Exercise 6-5: Remove a name from the table

Write a function undef that will remove a name and definition from the table maintained by lookup and install.

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 != '\0'; 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 */
}

/* 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;
}

Use it in a program whose input lines are commands: define name text installs name with the rest of the line, after one blank, as its text; undef name removes name; and lookup name prints name's text on a line of its own, or undefined if it has none. Names have no blanks, and lines are at most 1000 characters.