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

G2

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>
#include<string>

using namespace std;
string expr;
int step;
void analyze(int left,int right,char start);

int main()
{
    cout << "please input a correct expression
";
    getline(cin,expr);
    analyze(0,expr.size(),'E');
    return 0;
}
void analyze(int left,int right,char start)
{
    switch(start){
        case 'E':{
            int bracnt = 0;
            bool binope = false; // flag of + or -
            for(int i=right-1;i>=left;--i){
                if(expr[i] == '(')bracnt++;
                if(expr[i] == ')')bracnt--;
                if(!bracnt){
                    if(expr[i]=='+' || expr[i]=='-'){
                        binope = true;
                        cout << "step " << step << " : ";
                        cout << start << "->" << start << expr[i] << 'T' << endl;
                        step++;
                        analyze(left,i,'E');
                        analyze(i+1,right,'T');
                        break;
                    }
                }
            }
            if(!binope){
                cout << "step " << step << " : ";
                cout << start << "->" << 'T' << endl;
                step++;
                analyze(left,right,'T');
            }
            break;
        }
        case 'T':{
            int bracnt = 0;
            bool binope = false; // flag of * or /
            for(int i=right-1;i>=left;--i){
                if(expr[i] == '(')bracnt++;
                if(expr[i] == ')')bracnt--;
                if(!bracnt){
                    if(expr[i]=='*' || expr[i]=='/'){
                        binope = true;
                        cout << "step " << step << " : ";
                        cout << start << "->" << start << expr[i] << 'F' << endl;
                        step++;
                        analyze(left,i,'T');
                        analyze(i+1,right,'F');
                        break;
                    }
                }
            }
            if(!binope){
                cout << "step " << step << " : ";
                cout << start << "->" << 'F' << endl;
                step++;
                analyze(left,right,'F');
            }
            break;
        }
        case 'F':{
            if(expr[left] == '('){
                cout << "step " << step << " : ";
                cout << start << "->" << "(E)" << endl;
                step++;
                analyze(left+1,right,'E');
            }
            else{
                cout << "step " << step << " : ";
                cout << start << "->" << "n" << endl;
                step++;
            }
            break;
        }
        default:break;
    }
}