作业十二——实验二 递归下降语法分析
一、实验目的：
利用C语言编制递归下降分析程序,并对简单语言进行语法分析。
编制一个递归下降分析程序,实现对词法分析程序所提供的单词序列的语法检查和结构分析。
二、实验原理
每个非终结符都对应一个子程序。
该子程序根据下一个输入符号(SELECT集)来确定按照哪一个产生式进行处理，再根据该产生式的右端:
- 每遇到一个终结符，则判断当前读入的单词是否与该终结符相匹配，若匹配，再读取下一个单词继续分析；不匹配，则进行出错处理
- 每遇到一个非终结符，则调用相应的子程序
三、实验要求说明
输入单词串，以“＃”结束，如果是文法正确的句子，则输出成功信息，打印“success”,否则输出“error”，并指出语法错误的类型及位置。
例如：
输入begin a:=9;x:=2*3;b:=a+x end #
输出success
输入x:=a+b*c end #
输出‘end' error
四、实验步骤
1．待分析的语言的语法（参考P90）
2．将其改为文法表示，至少包含
–语句
–条件
–表达式
3. 消除其左递归
4. 提取公共左因子
5. SELECT集计算
6. LL(1)文法判断
7. 递归下降分析程序
连接上次博客（https://www.cnblogs.com/a131452/p/11646345.html）中的代码进行改进：
#include<conio.h>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
char duru[100], zuche[20];
char ch;
int fg,p,m,n,num;
char *index[35]={"begin","if","then","while","do","end","char","double","enum","float","int","long","short","signed",
"struct","union","unsigned","void","for","break","continue","else","goto","switch","case","default",
"return","auto","extern","register","static","const","sizeof","typedef","volatile"};
void E();
void T();
void E1();
void T1();
void F();
void error();
void scaner(){
for (n=0;n<20;n++)
zuche[n]=NULL;
m=0;
num=0;
ch=duru[p++];
while(ch==' '){ //跳过空格
ch=duru[p++];
}
if(ch>='a'&&ch<='z'){ //判断单词
while(ch>='a'&&ch<='z'){
zuche[m++]=ch;
ch=duru[p++];
}
fg=10;
p--;
for(n=0;n<6;n++){
if(strcmp(zuche,index[n])==0){
fg=n+1;
break;
}
}
}
else{
if(ch>='0'&&ch<='9'){ //判断数字
while(ch>='0'&&ch<='9'){
num=num*10+(ch-'0');
ch=duru[p++];
}
fg=11;
p--;
}
else{
switch(ch){
case '<':
zuche[m++]=ch;
ch=duru[p++];
if(ch=='>'){ //<>
fg=22;
zuche[m++]=ch;
}
else if(ch=='='){ //<=
fg=21;
zuche[m++]=ch;
}
else{ //<
fg=20;
p--;
}
break;
case '>':
m=0;
zuche[m++]=ch;
ch=duru[p++];
if(ch=='='){ //>=
fg=24;
zuche[m++]=ch;
}
else{ //>
fg=23;
p--;
}
break;
case ':':
m=0;
zuche[m++]=ch;
ch=duru[p++];
if(ch=='='){ //:=
fg=18;
zuche[m++]=ch;
}
else{ //:
fg=17;
p--;
}
break;
case '+':fg=13;zuche[0]=ch;break;
case '-':fg=14;zuche[0]=ch;break;
case '*':fg=15;zuche[0]=ch;break;
case '/':fg=16;zuche[0]=ch;break;
case '=':fg=25;zuche[0]=ch;break;
case ';':fg=26;zuche[0]=ch;break;
case '(':fg=27;zuche[0]=ch;break;
case ')':fg=28;zuche[0]=ch;break;
case '#':fg=0;zuche[0]=ch;break;
default:fg=-1;zuche[0]=ch;break;
}
}
}
}
void E(){
T();
E1();
}
void E1(){
if(fg==13){
scaner();
T();
E1();
}
else if (fg==0||fg==28){
}
else
error();
}
void T(){
F();
T1();
}
void T1(){
if(fg==15){
scaner();
F();
T1();
}
else if (fg==0||fg==28||fg==13){
}
else
error();
}
void F(){
if(fg==27){
scaner();
E();
if(fg==28)
scaner();
else
error();
}
else if (fg==11||fg==10)
scaner();
else
error();
}
void error(){
printf("\n(%s,出错！)",zuche);
}
int main(){
p=0;
printf("请输入符号串(End with '#'):\n");
do
{
ch=getchar();
duru[p++]=ch;
}while(ch!='#');
p=0;
do
{
scaner();
E();
}while(fg!=0);
getch(); //用于让程序停留在显示页面
}
结果截图：
2、
#include<conio.h>
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
char duru[100], zuche[20];
char ch;
int fg,p,m,n,num;
