10.消除左递归
1.将以下文法消除左递归,并分析符号串 i*i+i
并分别求FIRST集、FOLLOW集,和SELECT集
E -> E+T | T
T -> T*F | F
F -> (E) | i
E->TE'
E'->+TE'|ε
T->FT'
T'->*FT'|ε
F->(E)|i
FIRST集
FIRST(TE')={(,i }
FIRST(+TE')={ + }
FIRST(ε)={ ε }
FIRST(FT')={(,i }
FIRST(*FT')={ * }
FIRST((E))={( }
FIRST((E)|i)={ i }
FOLLOW集
FOLLOW(E)={#,)}
FOLLOW(E')={ )}
FOLLOW(T)={ + }
FOLLOW(T')={ + }
FOLLOW(F)={ * }
SELECT集
SELECT(E->TE')={(,i }
SELECT(E'->+TE')={ + }
SELECT(E'->ε)={ )}
SELECT(T->FT')={(,i }
SELECT(T'->*FT')={ * }
SELECT(T'->ε)={ + }
SELECT(F->(E))={( }
SELECT(F->i)={ i }
分析符号串 i*i+i
2.P101练习7(2)(3)文法改写,并分别求FIRST集、FOLLOW集,和SELECT集
(2)
A->aABe|a
B->Bb|d
A->aA'
A'->ABe|ε
B->dB'
B'->bB'|ε
FIRST集
FIRST(aA')={a}
FIRST(ABe)={a}
FIRST(ε)={ε}
FIRST(dB')={d}
FIRST(bB')={b}
FOLLOW集
FOLLOW(A)={#,d}
FOLLOW(A')={d}
FOLLOW(B)={e}
FOLLOW(B')={e}
SELECT集
SELECT(A->aA')={a}
SELECT(A'->ABe)={a}
SELECT(A'->ε)={d}
SELECT(B->dB')={d}
SELECT(B'->bB')={b}
SELECT(B'->ε)={e}
(3)
S->Aa|b
A->SB
B->ab
S->bS'
S'->BaS'|ε
B->ab
FIRST集
FIRST(bS')={b}
FIRST(BaS')={a}
FIRST(ε)={ε}
FIRST(a)={a}
FOLLOW集
FOLLOW(S)={#}
FOLLOW(S')={#}
FOLLOW(B)={a}
SELECT集
SELECT(S->bS')={b}
SELECT(S'->BaS')={a}
SELECT(S'->ε)={#}
SELECT(B->ab)={a}
课堂练习:
求以下文法的FIRST集、FOLLOW集和SELECT集。
S->Ap
A->a|ε
A->cA
A->aA
FIRST集
FIRST(Ap)={a,c,p}
FIRST(a)={a}
FIRST(ε)={ε}
FIRST(cA)={c}
FIRST(aA)={a}
FOLLOW集
FOLLOW(S)={#}
FOLLOW(A)={p}
SELECT集
SELECT(S->Ap)=FIRST(Ap)={a,c,p}
SELECT(A->a)=FIRST(a)={a}
SELECT(A->ε)=FIRST(ε) - {ε} U FOLLOW(A)={p}
SELECT(A->cA)=FIRST(cA)={c}
SELECT(A->aA)=FIRST(aA)={a}
S->Ap
S->Bq
A->a
A->cA
B->b
B->dB
FIRST集
FIRST(Ap)={a,c}
FIRST(Bq)={b,d}
FIRST(a)={a}
FIRST(cA)={c}
FIRST(b)={b}
FIRST(dB)={d}
FOLLOW集
FOLLOW(S)={#}
FOLLOW(A)={p}
FOLLOW(B)={q}
SELECT集
SELECT(S->Ap)=FIRST(Ap)={a,c}
SELECT(S->Bq)=FIRST(Bq)={b,d}
SELECT(A->a)=FIRST(a)={a}
SELECT(A->cA)=FIRST(cA)={c}
SELECT(B->b)=FIRST(b)={b}
SELECT(dB)=FIRST(dB)={d}