递归下降分析程序(语法分析)
作者:互联网
1. 文法 G(S):
(1)S -> AB
(2)A ->Da|ε
(3)B -> cC
(4)C -> aADC |ε
(5)D -> b|ε
验证文法 G(S)是不是 LL(1)文法?
FIRST集
FIRST(Da)={b,a}
FIRST(ε)={ε}
FIRST(cC)={c}
FIRST(aADC)={a}
FIRST(b)={b}
FOLLOW集
FOLLOW(A)={c,b,a,#}
其中#是FOLLOW(A)=FOLLOW(C)=FOLLOW(B)=FOLLOW(S)
FOLLOW(B)={#}
FOLLOW(C)={#}
FOLLOW(D={a,#}
SELECT集
SELECT(A->Da)=FIRST(Da)={b,a}
SELECT(A->ε)=FOLLOW(A)={c,b,a,#}
SELECT(C->aADC)=FIRST(aADC)={a}
SELECT(C->ε)=FOLLOW(C)={#}
SELECT(D->b)=FIRST(b)={b}
SELECT(D->ε)=FOLLOW(D)={a,#}
因为SELECT(A->Da)∩SELECT(A->ε)≠∅
SElECT(C->aADC)∩SELECT(C->ε)=∅
SELECT(D->b)∩SELECT(D->ε)=∅
所以G(S)不是LL(1)文法。
2.(上次作业)消除左递归之后的表达式文法是否是LL(1)文法?
消除左递归得:
E->TE’
E’->+TE’|ε
T->FT’
T’->*FT’|ε
F->(E)|i
FIRST集:
FIRST(TE’)={ (, i }
FIRST(+TE')={+,ε}
FIRST(FT’)={(,i}
FIRST(*FT')={ *,ε}
FIRST((E))={ (}
FIRST(i)={ i }
FOLLOW集:
FOLLOW(E)={ ),# }
FOLLOW(E')={ ),# }
FOLLOW(T)={+,),#}
FOLLOW(T')={+,),#}
FOLLOW(F)={*,+,),#}
SELECT集:
SELECT (E -> TE’) = FIRST(TE’) = { ( , i }
SELECT(E’ -> +TE’) = FIRST(+TE’) = { + }
SELECT(E’ -> ε) = FIRST(ε) - {ε} U FOLLOW(E’) = FOLLOW(E’) = { ) , # }
SELECT(T -> FT’) = FIRST(FT’) = { ( , i }
SELECT(T’ -> *FT’) = FIRST(*FT’) = { * }
SELECT(T’ -> ε) = FIRST(ε) - {ε} U FOLLOW(T’) = FOLLOW(T’) = { + , ) ,# }
SELECT(F -> (E)) = FIRST((E)) = { ( }
SELECT(F -> i) = FIRST(i) = { i }
SELECT(E’ -> +TE’) = FIRST(+TE’) = { + } ∩ SELECT(E’ -> ε) = FOLLOW(E’) = { ) , # }=∅
SELECT(T’ -> *FT’) = FIRST(*FT’) = { * } ∩ SELECT(T’ -> ε) =FOLLOW(T’) = { + , ) ,# }=∅
SELECT(F -> (E)) = FIRST((E)) = { ( } ∩ SELECT(F -> i) = FIRST(i) = { i }=∅
所以该文法为LL(1)文法。
3.接2,如果是LL(1)文法,写出它的递归下降语法分析程序代码。
E()
{T();
E'();
}
E'()
T()
T'()
F()
Void ParseE()
{
Switch(lookahead){
case ’(‘,’I’:
ParseT();
ParseE’()
break;
default:
printf("syntx error\n");
exit(0);
}
}
Void ParseE’()
{
Switch(lookahead){
case ‘+’:
MatchToken(+);
ParseT();
ParseE’();
break;
case ‘)’,’#’
break;
default:
printf("syntx error\n");
exit(0);
}
}
Void ParseT()
{
Switch(lookahead){
case ‘(’,’i’
ParseF();
ParseT’();
break;
default:
printf("syntx error\n");
exit(0);
}
}
Void ParseT’()
{
Switch(lookahead){
case ‘*’:
MatchToken(*);
ParseF();
ParseT’();
break;
case ‘+’ ,’ )’ ,’#’:
break;
default:
printf("syntx error\n");
exit(0);
}
}
void ParseF(){
switch(lookahead){
case '(':
MatchToken( '(' );
ParseE();
MatchToken( ')' );
break;
case 'i':
MatchToken( 'i' );
break;
default:
printf("syntx error\n");
exit(0);
}
}
标签:语法分析,递归,分析程序,break,FT,FOLLOW,TE,SELECT,FIRST 来源: https://www.cnblogs.com/zhff/p/11897801.html