编程实现计算FIRST集和FOLLOW集C++之（三）处理候选式:把空加入非终结符的First集

之前删去了右部为空的候选式。现在要实现把空加入到当前非终结符的First集中。

主要使用了boost的string的有关算法，trim处理字符串前后的空格。

string fzhongjiefu = line.substr(0,line.find("-"));             trim(fzhongjiefu);              preprocess.seekp(mark_now-line.length()-1);//把输出流定位到当前位置            string placeholder(line.length(),' ');            preprocess << placeholder  << endl;             addToFirstSet(fzhongjiefu,"ε");

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void addToFirstSet(string fzhongjiefu,string element){    fstream firstSet("first.txt", fstream::ate | fstream::in | fstream::out );    if(!firstSet){        cerr << "Can't Open file " << endl;    }    firstSet << fzhongjiefu+ ":" + element << endl;     firstSet.close();}

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main.cpp

#include<fstream>#include<iostream>#include<string>#include<vector>#include<regex>#include<cstdlib>#include <boost/algorithm/string.hpp>/**实现编译原理语法分析中计算非终结符的First集Follow集*候选式存于文件中**/using namespace std;using namespace boost;//定义候选式类class HXS{    public:        string left;        vector<string> right;};/**把第二个参数加入第一个参数的First集中*/void addToFirstSet(string fzhongjiefu,string element){    fstream firstSet("first.txt", fstream::ate | fstream::in | fstream::out );    if(!firstSet){        cerr << "Can't Open file " << endl;    }    firstSet << fzhongjiefu+ ":" + element << endl;     firstSet.close();}/**把有多个右部的候选式切分开,便于下一步进行处理*/void split(){    string line;    string symor("|");    size_t found;     ifstream infile("houxuanshi.txt");    ofstream processed("houxuanshi_processed.txt");    if(!infile.is_open()){        cout << "Error Open File!" << endl;     }    cout << "------------------------" << endl;    cout << "候选式:" << endl;    line = "";    while(getline(infile,line)){        cout << line << endl;           found =  line.find(symor);        if(found == string::npos){            processed << line << endl;              continue;           }else{        cout << "Has | " << endl;        processed << line.substr(0,found) << endl;        processed << line.substr(0,line.find(">")+1)+line.substr(found+1,line.length()-found) << endl;        }        line = "";      }    cout << "------------------------" << endl;    infile.close();    processed.close();}//预处理函数，先把候选式中由非终结符推导出空的候选式处理掉//先把原来的文件复制一份，操作新文件,//在用读写方式打开新文件，删除文件中候选式右部为空的候选式void preprocess_right_is_null(){    string line;    ifstream processed("houxuanshi_processed.txt");    ofstream preprocess_temp("preprocess.txt");    if(!processed.is_open()){        cerr << "Can't Open File" << endl;    }    preprocess_temp << processed.rdbuf();    processed.close();    preprocess_temp.close();    fstream preprocess("preprocess.txt",fstream::in | fstream::out);    if(!preprocess){        cerr << "Open File Error!" << endl;    }    while(getline(preprocess,line)){        //读取并记录文件流的位置        auto mark_now = preprocess.tellg();        if(string::npos==line.find("ε")){            cout << "  进入下一行" << endl;              continue;        }else{        //要把候选式集合中的右部为空的候选式删除(覆盖成空格)        //并且把空加入当前非终结符的First集中        //此处涉及文件的随机读写            string fzhongjiefu = line.substr(0,line.find("-"));             trim(fzhongjiefu);              preprocess.seekp(mark_now-line.length()-1);//把输出流定位到当前位置            string placeholder(line.length(),' ');            preprocess << placeholder  << endl;             addToFirstSet(fzhongjiefu,"ε");            cout << "把空加入"+ fzhongjiefu +"的First集" << endl;        }       }    preprocess.close(); /* *********************************以上是处理右部为空的候选式************************  */}//处理候选式左部为终结符的情况void preprocess_left(){}void FIRST(){    regex feizhongjiefu("[A-Z]");    cout << regex_match("E",feizhongjiefu) << endl;}void FOLLOW(){}int main(){//  split();//  FIRST();    preprocess_right_is_null();}