parser.cpp

编译原理作业

#include "global.h"

int lexan(); 
void expr();
void term();
void factor();
void match(int t);
void emit(int,int tval);
void error(char *m);

extern int tokenval;

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