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
2.P101练习7(2)(3)文法改写,并分别求FIRST集、FOLLOW集,和SELECT集
A → aABe|a
B → Bb|d
消除左递归
A→aA'
A'→ABe|ε
B→dB'
B'→bB'|ε
FIRST集:
FIRST(aA')={a}
FIRST(ABe)={a,ε}
FIRST(ε)={ε}
FIRST(dB')={d}
FIRST(bB')={b}
FIRST(ε)={ε}
FOLLOW集:
FOLLOW(A)={Be}
FOLLOW(A')={#}
FOLLOW(B)={e}
FOLLOW(B')={#}
SELECT集:
SELECT(A→aA')={a}
SELECT(A'→ABe)={a,ε}
SELECT(A'→ε)=(FIRST(ε)-{ε})∪FOLLOW(A')={#}
SELECT(B→dB')={d}
SELECT(B'→bB')={b}
SELECT(B'→ε)=(FIRST(ε)-{ε})∪FOLLOW(B')={#}
S → Aa|b
A → SB
B → ab
代入:
S→SBa|b
消除左递归:
S →bS'
S'→BaS'|ε
B→ab
FIRST集:
FIRST(bS')={b}
FIRST(BaS)={B}
FIRST(ε)={ε}
FIRST(ab)={a}
FOLLOW集:
FOLLOW(S)={a}
FOLLOW(S')={#}
FOLLOW(B)={a}
SELECT集:
SELECT(S→bS')={b}
SELECT(S'→BaS')={B}
SELECT(S'→ε)=(FIRST(ε)-{ε})∪FOLLOW(S')={#}
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)={a,c,p}
SELECT(A->a)={a}
SELECT(A->ε)=(FIRST(ε)-{ε})∪FOLLOW(A)={p}
SELECT(A->cA)={c}
SELECT(A->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)={a,c}
SELECT(S->Bq)={b,d}
SELECT(A->a)={a}
SELECT(A->cA)={c}
SELECT(B->b)={b}
SELECT(B->dB)={d}