作者：未知 来源：月光软件站 加入时间：2005-2-28　月光软件站

//6.1 
#include<iostream>//I/O
#include<string>   //字符串
#include<map>    //映射
#include<cctype>//isatpha()

using namespace std;
int no_of_errors;
double error(const string& s)
{
 no_of_errors++;
 cerr<<"error:";
 cerr<<s;
 cerr<<"/n";
 return 1;
}

enum Token_value
{
  NAME,NUMBER,END,PLUS='+',MINUS='-',MUL='*',
  DIV='/',PRINT=';',ASSING='=',LP='(',RP=')'
};
Token_value curr_tok=PRINT;
double number_value;
string string_value;
Token_value get_token()
{
 char ch;
 do{//跳过空白，除了'\n'
       if(!cin.get(ch))return curr_tok=END;
 }while(ch!='\n'&&isspace(ch));
 switch(ch){
 case ';':
 case '\n':
     return curr_tok=PRINT;
 case '*':
 case '/':
 case '+':
 case '-':
 case '(':
 case '=':
    return curr_tok=Token_value(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:     //NAME,NAME=,或者错误
  if(isalpha(ch)){
        string_value=ch;
        while(cin.get(ch)&&isalnum(ch))
              string_value.push_back(ch);
        cin.putback(ch);
        return curr_tok=NAME;
   }
  error("bad token");
  return curr_tok=PRINT;
 }
}
map<string,double> table;
double expr(bool);
double prim(bool get)  //处理初等式
{
    if(get) get_token();
    switch(curr_tok){
    case NUMBER:    //浮点常量
      {double v=number_value;
      get_token();
      return v;
      }
    case NAME:
      {double& v=table[string_value];
      if(get_token()==ASSING)v=expr(true);
      return v;
      }
    case MINUS:     //一元
      return -prim(true);
    case LP:
      {double e=expr(true);
      if(curr_tok!=RP)return error(") expected");
      get_token();
      return e;
      }
    default:
      return error("primary expected");
    }
}
double term(bool get)  //乘和除
{double left=prim(get);
     for(;;)
        switch(curr_tok){
        case MUL:
                  left*=prim(true);
                  break;
        case DIV:
                  if(double d=prim(true))
                  { left/=d;
                    break;
                  }
                 return error("divide by 0");
        default:
                return left;
        }
}
double expr(bool get)       //加和减
{
      double left=term(get);
          for(;;)
            switch(curr_tok){
            case PLUS:
                  left+=term(true);
                  break;
            case MINUS:
                  left-=term(true);
                 break;
            default:
                return left;
            }
}
int main()
{
   table["pi"]=3.1415926535897932385;
   table["e"]=2.7182818284590452354;
   while(cin){
       get_token();
       if(curr_tok==END)break;
       if(curr_tok==PRINT)continue;
       cout<<expr(false)<<"\n";
  }
  return no_of_errors;
}

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