CS50

Question

我正在做 CS50 拼写问题集。执行我的代码后，从文本文件加载的所有单词都显示为 "Misspelled"。

调试器：进入while循环后，我的光标变为NULL，因此立即跳出while循环。

我运行 Valgrind，没有内存泄漏。 Debug50 貌似索引是正确的，但是一进入while循环，cursor就变成了NULL。

问题一定出在 bool check() 函数中。

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

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

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

// initialize words counted in dictionary to 0
int words_counter = 0;
bool loaded = false;
node *head;

// 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();
        loaded = false;
        return false;
    }

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

    // Insert words into hash table
    while (fscanf(file, "%s", word) != EOF)
    {
        // TODO
        //malloc new node * for each new word,
        // node *head = malloc(sizeof(node));
        node *new_node = malloc(sizeof(node));

        // Check if pointer returns NULL
        if (new_node == NULL)
        {
            unload();
            return false;
        }

        //if malloc succeeds copy word into node
        strcpy(new_node->word, word);


        //Insert the node into linked list, starting at head

        if (head == NULL)
        {
            head = new_node;
            words_counter++;
        } else {
            new_node->next = head;
            head = new_node;
            words_counter++;
        }
    }

    // Close dictionary
    fclose(file);

    // Indicate success
    loaded = true;
    return true;

    }
bool check(const char *word)
{
    int index = hash(word);
    node* cursor = hashtable[index];

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

// Implements a spell-checker

#include <ctype.h>
#include <stdio.h>
#include <sys/resource.h>
#include <sys/time.h>

#include "dictionary.h"

// Undefine any definitions
#undef calculate
#undef getrusage

// Default dictionary
#define DICTIONARY "dictionaries/large"

// Prototype
double calculate(const struct rusage *b, const struct rusage *a);

int main(int argc, char *argv[])
{
    // Check for correct number of args
    if (argc != 2 && argc != 3)
    {
        printf("Usage: speller [dictionary] text\n");
        return 1;
    }

    // Structures for timing data
    struct rusage before, after;

    // Benchmarks
    double time_load = 0.0, time_check = 0.0, time_size = 0.0, time_unload = 0.0;

    // Determine dictionary to use
    char *dictionary = (argc == 3) ? argv[1] : DICTIONARY;

    // Load dictionary
    getrusage(RUSAGE_SELF, &before);
    bool loaded = load(dictionary);
    getrusage(RUSAGE_SELF, &after);

    // Exit if dictionary not loaded
    if (!loaded)
    {
        printf("Could not load %s.\n", dictionary);
        return 1;
    }

    // Calculate time to load dictionary
    time_load = calculate(&before, &after);

    // Try to open text
    char *text = (argc == 3) ? argv[2] : argv[1];
    FILE *file = fopen(text, "r");
    if (file == NULL)
    {
        printf("Could not open %s.\n", text);
        unload();
        return 1;
    }

    // Prepare to report misspellings
    printf("\nMISSPELLED WORDS\n\n");

    // Prepare to spell-check
    int index = 0, misspellings = 0, words = 0;
    char word[LENGTH + 1];

    // Spell-check each word in text
    for (int c = fgetc(file); c != EOF; c = fgetc(file))
    {
        // Allow only alphabetical characters and apostrophes
        if (isalpha(c) || (c == '\'' && index > 0))
        {
            // Append character to word
            word[index] = c;
            index++;

            // Ignore alphabetical strings too long to be words
            if (index > LENGTH)
            {
                // Consume remainder of alphabetical string
                while ((c = fgetc(file)) != EOF && isalpha(c));

                // Prepare for new word
                index = 0;
            }
        }

        // Ignore words with numbers (like MS Word can)
        else if (isdigit(c))
        {
            // Consume remainder of alphanumeric string
            while ((c = fgetc(file)) != EOF && isalnum(c));

            // Prepare for new word
            index = 0;
        }

        // We must have found a whole word
        else if (index > 0)
        {
            // Terminate current word
            word[index] = '[=11=]';

