对The C programming language一书第6.6节代码的理解

代码如下（基本与书中一致）

 1 #include <stdio.h>
 2 #include <string.h>
 3 #include <ctype.h>
 4 #include <stdlib.h>
 5
 6 #define HASHSIZE 101
 7
 8 struct nlist{
 9     struct nlist *next;
10     char *name;
11     char *defn;
12 };
13
14 static struct nlist *hashtab[HASHSIZE];
15 struct nlist *lookup(char *s);
16 struct nlist *install(char *name,char *defn);
17
18 int main(int argc, const char * argv[]) {
19     struct nlist list[] = {
20         {NULL,"abc","123"},
21         {NULL,"bcd","234"},
22         {NULL,"cde","345"},
23         {NULL,"def","567"},
24     };
25     int count = sizeof(list)/sizeof(struct nlist);
26     for (int i = 0; i< count; i++) {
27         struct nlist * p = install(list[i].name, list[i].defn);
28         if (p) {
29             printf("将%s添加到了哈希表里,值为%s\n",p->name,p->defn);
30         }
31     }
32     struct nlist *p = lookup("bcd");
33     if (p) {
34         printf("找到key为bcd时对应的值,值为%s\n",p->defn);
35     }else{
36         printf("没有找到key为bcd时对应的值\n");
37     }
38     return 0;
39 }
40
41 unsigned hash(char *s)
42 {
43     unsigned hashval;
44     for (hashval = 0; *s!='\0'; s++) {
45         hashval = *s+31*hashval;
46     }
47     return hashval%HASHSIZE;
48 }
49
50 struct nlist *lookup(char *s)
51 {
52     struct nlist *np;
53     for (np = hashtab[hash(s)]; np!=NULL; np = np->next) {
54         if (strcmp(s, np->name) == 0) {
55             return np;
56         }
57     }
58     return NULL;
59 }
60
61 char *strsave(char *s)
62 {
63     char *p;
64     p = (char*)malloc(strlen(s)+1);
65     if (p!=NULL) {
66         strcpy(p, s);
67     }
68     return p;
69 }
70
71 struct nlist *install(char *name,char *defn)
72 {
73     struct nlist *np;
74     unsigned hashval;
75     if ((np=lookup(name))==NULL) {
76         np = (struct nlist*)malloc(sizeof(struct nlist));
77         if (np == NULL||(np->name = strsave(name))==NULL) {
78             return NULL;
79         }
80         hashval = hash(name);
81         np->next = hashtab[hashval];
82         hashtab[hashval] = np;
83     }else{
84         free((void*)np->defn);
85     }
86     if ((np->defn = strsave(defn))==NULL) {
87         return NULL;
88     }
89     return np;
90 }

先看下输出：

这一节就是讲了表查找，别的没啥想说的，主要看的时候卡在了install方法的这两句代码:

np->next = hashtab[hashval];

hashtab[hashval] = np;

其实这么做的原因是链表是从后向前添加的，当第一次执行install方法时，hashtab[hashval]这个值是NULL，也就是将np->next的值设为了NULL,然后hashtab[hashval] = np 这样，

当第二次执行到np->next = hashtab[hashval]时就是将第一次执行的那个节点添加到了这个节点的next上，换句话说，第一次执行install方法添加的节点是链表的最后一个节点，然后依次向前添加。