DocScript SVN

#include <cctype>
#include <iostream>


class Input;

double expression(Input&);
double term(Input&);
double factor(Input&);


using namespace std;


class Input {
		char const *ch;
		int current;
		double num;
	public:
		enum { Number = 256 };
		int operator()()        { return current; }
		double number()         { return num; }
		Input& next();
		Input(char const *ch):  ch(ch) { next(); }
};

Input& Input::next() {
	while ( isspace(*ch) )
		++ch;
	if ( isdigit(*ch) ) {
		num = *ch++ - '0';
		while ( isdigit(*ch) )
			num = num * 10 + (*ch++ - '0');
		current = Number;
	}
	else switch ( *ch ) {
		case '+':   current = *ch++; break;
		case '-':   current = *ch++; break;
		case '*':   current = *ch++; break;
		case '/':   current = *ch++; break;
		case '(':   current = *ch++; break;
		case ')':   current = *ch++; break;
		case '\0':  current = *ch; break;
		default:
			throw "invalid character";
	}
	return *this;
}


double expression(Input& input)
{
	double value = term(input);
	while ( 1 )
		if ( input() == '+' )
			value += term(input.next());
		else if ( input() == '-' )
			value -= term(input.next());
		else
			break; 
	return value;
}

double term(Input& input)
{
	double value = factor(input);
	while ( 1 )
		if ( input() == '*' )
			value *= factor(input.next());
		else if ( input() == '/' )
			value /= factor(input.next());
		else
			break;
	return value;
}

double factor(Input& input)
{
	double value;
	if ( input() == Input::Number )
		value = input.number(), input.next();
	else if ( input() == '-' )
		value = - expression(input.next());
	else if ( input() == '(' ) {
		value = expression(input.next());
		if ( input() == ')' )
			input.next();
		else
			throw "')' expected"; }
	else
		throw "factor expected";
	return value;
}


int main(int argc, char *argv[])
{
	try {
		while ( *++argv ) {
			Input input(*argv);
			cout << *argv << '=' << expression(input) << endl;
		}
	}
	catch ( const char* exception ) {
		cerr << "Error: " << exception << endl;
		return 1;
	}
	return 0;
}