SimpleParser.cpp

#include<iostream>
#include<cstdlib>
#include<cctype>
#include<cstring>
using namespace std;
enum types { DELIMITER = 1, VARIABLE, NUMBER };
class SimpleParser
{
	char *exp_ptr; //Points to the expression
	char token[80]; //holds current token
	char tok_type; // holds token type
	void eval_exp2(double &result);
	void eval_exp3(double &result);
	void eval_exp4(double &result);
	void eval_exp5(double &result);
	void eval_exp6(double &result);
	void get_token();
	int isdelim(char c);
public:
	SimpleParser();
	double eval_exp(char *exp);
};
//Parser constructor
SimpleParser::SimpleParser()
{
	exp_ptr = NULL;
}
//Parser entry point
double SimpleParser::eval_exp(char *exp)
{
	double result;
	exp_ptr = exp;
	get_token();
	if(!*token)
	{
		serror(2); //No expression currently
		return 0.0;
	}
	eval_exp2(result);
	if(*token)
		serror(0); //last token must be NULL
	return result;
}
//Add or subtract
void SimpleParser::eval_exp2(double &result)
{
	register char op;
	double temp;
	eval_exp3(result);
	while((op = *token) == '+' || op == '-')
	{
		get_token();
		eval_exp3(temp);
		switch(op)
		{
		case '-':
			result = result - temp;
			break;
		case '+':
			result = result + temp;
			break;
		}
	}
}
//Multiply or divide
void SimpleParser::eval_exp3(double &result)
{
	register char op;
	double temp;
	eval_exp4(result);
	while((op = *token) == '*' || op == '/' || op == '%')
	{
		get_token();
		eval_exp4(temp);
		switch(op)
		{
		case '*':
			result = result * temp;
			break;
		case '/':
			result = result / temp;
			break;
		case '%':
			result = (int) result % (int) temp;
			break;
		}
	}
}
//Process an exponent
void SimpleParser::eval_exp4(double &result)
{
	double temp, ex;
	register int t;
	eval_exp5(result);
	if(*token== '^')
	{
		get_token();
		eval_exp4(temp);
		ex = result;
		if(temp==0.0)
		{
			result = 1.0;
			return;
		}
		for(t=(int)temp-1; t>0; --t)
			result = result * (double)ex;
	}
}
//Evaluate unary + or -
void SimpleParser::eval_exp5(double &result)
{
	register char op;
	op = 0;
	if((tok_type == DELIMITER) && *token=='+' || *token=='-')
	{
		op = *token;
		get_token();
	}
	eval_exp6(result);
	if(op=='-')
		result = -result;
}
//Process parenthesis expression
void SimpleParser::eval_exp6(double &result)
{
	if((*token == '('))
	{
		get_token();
		eval_exp2(result);
		if(*token != ')')
			serror(1);
		get_token();
	};
	else atom(result);
}
//Get the value of a number
void SimpleParser::atom(double &result)
{
	switch(tok_type)
	{
	case NUMBER:
		result = atof(token);
		get_token();
		return;
	default:
		serror(0);
	}
}
//Display syntax error
void SimpleParser::serror(int error)
{
	static char *e[]= {"Syntax error", "unbalanced parenthesis", "no expression present"};
	cout << e[error] << endl;
}
//Get the next token
void SimpleParser::get_token()
{
	register char *temp;
	tok_type = 0;
	temp = token;
	*temp = '\0';
	if(!*exp_ptr)
		return;
	while(isspace(*exp_ptr))
		++exp_ptr;
	if(strchr("+-*/%^=()", *exp_ptr))
	{
		tok_type = DELIMITER;
		*temp++ = *exp_ptr++;
	}
	else if(isalpha(*exp_ptr))
	{
		while(!isdelim(*exp_ptr))
			*temp++ = *exp_ptr++;
		tok_type = VARIABLE;
	}
	else if(isdigit(*exp_ptr))
	{
		while(!isdelim(*exp_ptr))
			*temp++ = *exp_ptr++;
		tok_type = NUMBER;
	}
	*temp = '\0';
}
//Return true if c is delimiter
int SimpleParser::isdelim(char c)
{
	if(strchr("+-*/%^=()", c || c==9 || c=='\r' || c==0)
		return 1;
	return 0;
}