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I am trying to implement the check function of Speller in CS50. The code below may or may not work. But the thing that gets me is that I don't understand the bit :


if strcasecmp(cursor-> word, tword) !=0)
    cursor= cursor-> next;

Doesn't this mean it is comparing against itself? tword is derived from word. So surely it is comparing against itself??


    // Implements a dictionary's functionality

#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include "dictionary.h"

// Represents number of buckets in a hash table
#define N 26

// Represents a node in a hash table
typedef struct node
{
    char word[LENGTH + 1];
    struct node *next;
}
node;

// declare words to count in dictionary
int dictionarysize = 0;

// Represents a hash table
node *hashtable[N];

// Hashes word to a number between 0 and 25, inclusive, based on its first letter
unsigned int hash(const char *word)
{
    return tolower(word[0]) - 'a';
}

// Loads dictionary into memory, returning true if successful else false
bool load(const char *dictionary)
{
    // Initialize hash table
    for (int i = 0; i < N; i++)
    {
        hashtable[i] = NULL;
    }

    // Open dictionary
    FILE *file = fopen(dictionary, "r");
    if (file == NULL)
    {
        unload();
        return false;
    }

    // Buffer for a word
    char word[LENGTH + 1];

    // Insert words into hash table
    while (fscanf(file, "%s", word) != EOF)
    {
        // TODO
        node *new_node = malloc(sizeof(node));
        if (new_node == NULL)
        {
            unload ();
            return false;
        }
        else
        {
            strcpy (new_node -> word, word);
        }
        //implement hash function to get an index
        int index = hash(word);

        // if corresponding index in hashtable is empty, assign it to the new node
        // else append new node to the start of the linked list
        if (hashtable[index]==NULL)
        {
            hashtable[index] = new_node;
        }
        else
        {
            new_node->next = hashtable [index];
            hashtable [index] = new_node;
        }
        dictionarysize++;
    }

    // Close dictionary
    fclose(file);

    // Indicate success
    return true;
}

// Returns number of words in dictionary if loaded else 0 if not yet loaded
unsigned int size(void)
{
    // TODO
    if (dictionarysize >= 1)
    {
        return dictionarysize;
    }
    else
    {
        return 0;
    }
}

// Returns true if word is in dictionary else false
bool check(const char *word)
{
    // TODO
    {
    //find out length of word
    int lengthword = strlen(word);

    //create an array to store word
    char tword[lengthword + 1];

    // make it lowercase before hashing
    for (int i = 0; i < lengthword; i++)
    {
        tword[i] = tolower(word[i]);
    }
    tword[lengthword] = '\0';

    // hash it and find out which bucket it belongs in
    int bucket = hash(tword);
    node *cursor = hashtable [bucket];

   // compare words
    while (cursor != NULL)
    {
        if (strcasecmp(cursor->word, tword) != 0)
            cursor = cursor->next;
        else
            return true;
    }
    return false;
}



// Unloads dictionary from memory, returning true if successful else false
bool unload(void)
{
    // TODO
    return false;
}




Any help gratefully accepted. Thanks.

1

No, not at all. You're conflating two different var names to be the same variable. Let's look at the code:

// Returns true if word is in dictionary else false
bool check(const char *word)
{
...
if strcasecmp(cursor-> word, tword) !=0)
    cursor= cursor-> next;

The variable tword is indeed derived from the variable word. But, this word is the name of the pointer variable that points to the text passed to the check function.

In the code above, cursor->word is something different. Here, word is the name of an element in a node struct. That particular struct is pointed to by the pointer cursor. So, cursor->word is not the same var as word because cursor->word is a variable in a struct, while word is a stand-alone string var.

Does that explain it for you?

If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)

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