First, the purpose of the experiment:
The use of C language compiled recursive descent parsing procedures, and simple language parsing.
The preparation of a recursive descent parser, syntax checking and realize the structure of the word lexical analysis program provided by sequence analysis.
Second, the experimental principle
Each nonterminal symbol corresponds to a subroutine.
This subroutine is determined according to a next input symbol (SELECT sets) which are processed in a production, then the right end of the production according to:
- Each encounter a terminator, it is determined whether the current read word matches the terminator, if matched, then the next word is read to continue the analysis; do not match, an error process is performed
- Each encounter a nonterminal, the appropriate subroutine is called
Third, the experiment asked for clarification
Enter the word string to "#" end if the grammar is correct sentences, the output success information, print "success", otherwise output "error", and pointed out grammatical errors of the type and location.
E.g:
Input begin a: = 9; x: = 2 * 3; b: = a + x end #
Output success
Input x: = a + b * c end #
Output 'end' error
Fourth, the experimental procedures
1. Grammar of the language to be analyzed (refer to P90)
2. Grammar to be expressed, at least comprising
- Statement
-condition
-expression
3. eliminate left recursion
4. Extract the left common factor
5. SELECT set computing
6. LL (1) grammar is determined
7. recursive descent parser
solution:
#include <stdio.h>
#include <the iostream>
#include <string.h>
the using namespace STD;
char * Reserved [. 6] = { "the begin", "IF", "the then", "the while", "do" , "End"};
char INPUT [80], Output [. 8];
char CH;
int SYN, p, I; // SYN code is a species other, p is the scanning adding
int m = 0, n, row , sum 0 =, COUNT = 0;
int flag = 0; // flag
void parseS (); // parse the statement
void ParseS1 ();
void ParseS2 ();
void ParseS3 ();
void ParseS4 ();
void PARSEC (); analysis conditions //
void ParseE (); // analytic expression
void ParseE1 ();
void PARSET (); // resolvables
void ParseT1 ();
void ParseF (); // parse factor
void Scanner () {
for (I = 0; i <8;i++ ){
output[i] = NULL;++ I) {
} // initialize output
ch = input[p++];
while( ch == ' ' ){
ch = input[p] ;
p ++ ;
}
//持续读入
if( ( ch >= 'a' && ch <= 'z' ) || ( ch >= 'A' && ch <= 'Z' ) ){
m = 0 ;
while( ( ch >= 'a' && ch <= 'z' ) || ( ch >= 'A' && ch <= 'Z' ) || ( ch >= '0' && ch <= '9' ) ){
output[m++] = ch ;
ch = input[p++];
}
output[m++] = '\0';
p -- ;
syn = 10 ;
for( n = 0 ; n < 6 ; n ++ ){
if( strcmp(output,reserved[n]) == 0 ){
syn = n + 1;
break;
}
}
}else if( ( ch >= '0' && ch <= '9' ) ){
sum = 0 ;
while( ch >= '0' && ch <= '9' ){
sum=sum*10+ch-'0' ;
ch = input[p++] ;
}
p -- ;
syn = 11;
}
else if(ch == '/'){
ch = input[p++] ;
if(ch == '/'){
while(ch != '\n'){
ch = input[p++] ;
}
scanner();
}
else if(ch == '*'){
ch = input[p++];
int flag=0;
// printf("%c %c",ch,input[p]);
