创建一个空哈希表

我正在尝试创建一个结构的空哈希表:

struct htab
{
   size_t capacity;
   size_t size;
   struct pair *data;
};

数据是结构对值的链表列表 . 这些链表包含标记(虚拟值)作为第一个元素 .

struct pair
{
   uint32_t hkey;
   char *key;
   void *value;
   struct pair *next;
};

所以我把它写成容量为4,大小为0.我怎样才能将'data'数组的所有单元格初始化为0?

struct htab *htab_new()
{
   struct htab *newtable = 
   malloc(sizeof(struct htab));
   if (newtable == NULL)
   {
       errx(1, "Not enough memory!");
   }
   newtable->capacity = 4;
   newtable->size = 0;
   newtable->data = calloc(// ??);
   return newtable;
}

另外,我怎么测试这实际上是否有效?

回答(2)

2 years ago

在评论中,OP提到他们从一个例子中学得更好 . (这不是一个答案本身,只是一个例子 . 请考虑这是一个扩展的评论,而不是答案 . )

让我们看一个简单的,现实世界的例子吧;但有些东西不能只是复制粘贴并呈现为自己的作品 .

假设我们需要一个文本或标记的哈希表,比方说

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

struct hashentry {
    struct hashentry   *next;
    size_t              hash;
    unsigned char       data[];
};

struct hashtable {
    size_t              size;
    struct hashentry  **slot;
};

表本身是一个指针数组,并通过链接解决哈希冲突 . 请注意,我实际上只使用键,而不是键值对;这是为了避免将此示例代码复制粘贴并呈现为某人自己的工作 . 我写这篇文章是为了帮助新程序员理解,而不是让作弊者作为他们的家庭作业提交 . (我不是指OP,请注意 . 这些问题通常是通过网络搜索找到的,我会为整个小组编写这些答案,而不仅仅是提问者 . )

表初始化为特定大小:

static inline void hashtable_init(struct hashtable *const ht, const size_t size)
{
    size_t  i;

    if (!ht) {
        fprintf(stderr, "hashtable_init(): No hashtable specified (ht == NULL).\n");
        exit(EXIT_FAILURE);
    } else
    if (size < 1) {
        fprintf(stderr, "hashtable_init(): Invalid hashtable size (size == %zu).\n", size);
        exit(EXIT_FAILURE);
    }

    /* Allocate an array of pointers. */
    ht->slot = calloc(size, sizeof ht->slot[0]);
    if (!ht->slot) {
        fprintf(stderr, "hashtable_init(): Failed to allocate an array of %zu slots.\n", size);
        exit(EXIT_FAILURE);
    }
    ht->size = size;

    /* Mark each slot unused. (On all current architectures, this is unnecessary,
       as calloc() does initialize the pointers to NULL, but let's do it anyway.) */
    for (i = 0; i < size; i++)
        ht->slot[i] = NULL;
}

对于哈希函数,我喜欢用于文本字符串的DJB2 Xor变体 . 它不是特别好(会有碰撞),但它非常简单快速:

static inline size_t  hash_data(const char *data, const size_t datalen)
{
    const char *const ends = data + datalen;
    size_t            hash = 5381;

    while (data < ends)
        hash = (33 * hash) ^ (unsigned char)(*(data++));

    return hash;
}

请注意,我使用 size_t 作为哈希的类型 . 你可以使用你想要的任何类型,但在大多数架构中,它与指针的大小相同,即 .

要向哈希表添加数据条目:

static inline void hashtable_add(struct hashtable *ht, const char *data, const size_t datalen)
{
    struct hashentry *entry;
    size_t            hash, slotnum;

    if (!ht) {
        fprintf(stderr, "hashtable_add(): No hashtable specified (ht == NULL).\n");
        exit(EXIT_FAILURE);
    } else
    if (ht->size < 1) {
        fprintf(stderr, "hashtable_add(): Hashtable has zero size.\n");
        exit(EXIT_FAILURE);
    } else
    if (!data && datalen > 0) {
        fprintf(stderr, "hashtable_add(): data is NULL, but datalen == %zu.\n", datalen);
        exit(EXIT_FAILURE);
    }

