一个计算器的C语言实现「建议收藏」

#include <stdio.h>
#include <stdlib.h>

char token;

double exp(void);
double term(void);
double factor(void);
char getPrintableChar(void);

void match(char expectedToken);
void error(void);


int main(void)
{
	double result;

	for (;;)
	{
		token = getPrintableChar();
		if (token == 'q')
			break;

		result = exp();
		if (token == '\n')
			printf("Result is: %g\n", result);
		else
			error();
	}

	return 0;
}

double exp(void)
{
	double temp = term();
	while (token == '+' || token == '-')
		switch(token)
		{
			case '+': match('+');
					  temp += term();
					  break;
			case '-': match('-');
					  temp -= term();
					  break;
		}
	return temp;
}

double term(void)
{
	double temp = factor();
	while (token == '*' || token == '/')
		switch(token)
		{
			case '*': match('*');
					  temp *= factor();
					  break;
			case '/': match('/');
					  temp /= factor();
					  break;
		}
	return temp;
}

double factor(void)
{
	double temp;
	if (token == '(')
	{
		match('(');
		temp = exp();
		match(')');
	} else if (isdigit(token))
	{
		ungetc(token, stdin);
		scanf("%lf", &temp);
		token = getPrintableChar();
	} else
		error();

	return temp;
}

void error(void)
{
	fprintf(stderr, "Error!\n");
	exit(EXIT_FAILURE);
}

void match(char expectedToken)
{
	if (expectedToken == token)
		token = getPrintableChar();
	else
		error();
}

char getPrintableChar(void)
{
	char temp;
	do
		temp = getchar();
	while (isblank(temp));

	return temp;
}

<exp> -> <term> { <addop> <term> }
<addop> -> + | -
<term> -> <factor> { <mulop> <factor> }
<mulop> -> * | /
<factor> -> ( <exp> ) | Number