1.将以下文法消除左递归,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
消除左递归
E→TE'
E'→+TE'|ε
T→FT'
T'→*FT'|ε
F→(E)|i
FIRST集:
FIRST(E)→FIRST(T)→FIRST(F)→{ ( , i }
FIRST(E')→{+,ε}
FIRST(T)→FIRST(F)→{ ( , i }
FIRST(T')→{*,ε}
FIRST(F)→{ ( , 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
提取左公因子:
A→aA'
A'→ABe|ε
消除公因式:
B→dB'
B'→bB'|ε
1.First集:
First(aA') = {a}
First(ABe) = {a}
First(ε) = {ε}
First(dB') = {d}
First(bB') = {b}
2.Follow集:
Follow(A) = {Be}
Follow(A') = {ε}
Follow(B) = {e}
Follow(B') = {ε}
3.Select集:
Select(A -> aA') = First(aA') = {a}
Select(A' -> ABe) = First(ABe) = {a}
Select(A' -> ε) = (First(ε)-{ε})∪Follow(A')) = {ε}
Select(B -> dB')= First(dB') = {d}
Select(B' -> bB') = First(bB') = {b}
Select(B' -> ε) = (First(ε)-{ε})∪Follow(B')) = {ε}
(3) S->Aa|b
A->SB
B->ab
消除左递归:
S->bS'
S'->BaS'|ε
B->ab
FIRST集:
First(SBa) = {S}
First(b) = {b}
First(bS') = {b}
First(BaS') = {B}
First(ε) = {ε}
First(ab) = {a}
② FOLLOW集:
Follow(S) = {B}
Follow(S') = {#}
Follow(B) = {a}
③ SELECT集:
Select(S->SBa) = First(SBa) = {S}
Select(S->b) = First(b) = {b}
Select(S->bS') = First(bS') = {b}
Select(S'->BaS') = First(BaS') = {B}
Select(S->ε) = First(ε) = {ε}∪Follow(S') = {#}
Select(B->ab) = First(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(a)={a}
FIRST(ε)={ε}
FIRST(cA)={c}
FIRST(aA)={a}
FIRST(Ap)={a,c,p}
FOLLOW(S)={#}
FOLLOW(A)= {p}
SELECT(S→Ap)=FIEST(Ap)={A}
SELECT(A→a)=FIRST(a)={a}
SELECT(A→ε)=(FIRST(ε)-{ε})∪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(a)={a}
FIRST(cA)={c}
FIRST(b)={b}
FIRST(dB)={d}
FIRST(Ap)={a,c}
FIRST(Bq)={b,d}
FOLLOW(S)={#}
FOLLOW(A)={p}
FOLLOW(B)={q}
SELECT(S→Ap)=FIEST(Ap)={A}
SELECT(S→Bq)=FIRST(a)={a}
SELECT(A→a)=FIRST(a)={a}
SELECT(A→cA)=FIRST(cA)={c}
SELECT(B→b)=FIRST(b)={b}
SELECT(B→dB)=FIRST(dB)={d}