    /* Allocate memory for the entry, including the data, and the string-terminating nul '\0'. */
    entry = malloc(sizeof (struct hashentry) + datalen + 1);
    if (!entry) {
        fprintf(stderr, "hashtable_add(): Out of memory (datalen = %zu).\n", datalen);        
        exit(EXIT_FAILURE);
    }

    /* Copy the data, if any. */
    if (datalen > 0)
        memcpy(entry->data, data, datalen);

    /* Ensure the data is terminated with a nul, '\0'. */
    entry->data[datalen] = '\0';

    /* Compute the hash. */
    hash = hash_data(data, datalen);
    entry->hash = hash;

    /* The slot number is the hash modulo hash table size. */
    slotnum = hash % ht->size;

    /* Prepend entry to the corresponding slot chain. */
    entry->next = ht->slot[slotnum];
    ht->slot[slotnum] = entry;
}

当我最初编写如上所述的代码时,我总是将其编写为测试程序,并对其进行测试 . (这在技术上属于_4921范式 . )

在这种情况下,我们可以简单地将插槽数作为命令行参数,并从标准输入读取每一行作为要添加到散列表的数据 .

因为标准C没有实现 getline() ,所以我们最好使用 fgets() ,而使用固定大小的行缓冲区 . 如果我们宣布

#ifndef  MAX_LINE_LEN
#define  MAX_LINE_LEN  16383
#endif

我们有一个预处理器宏 MAX_LINE_LEN ,默认为16383,但可以在编译时使用编译器选项覆盖 . (使用GCC,Intel CC和clang,您可以使用例如 -DMAX_LINE_LEN=8191 将其减半 . )

main() 中,如果参数计数不正确,或者 -h--help 是第一个参数,我喜欢打印用法:

int main(int argc, char *argv[])
{
    char              buffer[MAX_LINE_LEN + 1];
    char             *line;
    size_t            size, i;
    struct hashtable  table;
    char              dummy;

    if (argc != 2 || !strcmp(argv[1], "-h") || !strcmp(argv[1], "--help")) {
        fprintf(stderr, "\n");
        fprintf(stderr, "Usage: %s [ -h | --help ]\n", argv[0]);
        fprintf(stderr, "       %s ENTRIES < DATA-FILE > DOT-FILE\n", argv[0]);
        fprintf(stderr, "\n");
        fprintf(stderr, "This program reads lines from DATA-FILE, adding them to\n");
        fprintf(stderr, "a hash table with ENTRIES slots and hash chaining.\n");
        fprintf(stderr, "When all input lines have been read, the contents of the\n");
        fprintf(stderr, "hash table slots will be output as a Graphviz DOT format graph.\n");
        fprintf(stderr, "\n");
        return EXIT_SUCCESS;
    }

接下来,我们可以尝试将第一个命令行参数解析为 size_t size; . 我喜欢使用"sentinel"字符来检测参数在值之后是否有垃圾(除了空白):

if (sscanf(argv[1], "%zu %c", &size, &dummy) != 1 || size < 1) {
        fprintf(stderr, "%s: Invalid number of hash table entries.\n", argv[1]);
        return EXIT_FAILURE;
    }
    hashtable_init(&table, size);

下一部分是从标准输入中读取每一行,并将它们添加到哈希表中 .

while (1) {

        line = fgets(buffer, sizeof buffer, stdin);
        if (!line)
            break;

        /* Skip leading ASCII whitespace. */
        line += strspn(line, "\t\n\v\f\r ");

        /* Find out the remaining length of the line. */
        size = strlen(line);

        /* Ignore trailing ASCII whitespace. */
        while (size > 0 && (line[size-1] == '\t' || line[size-1] == '\n' ||
                            line[size-1] == '\v' || line[size-1] == '\f' ||
                            line[size-1] == '\r' || line[size-1] == ' '))
            size--;

        /* Ignore empty lines. */
        if (size < 1)
            continue;

        /* Add line to the hash table. */
        hashtable_add(&table, line, size);
    }

    /* Check if fgets() failed due to an error, and not EOF. */
    if (ferror(stdin) || !feof(stdin)) {
        fprintf(stderr, "Error reading from standard input.\n");
        return EXIT_FAILURE;
    }

此时,我们有 tablesize 个插槽 . 我编写测试程序来编写纯文本输出(如果简单)或Graphviz DOT格式输出(如果结构像图形) . 在这种情况下,图形输出格式听起来更好 .

