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
Codes are as follows:
#include<stdio.h> #include<string.h> #include<stdlib.h>char wsym[80],ssym[80]; char ch; int row,syn,sum,m,i,p; char *word[6]={"begin","if","then","while","do","end"}; int flag=0;E (); void void T(); void E1(); void T1(); void F(); void getsyn(){ for(i=0;i<8;i++) ssym[i]=NULL; ch=wsym[p++]; while(ch==' '){ ch=wsym[p]; p++; } if((ch>='a'&&ch<='z')||(ch>='A'&&ch<='Z')){ m=0; while((ch>='0'&&ch<='9')||(ch>='a'&&ch<='z')||(ch>='A'&&ch<='Z')){ ssym[m++]=ch; ch=wsym[p++]; } ssym[m++]='\0'; son- p;=10; for(i=0;i<6;i++){ if(strcmp(word[i],ssym)==0){ syn=i+1; break; } } }else if(ch>='0'&&ch<='9'){ sum=0; while(ch>='0'&&ch<='9'){ sum=sum*10+(ch-'0'); ch=wsym[p++]; } p--; syn=11; }else switch(ch){ case '+': ssym[0]=ch;syn=13; break; case '-': ssym[0]=ch;syn=14; break; case '*': ssym[0]=ch;syn=15; break; case '/': ssym[0]=ch;syn=16; break; case ':': i=0; ssym[i++]=ch; ch=wsym[p++]; if(ch=='='){ ssym[i++]=ch; syn=18; }else{ syn=17; p--; } break; case '<': i=0; ssym[i++]=ch; ch=wsym[p++]; if(ch=='='){ ssym[i++]=ch; syn=21; }else if(ch=='>'){ ssym[i++]=ch; syn=22; }else{ syn=20; p--; } break; case '>': i=0; ssym[i++]=ch; ch=wsym[p++]; if(ch=='='){ ssym[i++]=ch; syn=24; }else{ syn=23; p--; } break; case '=': ssym[0]=ch;syn=25; break; case ';': ssym[0]=ch;syn=26; break; case '(': ssym[0]=ch;syn=27; break; case ')': ssym[0]=ch;syn=28; break; case '#': ssym[0]=ch;syn=0; break; case '\n': syn=100; break; default: syn=-1; break; } } void E(){ if(syn==1){ getsyn(); T(); while(syn==26){ getsyn(); T(); } if(syn==6){ getsyn(); if(syn==0 && flag==0){ the printf ( getsyn ();" ! Success \ n- " ); } } the else { the printf ( " syntax error, missing End \ n-! " ); } } The else { the printf ( " ! Errors syntax, missing the begin \ n- " ); In Flag = . 1 ; } return ; } void T () { IF (SYN == 10 ) { getsyn (); IF (SYN == 18 is ) { E1 (); } the else { the printf ( " Expression of a syntax error Error:!% S \ n- " , ssym); In Flag = . 1 ; } } the else { IF (! = In Flag . 1 ) { the printf ( " expression syntax error, the error is:% S \ n- " , ssym); In Flag = . 1 ; } } } void E1 () { Tl (); the while (SYN == 13 is || SYN == 14 ) { getsyn (); Tl ( ); } return ; } voidTl () { F. (); The while (SYN == 15 || SYN == 16 ) { getsyn (); F. (); } Return ; } void F. () { IF (SYN == 10 || SYN == . 11 ) { getsyn (); } the else IF (SYN == 27 ) { getsyn (); E1 (); IF (SYN == 28 ) getsyn (); the else { the printf ( " no ''), a syntax error! \ the n- " ); Flag= . 1 ; } } the else { the printf ( " Expression of a syntax error Error:!% S \ n- " , ssym); In Flag = . 1 ; } return ; } int main () { P = 0 ; the printf ( " Enter paragraph statement: " ); do { CH = getchar (); wsym [P ++] = CH; } the while (CH =! ' # '); P = 0; Getsyn (); ; E () (printf " analysis is finished! " ); Getchar (); }
Screenshot operating results as follows:
