calculator from Bjarnes book

[adam2016]

Hi guys so I've come across a snag when trying to get my code working for a calculator using tokens anyway in the book Bjarne helps you and gives you the code for the primary() term() and expression() but he leaves out the getToken() function so I tried to implement this myself

after working on this with a piece of paper for 2 hours I can't seem to get the code working on paper it should work but when I run the program all it prints is the first number I entered in

for example

enter an expression
4 + 2 + 2 q // input
4 // output
enter an expression
error
5.18496e+006

#include <iostream>
#include <vector>
#include <exception>
using namespace std;

double term();
int primary();

class Token{

public:

	char kind;
	double value;

	Token(char kind)
	:kind(kind),value(0)
	{}
	Token(char kind,double value)
	:kind(kind),value(value)
	{}
};

vector<Token> tokens;
int number = 0;

void populateTokens(){

	cout << "enter an expression" << endl;

	int number;
	char op;

	while(cin >> number >> op){

		Token token1('8',number);
		tokens.push_back(token1);

		if(op == 'q'){
			return;
		}

		Token token2(op);
		tokens.push_back(token2);
	}
}

Token getToken(){

	for(int i = 0; i < tokens.size(); i++){

		if(i == number){

			return tokens[i];
		}
	}
	return NULL;
}

double expression(){

	int left = term();
	Token t = getToken();
    number++;
    char kind = t.kind;

    while(true){

    	switch(kind){

    	case '+': left += term();
    	kind = '0';
    	break;
    	case '-': left -= term();
    	break;
    	default : return left;

    	}
    }
}

double term(){

	double left = primary();
	Token t = getToken();
	number++;

	while(true){

		switch(t.kind){

		case '*' :
			left *= primary();
			break;
		case '/' :
			left /= primary();
			break;
		default :
			return left;
		}
	}
}

int primary(){

      Token t = getToken();
      number++;
      switch(t.kind){
      case '(' :{
    	  double d = expression();
    	  t = getToken();
    	  number++;
          if(t.kind != ')'){
        	  cout << "error" << endl;
          }
          return d;
      }
      case '8':
    	  return t.value;
      default: cout << "error" << endl;

      }
      }

int main() {

   try{
   while(cin){

	   populateTokens();
	   cout << expression() << endl;

   }
   }catch(exception& e){

	   cerr << e.what() << endl;
   }
   catch(...){

	   cout << "exception" << endl;
   }
}

this is a pretty tricky one

thanks

[kbw]

http://www.stroustrup.com/dc.c
linked from here
http://www.stroustrup.com/3rd_code.html

[adam2016]

thanks kbw much appreciated

but for my own sanity how would I get my code to work AKA write my own getToken() function,I mean I populate the vector of Tokens with the populateTokens() function then I use the getToken() function to get the next Token in the vector,

any ideas how I can tweak my code to make it work,or at least so it calculates the sum (doesn't have to be in the correct order just want to get it working)

thanks

[adam2016]

I thought when I incremented the global number variable this would make the program work but to no avail


also the token function is for later code as far as I know it's for the third version of the program I'm working on the first

thanks

[adam2016]

hi people for anybody reading this in the future I have came up with a solution

It took me about 90 mins but I reviewed the code and found I could use an incremental solution this isn't the most pretty solution but it works

Bjarne implements his own token stream in the book anyway I'm pretty happy I was able to implement my own

#include <iostream>
#include <vector>
#include <exception>
using namespace std;

double term();
int primary();

class Token {

public:

	char kind;
	double value;

	Token(char kind) :
			kind(kind), value(0) {
	}
	Token(char kind, double value) :
			kind(kind), value(value) {
	}
};

vector<Token> tokens;
int number = 0;

void populateTokens() {

	cout << "enter an expression" << endl;

	int number;
	char op;

	while (cin >> number >> op) {

		Token token1('8', number);
		tokens.push_back(token1);

		if (op == 'q') {

			Token quitToken(op);
			tokens.push_back(quitToken);
			return;
		}

		Token token2(op);
		tokens.push_back(token2);

	}
}

Token getToken() {

	for (int i = 0; i < tokens.size(); i++) {

		if (i == number) {

			return tokens[i];
		}
	}

	return NULL;
}

double expression() {

	int left = term();
	Token t = getToken();

	while (true) {

		switch (t.kind) {

		case '+':
			number++;
			left += term();
			t = getToken();
			break;
		case '-':
			number++;
			left -= term();
			t = getToken();
			break;
		default:
			number--;
			return left;

		}
	}
}

double term() {

	double left = primary();
	Token t = getToken();

	while (true) {

		switch (t.kind) {

		case '*':
			number++;
			left *= primary();
			t = getToken();
			break;
		case '/':
			number++;
			left /= primary();
			t = getToken();
			break;
		default:
			return left;

		}
	}
}

int primary() {

	Token t = getToken();
// maybe remove
	number++;

	switch (t.kind) {
	case '(': {
		double d = expression();
		t = getToken();

		if (t.kind != ')') {
			cout << "error" << endl;
		}
		return d;
	}
	case '8':
		return t.value;
	default:
		cout << "error" << endl;

	}
}

int main() {

	try {
		while (cin) {

			populateTokens();
			cout << expression() << endl;
			number = 0;

		}
	} catch (exception& e) {

		cerr << e.what() << endl;

	} catch (...) {
		cout << "exception" << endl;
	}
}

[adam2016]

there is a major flaw with this still,if anybody wants to try solve it feel free =)

and if you solve whats wrong (I already know) please tell me your solution if you have one =)

thanks

[kbw]

I'm not quite sure what the problem is that you've found.