ll.cpp

一个递归下降语法分析器。 测试数据为 i a + nul ( nul i b * nul i c ) nul # nul

#include <fstream.h>
#include <stdio.h>

struct _SYM
{
	char code;
	char val[10];
}SYM;

void E();
void T();
void F();
void ADVANCE();

ifstream cinf("LEX.TXT");

void main()
{
	ADVANCE();
	E();
	printf("\n%s\n", (SYM.code == '#')?"END":"ERROR!");
}

void ADVANCE()
{
	cinf>>SYM.code>>SYM.val;
	printf("%c", SYM.code);
}

void E()			//E→T|{+T}
{
	T();
	while (SYM.code == '+')
	{
		ADVANCE();
		T();
	}
}

void T()			//F→F{*F}
{
	F();
	while (SYM.code == '*')
	{
		ADVANCE();
		F();
	}
}

void F()			//F→i|(E)
{
	if (SYM.code == 'i')
		ADVANCE();
	else
	{
		if (SYM.code == '(')
		{
			ADVANCE();
			E();
			if (SYM.code == ')')
				ADVANCE();
			else
				printf("ERROR!\n");
		}
		else
			printf("ERROR!\n");
	}
}