/* Print the hash table contents as a directed graph, with slots as boxes. */
    printf("digraph {\n");

    for (i = 0; i < table.size; i++) {
        struct hashentry *next = table.slot[i];

        /* The slot box. */
        printf("    \"%zu\" [ shape=\"box\", label=\"%zu\" ];\n", i, i);

        if (next) {

            /* The edge from the slot box to the entry oval. */
            printf("    \"%zu\" -> \"%p\";\n", i, (void *)next);

            while (next) {
                struct hashentry *curr = next;

                /* Each entry oval; text shown is the value read from the file. */
                printf("    \"%p\" [ shape=\"oval\", label=\"%s\" ];\n", (void *)curr, curr->data);

                next = next->next;

                /* The edge to the next oval, if any. */
                if (next)
                    printf("    \"%p\" -> \"%p\";\n", (void *)curr, (void *)next);
            }
        } 
    }

    printf("}\n");
    return EXIT_SUCCESS;
}

而已 . 如果使用 10 作为命令行参数编译并运行上述程序,并使用feed

one
two
three
four
five
six
seven
eight
nine
ten

对于它的标准输入,它将输出

digraph {
    "0" [ shape="box", label="0" ];
    "1" [ shape="box", label="1" ];
    "1" -> "0xb460c0";
    "0xb460c0" [ shape="oval", label="three" ];
    "0xb460c0" -> "0xb46080";
    "0xb46080" [ shape="oval", label="one" ];
    "2" [ shape="box", label="2" ];
    "3" [ shape="box", label="3" ];
    "3" -> "0xb46180";
    "0xb46180" [ shape="oval", label="nine" ];
    "0xb46180" -> "0xb460a0";
    "0xb460a0" [ shape="oval", label="two" ];
    "4" [ shape="box", label="4" ];
    "4" -> "0xb46160";
    "0xb46160" [ shape="oval", label="eight" ];
    "0xb46160" -> "0xb46140";
    "0xb46140" [ shape="oval", label="seven" ];
    "5" [ shape="box", label="5" ];
    "5" -> "0xb46100";
    "0xb46100" [ shape="oval", label="five" ];
    "6" [ shape="box", label="6" ];
    "6" -> "0xb461a0";
    "0xb461a0" [ shape="oval", label="ten" ];
    "7" [ shape="box", label="7" ];
    "7" -> "0xb46120";
    "0xb46120" [ shape="oval", label="six" ];
    "0xb46120" -> "0xb460e0";
    "0xb460e0" [ shape="oval", label="four" ];
    "8" [ shape="box", label="8" ];
    "9" [ shape="box", label="9" ];
}

喂给Graphviz dot 会生成一个漂亮的图表:

Hash table graph

如果要查看数据字符串上方的实际哈希值,请更改为

/* Each entry oval; text shown is the value read from the file. */
                printf("    \"%p\" [ shape=oval, label=\"%zu:\\n%s\" ];\n", (void *)curr, curr->hash, curr->data);

正如我所说,DJB2 Xor哈希并不是特别好,对于上面的输入,你需要至少43个哈希表槽来避免链接 .

2 years ago

这些链接列表包含作为第一个元素的标记(虚拟值) .

newtable->data = calloc(capacity, sizeof(*newtable->data));
// but you should check the result of calloc as well, just as you did for malloc!

你会发现钥匙已经分配了一个值实际使用的插槽吗?然后,您将无法使用空指针作为键 . 如果只使用 next 指针,那么为什么要使用结构呢?你可以有一个指针数组,然后sentinel将是一个空指针:

struct pair** data;

有趣的是,有了这个,你就不需要像上面给出的那样改变对calloc的调用( sizeof(*data) 现在将是指针的大小......) .

旁注:见calloc and pointers ......