            // Update counter
            words++;

            // Check word's spelling
            getrusage(RUSAGE_SELF, &before);
            bool misspelled = !check(word);
            getrusage(RUSAGE_SELF, &after);

            // Update benchmark
            time_check += calculate(&before, &after);

            // Print word if misspelled
            if (misspelled)
            {
                printf("%s\n", word);
                misspellings++;
            }

            // Prepare for next word
            index = 0;
        }
    }

    // Check whether there was an error
    if (ferror(file))
    {
        fclose(file);
        printf("Error reading %s.\n", text);
        unload();
        return 1;
    }

    // Close text
    fclose(file);

    // Determine dictionary's size
    getrusage(RUSAGE_SELF, &before);
    unsigned int n = size();
    getrusage(RUSAGE_SELF, &after);

    // Calculate time to determine dictionary's size
    time_size = calculate(&before, &after);

    // Unload dictionary
    getrusage(RUSAGE_SELF, &before);
    bool unloaded = unload();
    getrusage(RUSAGE_SELF, &after);

    // Abort if dictionary not unloaded
    if (!unloaded)
    {
        printf("Could not unload %s.\n", dictionary);
        return 1;
    }

    // Calculate time to unload dictionary
    time_unload = calculate(&before, &after);

    // Report benchmarks
    printf("\nWORDS MISSPELLED:     %d\n", misspellings);
    printf("WORDS IN DICTIONARY:  %d\n", n);
    printf("WORDS IN TEXT:        %d\n", words);
    printf("TIME IN load:         %.2f\n", time_load);
    printf("TIME IN check:        %.2f\n", time_check);
    printf("TIME IN size:         %.2f\n", time_size);
    printf("TIME IN unload:       %.2f\n", time_unload);
    printf("TIME IN TOTAL:        %.2f\n\n",
           time_load + time_check + time_size + time_unload);

    // Success
    return 0;
}

// Returns number of seconds between b and a
double calculate(const struct rusage *b, const struct rusage *a)
{
    if (b == NULL || a == NULL)
    {
        return 0.0;
    }
    else
    {
        return ((((a->ru_utime.tv_sec * 1000000 + a->ru_utime.tv_usec) -
                  (b->ru_utime.tv_sec * 1000000 + b->ru_utime.tv_usec)) +
                 ((a->ru_stime.tv_sec * 1000000 + a->ru_stime.tv_usec) -
                  (b->ru_stime.tv_sec * 1000000 + b->ru_stime.tv_usec)))
                / 1000000.0);
    }
}

// Unloads dictionary from memory, returning true if successful 
bool unload(void)
{
// TODO
    for (int i = 0; i < N; i++)
    {
        node *cursor = hashtable[i];
        while (cursor != NULL)
        {
            node *temp = cursor;
            cursor = cursor->next;
            free(temp);
        }    
    }
    return true;
}

// Dictionary.h - Declares a dictionary's functionality

#ifndef DICTIONARY_H
#define DICTIONARY_H

#include <stdbool.h>

// Maximum length for a word
// (e.g., pneumonoultramicroscopicsilicovolcanoconiosis)
#define LENGTH 45

// Prototypes
bool load(const char *dictionary);
unsigned int size(void);
bool check(const char *word);
bool unload(void);

#endif // DICTIONARY_H
~~~

Answer 1

您的 check 函数使用 hashtable 从字典中查找具有给定散列的单词的链接列表。但是，您没有任何代码将加载的单词字典放入 hashtable.

您要么需要在 load 期间执行此操作（这很可能，因为 load 将 hashtable 初始化为空值），或者添加一个额外的步骤来加载字典进入哈希表。

CS50 - 拼写器 - 所有单词都拼错了 - 调试显示光标为 NULL，为什么？

CS50 - speller - All words Misspelled - debug shows cursor to be NULL, why?

c

spell-checking

nodes