编译原理语法分析程序（最左推导）

文法G2的最左推导和最右推导

E->E+T | E-T | T

T->T*F | T/F | F

F->(E) | n

G2的终结符集合VT

+，-，*，/，(，)，

G2的非终结符集合VN

E，T，F，n

句子3+2*4最左推导

E
E	+	T
F	+	T
N	+	T
N	+	T	*	F
N	+	F	*	F
N	+	N	*	F
N	+	N	*	N

句子3+2*4最右推导：

E
E	+	T
E	+	T	*	F
E	+	T	*	N
E	+	F	*	N
E	+	N	*	N
F	+	N	*	N
N	+	N	*	N

语法树：

代码实现：

 本人笔记的课程作业程序，不要抄袭
 1 #include <iostream>
 2 #include<string>
 3 
 4 using namespace std;
 5 string expr;
 6 int step;
 7 void analyze(int left,int right,char start);
 8 
 9 int main()
10 {
11     cout << "please input a correct expression\n";
12     getline(cin,expr);
13     analyze(0,expr.size(),\'E\');
14     return 0;
15 }
16 void analyze(int left,int right,char start)
17 {
18     switch(start){
19         case \'E\':{
20             int bracnt = 0;
21             bool binope = false; // flag of + or -
22             for(int i=right-1;i>=left;--i){
23                 if(expr[i] == \'(\')bracnt++;
24                 if(expr[i] == \')\')bracnt--;
25                 if(!bracnt){
26                     if(expr[i]==\'+\' || expr[i]==\'-\'){
27                         binope = true;
28                         cout << "step " << step << " : ";
29                         cout << start << "->" << start << expr[i] << \'T\' << endl;
30                         step++;
31                         analyze(left,i,\'E\');
32                         analyze(i+1,right,\'T\');
33                         break;
34                     }
35                 }
36             }
37             if(!binope){
38                 cout << "step " << step << " : ";
39                 cout << start << "->" << \'T\' << endl;
40                 step++;
41                 analyze(left,right,\'T\');
42             }
43             break;
44         }
45         case \'T\':{
46             int bracnt = 0;
47             bool binope = false; // flag of * or /
48             for(int i=right-1;i>=left;--i){
49                 if(expr[i] == \'(\')bracnt++;
50                 if(expr[i] == \')\')bracnt--;
51                 if(!bracnt){
52                     if(expr[i]==\'*\' || expr[i]==\'/\'){
53                         binope = true;
54                         cout << "step " << step << " : ";
55                         cout << start << "->" << start << expr[i] << \'F\' << endl;
56                         step++;
57                         analyze(left,i,\'T\');
58                         analyze(i+1,right,\'F\');
59                         break;
60                     }
61                 }
62             }
63             if(!binope){
64                 cout << "step " << step << " : ";
65                 cout << start << "->" << \'F\' << endl;
66                 step++;
67                 analyze(left,right,\'F\');
68             }
69             break;
70         }
71         case \'F\':{
72             if(expr[left] == \'(\'){
73                 cout << "step " << step << " : ";
74                 cout << start << "->" << "(E)" << endl;
75                 step++;
76                 analyze(left+1,right,\'E\');
77             }
78             else{
79                 cout << "step " << step << " : ";
80                 cout << start << "->" << "n" << endl;
81                 step++;
82             }
83             break;
84         }
85         default:break;
86     }
87 }