parser.c

一个简单的识别运算的编译器程序

#include "global.h"
#include "lexer.c"

int lookahead;

void parse()
{
	lookahead =lexan();
	while(lookahead!=DONE){
		expr();match(';');
	}
}


void expr()
{
	int t;
	term();
	while(1)
		switch(lookahead){
		case '+': 
		case '-': 
			t=lookahead;
			match(lookahead);term();emit(t,NONE);
			continue;
		default:
			return;
	}
}


void term()
{

	int t;
	factor();
	while(1)
		switch(lookahead){
		case'*':
		case"/":
		case DIV:
		case MOD:
			t=lookahead;
			match(lookahead);factor();emit(t,NONE);
			continue;
		default:
			return;
	}
}


void factor()
{
	switch(lookahead){
	case'(':
		match('(');expr();match(')');break;
	case NUM:
		emit(NUM,tokenval);match(NUM);break;
	case ID:
		emit(ID,tokenval);match(ID);break;
	default:
		error("syntax error");
	}
}

void match(t)
     int t;
{
	if(lookahead==t)
		lookahead=lexan();
	else
		error("syntax error");
}