表达式文法以及parse实现

来源：互联网发布：php空间编辑：程序博客网时间：2024/05/22 00:47

1.文法

1)E->E+T
2)E->E-T
3)E->T
4)T->T*F
5)T->T/F
6)T->F
7)F->i
8)F->(E)

2.去左递归、提取左因子后的改写文法

1) E -> TE'
2) E'-> +TE' | -TE' | e
3) T -> FT'
3) T'-> *FT' | /FT' | e
4) F -> int | (E)

3. 代码

class Exp {private:string s;int i;enum Token { PLUS, MINUS, TIME, DIVIDE, OPEN, CLOSE, INT };map<Token, char> op;bool E() {return T() && EA();}bool EA() {int save = i;if (term(PLUS)) return T() && EA();i = save;if (term(MINUS)) return T() && EA();i = save;return true;}bool T() {return F() && TA();}bool TA() {int save = i;if (term(TIME)) return F() && TA();i = save;if (term(DIVIDE)) return F() &&TA();i = save;return true;}bool F() {int save = i;if (term(INT)) return true;i = save;return term(OPEN) && E() && term(CLOSE);}bool term(Token t) {if (i == s.size()) return false;if (t < INT) return (s[i++] == op[t]);int save = i;for (; i < s.size() && isdigit(s[i]); ++i);return i > save;}public:bool valid;Exp(string exp) : s(exp), i(0), valid(false), op({ { PLUS, '+' }, { MINUS, '-' }, { TIME, '*' }, { DIVIDE, '/' }, { OPEN, '(' }, { CLOSE, ')' } }) {valid = E() && (i == s.size());}};

0 0