在 C 中实现字典的快速方法 [英] Quick Way to Implement Dictionary in C

问题描述

我在用 C 编写程序时想念的一件事是字典数据结构.在 C 中实现一个最方便的方法是什么?我不是在寻找性能，而是从头开始编写代码的难易程度.我也不希望它是通用的——像 char*→int 这样的东西就可以了.但我确实希望它能够存储任意数量的项目.

One of the things which I miss while writing programs in C is a dictionary data structure. What's the most convenient way to implement one in C? I am not looking for performance, but ease of coding it from scratch. I don't want it to be generic either -- something like char*→int will do. But I do want it to be able to store an arbitrary number of items.

这更像是一个练习.我知道有 3rd 方库可供使用.但是请考虑一下，它们不存在.在这种情况下，实现满足上述要求的字典最快的方法是什么.

This is intended more as an exercise. I know that there are 3rd party libraries available which one can use. But consider for a moment, that they don't exist. In such a situation what's the quickest way you can implement a dictionary satisfying the above requirements.

推荐答案

Section 6.6 of The C Programming Language 展示了一个简单的字典(哈希表)数据结构.我认为没有比这更简单的有用的字典实现了.为了您的方便，我在这里复制了代码.

Section 6.6 of The C Programming Language presents a simple dictionary (hashtable) data structure. I don't think a useful dictionary implementation could get any simpler than this. For your convenience, I reproduce the code here.

struct nlist { /* table entry: */
    struct nlist *next; /* next entry in chain */
    char *name; /* defined name */
    char *defn; /* replacement text */
};

#define HASHSIZE 101
static struct nlist *hashtab[HASHSIZE]; /* pointer table */

/* hash: form hash value for string s */
unsigned hash(char *s)
{
    unsigned hashval;
    for (hashval = 0; *s != ''; s++)
      hashval = *s + 31 * hashval;
    return hashval % HASHSIZE;
}

/* lookup: look for s in hashtab */
struct nlist *lookup(char *s)
{
    struct nlist *np;
    for (np = hashtab[hash(s)]; np != NULL; np = np->next)
        if (strcmp(s, np->name) == 0)
          return np; /* found */
    return NULL; /* not found */
}

char *strdup(char *);
/* install: put (name, defn) in hashtab */
struct nlist *install(char *name, char *defn)
{
    struct nlist *np;
    unsigned hashval;
    if ((np = lookup(name)) == NULL) { /* not found */
        np = (struct nlist *) malloc(sizeof(*np));
        if (np == NULL || (np->name = strdup(name)) == NULL)
          return NULL;
        hashval = hash(name);
        np->next = hashtab[hashval];
        hashtab[hashval] = np;
    } else /* already there */
        free((void *) np->defn); /*free previous defn */
    if ((np->defn = strdup(defn)) == NULL)
       return NULL;
    return np;
}

char *strdup(char *s) /* make a duplicate of s */
{
    char *p;
    p = (char *) malloc(strlen(s)+1); /* +1 for ’’ */
    if (p != NULL)
       strcpy(p, s);
    return p;
}

请注意，如果两个字符串的哈希值发生冲突，可能会导致 O(n) 查找时间.您可以通过增加 HASHSIZE 的值来降低发生冲突的可能性.有关数据结构的完整讨论，请参阅本书.

Note that if the hashes of two strings collide, it may lead to an O(n) lookup time. You can reduce the likelihood of collisions by increasing the value of HASHSIZE. For a complete discussion of the data structure, please consult the book.

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