exercise in the c++ programming language 3rd edition - desk calculator

I'm not sure what Bjarne meant with this exercise:

"Convert the desk calculator to use a symbol structure instead of using the static variables number_value and string_value."

Did he mean puting those 2 variables inside a structure and then using them through a structure?

Edit: also one exercise related to the clculator and it says: "Allow a user to define functions in the claculator. Hint: Define a function as a sequence of operations just as a user would ave typed them. Such a sequence can be stored either as a char string or as a list of tokens. Then read and execute those operations when the function is called. If you want a user-defined function to take args, you'll have to invent a notation for that."

Could anyone give ma an example of what it means? What sort of functions should user define, i dont get it. Ability to define for example function that returns a ^2 value from its argument for example?

Here's the code.

#include <iostream>
#include <map>

using namespace std;

double term(bool);
double expr(bool);
double prim(bool);
double error(const string&);

double number_value;
string string_value;

int no_of_errors;
map<string, double> table;

enum Token_value {
     NAME,          NUMBER,          END,
     PLUS = '+',    MINUS = '-',     MUL = '*',    DIV = '/',
     PRINT = ';',   ASSIGN = '=',    LP = '(',     RP = ')'
};

Token_value curr_tok = PRINT;
Token_value get_token();

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

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

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

       for(;;) 
    开发者_开发知识库       switch(curr_tok) {
              case MUL:
                   left *= prim(true);
                   break;
              case DIV:
                   if(double d = prim(true)) {
                       left /= d;
                       break;
                   }
                   return error("divide by zero");
              default:
                      return left;
           }
}

double prim(bool get)
{
       if(get)
          get_token();

       switch(curr_tok) {
           case NUMBER: {
                double v = number_value;
                get_token();
                return v;
           }
           case NAME: {
                double &v = table[string_value];
                if(get_token() == ASSIGN)
                   v = expr(true);
                return v;
           }
           case MINUS:
                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");
       }
}

Token_value get_token()
{
       char ch = 0;
       do {
           if(!cin.get(ch)) 
              return curr_tok = END;
       } while(ch != '\n' && isspace(ch));

       switch(ch) {
           case 0:
                return curr_tok = END;
           case ';':
           case '\n':
                return curr_tok = PRINT;
           case '+':
           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:
                   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;
       }
}

double error(const string &s)
{
       no_of_errors++;
       cerr << "error: " << s << '\n';
       return 1;
}

int main()
{
      table["pi"] = 3.14;
      table["e"] = 2.71;

      while(cin) {
          get_token();
          if(curr_tok == END)
             break;
          if(curr_tok == PRINT)
             continue;
          cout << expr(false) << endl;
      }
      return no_of_errors;
}

That's exactly what he means, in my opinion. Also the structure shouldn't be global either, it should be passed as a parameter (by value or reference appropriately).