k&r(6.4): Pointers to Structures

/* k&r(6.4): Pointers to Structures
   created on Mar 27, 2011
   */

#include    "stdio.h"
#include    "ctype.h"
#include    "string.h"
#define     MAXWORD 100

#define     NKEYS (sizeof keytab / sizeof keytab[0])

struct key {
    char *word;
    int count;
} keytab[] = {
    "auto", 0,
    "break", 0,
    "case", 0,
    /* ...*/
    "while", 0
};

int getword(char *, int);
struct key *binsearch(char *, struct key *, int);

/* count C keywords; pointer version  */
int main()
{
    char word[MAXWORD];
    struct key *p;

    while (getword(word, MAXWORD) != EOF)
      if (isalpha(word[0]))
        if ((p = binsearch(word, keytab, NKEYS)) != NULL)
         p->count++;
    for (p = keytab; p < keytab + NKEYS; p++)
      if (p->count > 0)
        printf ("%4d %s\n", p->count, p->word);
    return 0;
}

/* binsearch: find word in tab[0]...tab[n-1] */
struct key *binsearch(char *word, struct key *tab, int n)
{
    int cond;
    struct key *low = &tab[0];
    struct key *high = &tab[n];
    struct key *mid ;

    while (low < high) {
        mid = low + (high-low)/2;
        if ((cond = strcmp(word, mid->word)) < 0)
          high = mid;
        else if (cond > 0)
          low = mid + 1;
        else
          return mid;
    }
    return NULL;
}

 /* getword: get next word or character from input */
int getword(char *word, int lim)
{
    int c, getch(void);
    void ungetch(int);
    char *w = word;

    while (isspace(c = getch())) //not getchar() here
      ;
    if (c != EOF)
      *w++ = c;
    if (!isalpha(c)) {
        *w = '\0';
        return c;
    }
    for (; --lim > 0; w++)
      if (!isalnum(*w = getch())) {
          ungetch(*w);
          break;
      }
    *w = '\0';
    return word[0];
}