Справочное руководство по C++ — страница 37 из 41

name* look(char* p, int ins = 0)
{
 int ii= 0;
 char *pp = p;
 while (*pp) ii = ii‹‹1 ^ *pp++;
 if (ii ‹ 0) ii = -ii;
 ii %= TBLSZ;

 for (name* n=table [ii]; n; n=n-›next)
  if (strcmp(p,n-›string) == 0) return n;

 if (ins == 0) error("name not found");

 name* nn = new name;
 nn-›string = new char[strlen(p) + 1];
 strcpy(nn-›string,p);
 nn-›value = 1;
 nn-›next = table[ii];
 table[ii] = nn;
 return nn;
}

inline name* insert(char* s) { return look (s,1); }

token_value get_token();
double term();

double expr()
{
 double left = term();

 for (;;)
  switch (curr_tok) {
  case PLUS:
   get_token();
   left += term();
   break;
  case MINUS:
   get_token();
   left -= term();
   break;
  default:
   return left;
  }
}

double prim();

double term()
{
 double left = prim();

 for (;;)
  switch (curr_tok) {
  case MUL:
   get_token();
   left *= prim();
   break;
  case DIV:
   get_token();
   double d = prim();
   if (d == 0) return error("divide by 0");
   left /= d;
   break;
  default:
   return left;
  }
}
int number_value;
char name_string[80];

double prim()
{
 switch (curr_tok) {
 case NUMBER:
  get_token();
  return number_value;
 case NAME:
  if (get_token() == ASSIGN) {
   name* n = insert(name_string);
   get_token();
   n-›value = expr();
   return n-›value;
  }
  return look(name_string)-›value;
 case MINUS:
  get_token();
  return -prim();
 case LP:
  get_token();
  double e = expr();
  if (curr_tok != RP) return error(") expected");
  get_token();
  return e;
 case END:
  return 1;
 default:
  return error ("primary expected");
 }
}

token_value get_token()
{
 char ch = 0;

 do {
  if (!cin.get(ch)) return curr_tok = END;
 } while (ch !='\n' && isspace(ch));

 switch (ch) {
 case ';':
 case '\n':
  cin ›› WS;
  return curr_tok=PRINT;
 case '*':
 case '/':
 case '+':
 case '-':
 case '(':
 case ')':
 case '=':
  return curr_tok=ch;
 case '0': case '1': case '2': case '3': case '4':
 case '5': case '6': case '7': case '8': case '9':
 case '.':
  cin.putback(ch);
  cin ›› number_value;
  return curr_tok=NUMBER;
 default:
  if (isalpha(ch)) {
   char* p = name_string;
   *p++ = ch;
   while (cin.get(ch) && isalnum(ch)) *p++ = ch;
   cin.putback(ch);
   *p = 0;
   return curr_tok=NAME;
  }
  error ("bad token");
  return curr_tok=PRINT;
 }
}

int main(int argc, char* argv[])
{
 switch (argc) {
 case 1:
  break;
 case 2:
  cin = *new istream(strlen(argv[1]),argv[1]);
  break;
 default:
  error("too many arguments");
  return 1;
 }

 // insert predefined names:
 insert("pi")-›value = 3.1415926535897932385;
 insert("e")-›value = 2.7182818284590452354;

 while (1) {
  get_token();
  if (curr_tok == END) break;
  if (curr_tok == PRINT) continue;
  cout ‹‹ expr() ‹‹ "\n";
 }

 return no_of_errors;
}

b3_2_6a.cxx

extern void strcpy(char *,char *);
extern void exit(int);
extern int strlen(char *);

char *save_string(char* p)
{
 char* s = new char[strlen(p)+1];
 strcpy(s,p);
 return s;
}

int main (int argc, char* argv[])
{
 if (argc ‹ 2) exit(1);
 int size = strlen(argv[1])+1;
 char* p = save_string (argv[1]);
 delete[size] p;
}

b3_2_6b.cxx

#include ‹stream.hxx›

extern void exit(int);