Source : Free On-Line Dictionary of Computing

binary search
     
         A search {algorithm} which repeatedly divides an
        ordered {search space} in half according to how the required
        (key) value compares with the middle element.
     
        The following pseudo-{C} routine performs a binary search
        return the index of the element of vector "thing[first..last]"
        equal to "target":
     
         if (target < thing[first] || target > thing[last])
           return NOT_FOUND;
         while (first < last)
         {
           mid = (first+last)/2;	/* truncate to integer */
           if (target == thing[mid])
             return mid;
           if (target < thing[mid])
             last = mid-1;
           else
             first = mid+1;
         }
         if (target == thing[last])
           return last;
         return NOT_FOUND;
     
        (2003-01-14)