c++代码看不太懂，有朋友能注释一下嘛

题目
在文件中question.txt存放了以下运算表达式
add(23,45)
sub(44,12)
muti(3,5)
div(54,9)
doubleMe(5)
编写一个程序,从文件中读取每行的运算表达式,将计算结果存放于文件answer.txt中
add(23,45)=68
sub(44,12)=32
muti(3,5)=15
div(54,9)=6
doubleMe(5)=10
代码

 
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
using namespace std;
int main()
{
    ifstream myfile("question.txt");
    if (!myfile.is_open())
    {
        cout << "can not open this file" << endl;
        return 0;
    }
    ofstream outfile("answer.txt", ios::trunc);
    string s, fun;
    double a, b;
    string c, d;
    while(getline(myfile,s))
    {
        string fun = s.substr(0, s.find_first_of('('));
        if (s.find_first_of(',') == string::npos){
            c = s.substr(s.find_first_of('(') + 1, s.find_first_of(')') -s.find_first_of('(') -1);
            istringstream iss(c);
            iss >> a;
        }
        else{
            c = s.substr(s.find_first_of('(') + 1, s.find_first_of(',') -s.find_first_of('(') -1);
            d = s.substr(s.find_first_of(',') + 1, s.find_first_of(')') -s.find_first_of(',') -1);
            istringstream issa(c);
            issa >> a;
            istringstream issb(d);
            issb >> b;
        }
        double res = 0;
        if(fun.compare("add") == 0){
            res=a+b;
        }
        else if(fun.compare("sub") == 0){
            res=a-b;
        }
        else if(fun.compare("muti") == 0){
            res=a*b;
        }
        else if(fun.compare("div") == 0){
            res=a/b;
        }
        else if(fun.compare("doubleMe") == 0){
            res=a*2;
        }
        outfile  << s <<"="<<res<<endl;
    }
    myfile.close();
    outfile.close();
   return 0;
}

 
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
using namespace std;
int main()
{
    ifstream myfile("question.txt");
    if (!myfile.is_open())
    {
        cout << "can not open this file" << endl;
        return 0;
    }
    ofstream outfile("answer.txt", ios::trunc);
    string s, fun;
    double a, b;
    string c, d;
    //从文件中读取一行到s
    while(getline(myfile,s))
    {
        //获取函数名
        string fun = s.substr(0, s.find_first_of('('));
        //逗号是否存在
        if (s.find_first_of(',') == string::npos){
           //不存在 说明参数只有一个
            c = s.substr(s.find_first_of('(') + 1, s.find_first_of(')') -s.find_first_of('(') -1);
           //定义整形字符流
            istringstream iss(c);
            //参数存到a
            iss >> a;
        }
        else{
            //存在 有两个参数
            c = s.substr(s.find_first_of('(') + 1, s.find_first_of(',') -s.find_first_of('(') -1);
            d = s.substr(s.find_first_of(',') + 1, s.find_first_of(')') -s.find_first_of(',') -1);
            istringstream issa(c);
            issa >> a;
            istringstream issb(d);
            issb >> b;
        }
        double res = 0;
        //判断函数的作用
        if(fun.compare("add") == 0){
            res=a+b;
        }
        else if(fun.compare("sub") == 0){
            res=a-b;
        }
        else if(fun.compare("muti") == 0){
            res=a*b;
        }
        else if(fun.compare("div") == 0){
            res=a/b;
        }
        else if(fun.compare("doubleMe") == 0){
            res=a*2;
        }
        //输出到文件
        outfile  << s <<"="<<res<<endl;
    }
   //关闭文件
    myfile.close();
    outfile.close();
   return 0;
}