折半查找函数

该函数用于判定已排序的数组v中是否存在某个特定的值x。数组v的元素必须以升序排列。如果v中包含x，则该函数返回x在v中的位置（介于0～n-1之间的一个整数）；否则，该函数返回-1。

/* binsearch:  find x in v[0] <= v[1] <= ... <= v[n-1] */ 
   int binsearch(int x, int v[], int n) 
   { 
       int low, high, mid; 
 
       low = 0; 
       high = n - 1; 
       while (low <= high) { 
           mid = (low+high)/2; 
           if (x < v[mid]) 
               high = mid - 1; 
           else if (x  > v[mid]) 
               low = mid + 1; 
           else    /* found match */ 
               return mid; 
       } 
       return -1;   /* no match */ 
   }

int binsearch(char *word, struct key tab[], int n) 
{ 
       int cond; 
       int low, high, mid; 
 
       low = 0; 
       high = n - 1; 
       while (low <= high) { 
           mid = (low+high) / 2; 
           if ((cond = strcmp(word, tab[mid].word)) < 0) 
               high = mid - 1; 
           else if (cond > 0) 
               low = mid + 1; 
           else 
               return mid; 
       } 
       return -1; 
   }

struct key *binsearch(char *word, struck 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; 
   }