parser.c

简单编译器

int lookahead;
void match(int t);
void term();
void expr();
void factor();
void parse();
void match(int t)
{
	if(lookahead == t)
	{ 
		if(t==';')
			if((s.top-s.base)==1) 
			{
        	    pop();
            	printf("the result is %d\n",topvalue);
			}
    lookahead = lexan();  }                                  
	else error(" match syntax error");
	
}
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 expr()
{
	int t;
	term();
	while(1)
		switch (lookahead){
		case '+':case '-':
			t=lookahead;
			match(lookahead);term();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("factor syntax error");
	}
}


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