char *index[35]={"begin","if","then","while","do","end","char","double","enum","float","int","long","short","signed",
"struct","union","unsigned","void","for","break","continue","else","goto","switch","case","default",
"return","auto","extern","register","static","const","sizeof","typedef","volatile"};
int kk=0; //判断是否有错误
void scaner();
void panduan();
void yinzi();
void yuju();
void yucuan();
void tiaojian();
void biaoda();
void xiang();
void scaner(){
for (n=0;n<20;n++)
zuche[n]=NULL;
m=0;
num=0;
ch=duru[p++];
while(ch==' '){ //跳过空格
ch=duru[p++];
}
if(ch>='a'&&ch<='z'){ //判断单词
while(ch>='a'&&ch<='z'){
zuche[m++]=ch;
ch=duru[p++];
}
fg=10;
p--;
for(n=0;n<6;n++){
if(strcmp(zuche,index[n])==0){
fg=n+1;
break;
}
}
}
else{
if(ch>='0'&&ch<='9'){ //判断数字
while(ch>='0'&&ch<='9'){
num=num*10+(ch-'0');
ch=duru[p++];
}
fg=11;
p--;
}
else{
switch(ch){
case '<':
zuche[m++]=ch;
ch=duru[p++];
if(ch=='>'){ //<>
fg=22;
zuche[m++]=ch;
}
else if(ch=='='){ //<=
fg=21;
zuche[m++]=ch;
}
else{ //<
fg=20;
p--;
}
break;
case '>':
m=0;
zuche[m++]=ch;
ch=duru[p++];
if(ch=='='){ //>=
fg=24;
zuche[m++]=ch;
}
else{ //>
fg=23;
p--;
}
break;
case ':':
m=0;
zuche[m++]=ch;
ch=duru[p++];
if(ch=='='){ //:=
fg=18;
zuche[m++]=ch;
}
else{ //:
fg=17;
p--;
}
break;
case '+':fg=13;zuche[0]=ch;break;
case '-':fg=14;zuche[0]=ch;break;
case '*':fg=15;zuche[0]=ch;break;
case '/':fg=16;zuche[0]=ch;break;
case '=':fg=25;zuche[0]=ch;break;
case ';':fg=26;zuche[0]=ch;break;
case '(':fg=27;zuche[0]=ch;break;
case ')':fg=28;zuche[0]=ch;break;
case '#':fg=0;zuche[0]=ch;break;
default:fg=-1;zuche[0]=ch;break;
}
}
}
}
//程序，判断是否以begin开始,end #结束
void panduan(){
if (fg==1) { //begin
scaner();
yucuan();
if (fg==6) { //end
scaner();
if (fg==0 && kk==0){
printf("success \n");
}
}
else {
printf("error,lose 'end' ! \n");
kk=1;
exit(0);
}
}
else {
printf("error,lose 'begin' ! \n");
kk=1;
exit(0);
}
return;
}
//语句串
void yucuan() {
yuju();
while(fg==26) {
scaner();
yuju();
}
return;
}
//语句
void yuju(){
if (fg==10) { //为标识符
scaner();
if (fg==18) { // :=
scaner();
biaoda();
}
else {
printf("error!");
kk=1;
exit(0);
}
}
else if(fg==2){ // if
tiaojian();
scaner();
if(fg==3){
yuju();
}
else{
printf("error,lose 'then' ! \n");
kk=1;
exit(0);
}
}
else {
printf("error!");
kk=1;
exit(0);
}
return;
}
//条件
void tiaojian(){
biaoda();
if(fg==25||fg==0||fg==20||fg==21||fg==23||fg==24){
scaner();
}
else{
printf("error! \n");
kk=1;
exit(0);
}
biaoda();
return;
}
//表达式
void biaoda(){
xiang();
while(fg==13 || fg==14) {
scaner();
xiang();
}
return;
}
//项
void xiang(){
yinzi();
while(fg==15 || fg==16) {
scaner();
yinzi();
}
return;
}
//因子
void yinzi(){
if(fg==10 || fg==11){
scaner(); //为标识符或整常数时，读下一个单词符号
}
else if(fg==27) {
scaner();
biaoda();
if(fg==28){
scaner();
}
else {
printf(" ')' 错误\n");
kk=1;
exit(0);
}
} else {
printf("表达式错误\n");
kk=1;
exit(0);
}
return;
}
int main(void){
p=0;
int i;
printf("请输入源程序:\n");
do {
scanf("%c",&ch);
duru[p++]=ch;
} while(ch!='#');
p=0;
do{
scaner();
panduan();
}while(fg!=0);
printf("语法分析结束！\n");
}
截图如下：
