编译原理—实验五 LL(1)语法分析（四）

一、实验目的
1．熟悉LL(1)语法分析的基本原理，语法分析的过程，以及语法分析中要注意的一些问题。
2. 复习高级语言及线性表、栈、图等典型数据结构，进一步加强用高级语言来解决实际问题的能力。
二、实验内容
将实验四编写的程序的基础之上，实现下面的功能：
按照教材5.2节算法，输入一个符号串，根据实验四的预测分析表，输出该符号串的最左推导，如果不能构造，提示字符串的出错位置。

S->a
S->^
S->(T)
T->SN
N->,SN
N->@

代码如下：

#include<bits/stdc++.h>
using namespace std;
int main()
{
  typedef pair<char, pair<string, string> >PPP;
  vector<PPP >SELECT; //SELECT集
  SELECT.push_back(make_pair('S', make_pair("a","a")));
  SELECT.push_back(make_pair('S', make_pair("^","^")));
  SELECT.push_back(make_pair('S', make_pair("(T)","(")));
  SELECT.push_back(make_pair('T', make_pair("SN","a")));
  SELECT.push_back(make_pair('T', make_pair("SN","^")));
  SELECT.push_back(make_pair('T', make_pair("SN","(")));
  SELECT.push_back(make_pair('N', make_pair("@",")")));
  SELECT.push_back(make_pair('N', make_pair(",SN",",")));
  vector<char >Sentence;//要识别的句子
  cout << "请输入要识别的串: " << endl;
  string str;
  cin >> str;
  for (int i = 0; i < str.size(); i++)
  {
    Sentence.push_back(str[i]);
  }
  Sentence.push_back('#');
//  for(int i=0;i<Sentence.size();i++)
//  {
//    cout<<Sentence[i];
//  }
  vector<char>::iterator a = Sentence.begin(), ii;
  stack<char>S;
  vector<char>vet;
  S.push('#');
  S.push('S');//分析栈
  vet.push_back('#');//存储分析栈
  vet.push_back('S');
  cout << "分析栈" << "              " << "剩余输入串" << "              " << "推导所用的产生式或匹配" << endl;
  int EMPTY = 7;
  string ac[51];
  int k=0;
  while (!S.empty())
  {
    for (int i = 0; i < vet.size(); i++)   //分析栈
    {
      cout << vet[i] << " ";
    }
    for (int i = vet.size(); i <= EMPTY+2; i++)
      cout << "  ";
    int count = 0;
    for (ii = a; ii != Sentence.end(); ii++)   //剩余输入串
    {
      cout << (*ii) << " ";
      count++;
    }
    for (; count <= EMPTY; count++)
      cout << "  ";
    char X = S.top();//栈顶符号  *a当前输入符号
    if (X == (*a))
    {
      S.pop();
      vet.pop_back();
      a++;
      for (int i = 0; i <= EMPTY; i++)cout << "  ";
      if(X=='#')
        cout<<"接受"<<endl;
      else
        cout << "匹配" << endl;
    }
    else
    {
      vector<PPP >::iterator it;
      string ss = "";
      bool flag = false;
      for (it = SELECT.begin(); it != SELECT.end(); it++)
      {
        if (it->first == X)
        {
          ss = it->second.first;//产生式右部 
          for (int i = 0; i < (int)it->second.second.size(); i++)
          {
            if (it->second.second[i] == (*a))
            {
              flag = true;
              break;
            }
          }
          if (flag)break;
        }
      }
      for (int i = 0; i <= EMPTY; i++)cout << "  ";
      if (!flag)
      {
        cout << "ERROR!!!" << endl;
        return 0;
      }
      cout << X << "->" << ss << endl;
      ac[k++]=X+ss;//最左推导存储 
      reverse(ss.begin(), ss.end()); //反转
      if (ss == "@")
      {
        S.pop();
        vet.pop_back();
      }
      else
      {
        S.pop();
        vet.pop_back();
        for (int i = 0; i < (int)ss.size(); i++)
        {
          S.push(ss[i]);
          vet.push_back(ss[i]);
        }
      }
    }
  }
  cout << "最左推导数：" << endl;
  cout<<"S=>"<<ac[0].substr(1);
  string qq=ac[0].substr(1);
  for(int i=1; i<k; i++)
  {
    cout<<"=>";
    string str1="";
    for(int j=0; j<qq.size(); j++)
    {
      if(ac[i][0]==qq[j])
      {
        if(ac[i].substr(1)=="@")
        {
          str1+="";
        }
        else
          str1+=ac[i].substr(1);
      }
      else
        str1+=qq[j];
    }
    qq=str1;
    cout<<str1;
  }
}