calculator.c

unix环境下实现的cmm语言编译器

/*calculator.c	Simple integer arithmetic calculator
	according to the EBNF:
	
	<exp> -> <term> { <addop> <term>}
	<addop> -> + | -
	<term> -> <pow> {<mulop> <pow>}
	<mulop> -> * | / | %
	<pow> -><factor> [<powop> <factor]
	<powop> -> ^
	<factor> ->[-]( ( <exp> ) | Number)
	
	Inputs a line of text from stdin
	Outputs "Eoor" or the result.
 */

/*标准库函数声明*/
int getchar(void);
int printf(char *fmt, int);
void exit(int);
int puts(char *);
int isdigit(char s);
int sscanf(char *s, char *fmt, int *temp);

char token; /* global token variable */

/* function  prototypes for recursive calles */
int expi(void);
int	term(void);
int factor(void);
int powi(void);
int pow(int, int);

void error(void)
{
	puts("Error\n");
	exit(1);
}
void match(char expect)
{
	if(token == expect)
		token = getchar();
	else
		error();
}
int main()
{
	int result;
	token = getchar();	/* load token with first 
							character for lookahead */

	result = expi();
	if( token == '\n')	/*check for end of line */
		printf("Result = %d\n", result);
	else
		error();

	return 0;
}

int expi(void)
{
	int temp = term();
	while (token != '\n') {
		if (token != '+')
			if (token != '-')
				break;
		if (token == '+') {
			match('+');
			temp =  temp + term();
			continue;
		}
		if (token == '-') {
			match('-');
			temp = temp - term();
			continue;
		}
	}
	return temp;
}

int term(void)
{
	int temp = powi();

	while (token != '\n') {
		if (token != '*') if (token != '/') if (token != '%')
			break;
		if (token == '*') {
			match('*');
			temp = temp * powi();
			continue;
		}
		if (token == '/') {
			match('/');
			temp = temp / powi();
			continue;
		}
		if (token == '%') {
			match('%');
			temp = temp % powi();
			continue;
		}
	}

	return temp;
}

int powi(void)
{
	int temp;

	temp = factor();
	if(token == '^') {
		match('^');
		temp = pow(temp, powi());
	}
	return temp;
}

int factor(void)
{
	int temp;
	char sig = 0;

	if(token == '-') {
		sig = '-';
		match('-');		
	}

	if(token == '(') {
		match('(');
		temp = expi();
		match(')');
	}
	else if(isdigit(token)) {
		char buf[25];
		int i = 0;
		while (i < sizeof buf){
			buf[i] = token;
			token = getchar();
			i = i + 1;
			if (!isdigit(token))
				break;
		}
		buf[i] = 0;
		sscanf(buf, "%d", &temp);
	}	
	else
		error();
	
	if(sig == '-')
		temp = -temp;
	return  temp;
}
/*pow:计算x^n*/
int pow(int x, int n)
{
	int r = 1;
	if (n == 0)
		return 1;
	while (n) {
		r = r * x;
		n = n - 1;
	}
	return r;
}