > 0) { totalWords++; if (!member(word, table)) { uniqueWords++; insert(word, table); } } } fclose(file); printf("Total Words: %d\nUnique Words: %d\n", totalWords, uniqueWords); }

Debugging and Testing the Program

Checking the Hash Table Structure

After inserting all words, print the hash table to see how well the words are distributed.

void printTable(OpenHash* table) { for (int i = 0; i < table->size; i++) { printf("Row %d: ", i); Node* temp = table->array[i]; while (temp) { printf("%s -> ", temp->word); temp = temp->next; } printf("NULL\n"); } }

Conclusion

By following this structured approach, solving assignments involving Open Hash Tables becomes more manageable. Understanding the problem, designing a plan, implementing in small parts, debugging, and optimizing ensures a successful solution. Apply these steps, and you'll efficiently tackle similar problems in future assignments!