while((ch == '*' && input[p] != '/')||(ch != '*' && input[p] == '/') || (ch != '*' && input[p] != '/')){
ch = input[p++] ;
if(p==count-2 && (ch != '*' && input[p] != '/')){
flag =1;
syn = 100;
break;
}
}
p++;
if(flag==0){
scanner();
}
}else{
p = p - 2 ;
ch = input[p++] ;
output[0] = ch ;
syn = 16 ;
}
}else switch(ch){
case '+':
output[0] = ch ;
syn = 13;
break;
case '-':
output[0] = ch ;
syn = 14;
break;
case '*':
output[0] = ch ;
syn = 15 ;
break;
case '/':
output[0] = ch ;
syn = 16 ;
break;
case ':':
i = 0;
output[i++] = ch ;
ch = input[p++];
if( ch == '=' ){
output[i++] = ch;
syn = 18 ;
}else{
syn = 17;
p-- ;
}
break;
case '<':
i = 0 ;
output[i++] = ch ;
ch = input[p++] ;
if( ch == '=' ){
output[i++] = ch ;
syn = 21 ;
}else if( ch == '>' ){
output[i++] = ch ;
syn = 22 ;
}else{
syn = 20 ;
p-- ;
}
break;
case '>':
i = 0;
output[i++] = ch ;
ch = input[p++] ;
if( ch == '=' ){
output[i++] = ch;
syn=24;
}else{
syn=23;
p--;
}
break;
case '=':
output[0] = ch ;
syn = 25 ;
break;
case ';':
output[0] = ch ;
syn = 26 ;
break;
case '(':
output[0] = ch ;
syn = 27 ;
break;
case ')':
output[0] = ch ;
syn = 28 ;
break;
case '#':
output[0] = ch ;
syn = 0;
break;
case '\n':
syn = 99 ;
break;
default:
syn = -1 ;
break;
}
}
void ParseS(){
if(flag != 0){
if(syn == 10){ //<id>:=<表达式>
scanner();
ParseS1();
}
else if(syn == 2){ //if
scanner();
ParseS2();
}
else if(syn == 4){//while
scanner();
ParseS3();
}
else if(syn == 0){
}
else{
printf("statement syntx error S\n");
exit(0);
}
}
else{
if(syn == 1){//begin
scanner();
flag = 1;
ParseS4();
}else{
printf("error,缺少begin!\n");
exit(0);
}
}
}
void ParseS1(){
if(syn==18){
scanner();
ParseE();
}
else{
printf("statement syntx error S1\n");
exit(0);
}
}
void ParseS2(){//if
ParseC();
if(syn== 3 ){
scanner();
ParseS();
}
else{
printf("statement syntx error S2\n");
exit(0);
}
}
void ParseS3(){//while
ParseC();
if(syn== 5 ){
scanner();
ParseS();
}
else{
printf("statement syntx error S3 \n");
exit(0);
}
}
void ParseS4(){//begin
ParseS();
while(syn == 26){
scanner();
ParseS();
}
if(syn == 6){
scanner();
if(syn == 0){
printf("success!");
}
else{
printf("statement syntx error S4\n");
}
}
else{
printf("error,缺少end\n");
exit(0);
}
}
void ParseC(){
ParseE();
if(syn==25||syn==0||syn==20||syn==21||syn==23||syn==24){
scanner();
scanner();
}
else{
printf("condition syntx error C\n");
exit(0);
}
ParseE();
}
void ParseE(){
ParseT();
ParseE1();
}
void ParseE1(){
if(syn == 13 || syn == 14){
scanner();
ParseT();
ParseE1();
}
else if(syn == 28 || syn == 0){
}
}
void ParseT(){
ParseF();
ParseT1();
}
void ParseT1(){
if(syn == 15 || syn == 16){
scanner();
ParseF();
ParseT1();
}
else if(syn == 13 ||syn == 14 || syn == 28 || syn == 0){
}
}
void ParseF(){
if(syn == 27){
scanner();
ParseE();
if(syn == 28){
scanner();
}
else{
printf("factor syntx error F\n");
exit(0);
}
}
else if(syn == 10 || syn == 11){
scanner();
}
else{
printf("factor syntx error F\n");
exit(0);
}
}
int main() {
p = 0;
printf("请输入源程序：");
do{
ch = getchar();
input[p++] = ch;
}while(ch != '#');
count = p;
p = 0;
scanner();
ParseS();
return 0;
}
