简单计算器C++

我为家庭作业编写了这个程序。我没有得到正确的输出。有人告诉我，这是因为我不会在两组计算之间重置PreviousResult，但我不知道如何做到这一点。输出应为：

请输入文件名：calc.txt

计算1的结果是：26

计算2的结果是：2

计算3的结果是：0

相反，我得到了4次计算，其中计算3=1，4=0

calc.txt文件为：

3加5 4添加_修订4mul_prev2

1sub 3 1

1div_prev 2

我的代码：

// Header Files
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
// Declaring variables
int PreviousResult = 0;
string command;
string filename = "calc.txt";
int x,y,z;
int result = 0;
int numCommands = 0;
int operation = 1;
//Prompts the user to enter the text file name.
cout << "Please enter a filename: ";
cin >> filename;
//Declaring an ifstream object and opening the file
ifstream myfile(filename.c_str());
myfile.open("calc.txt");
//Will check to see if file exists. If not, it will output the following.
if(!myfile)
{
    cout << "The file doesn't exist." <<endl;
    return 0;
}
//While loop- read the file until the end is reached.
while(!myfile.eof())
{
    myfile >> numCommands;
    for(int i = 0; i < numCommands; i++)
    {
        myfile >> command;
        //Addition
        if (command=="add")
        {
            myfile >> x >> y;
            PreviousResult = x + y;
        }
        //Subtraction
        else if (command == "sub")
        {
            myfile >> x >> y;
            PreviousResult = x - y;
        }
        //Multiplication
        else if (command == "mul")
        {
            myfile >> x >> y;
            PreviousResult=x*y;
        } 
        //Division
        else if(command=="div")
        {
            myfile >> x >> y;
            PreviousResult = x / y;
        }
        else if (command == "add_prev")
        {
            myfile >> z;
            PreviousResult += z;
        }
        else if (command == "sub_prev")
        {
            myfile >> z;
            PreviousResult -= z;
        }
        else if (command == "mul_prev")
        {
            myfile >> z;
            PreviousResult *= z;
        }
        else if (command == "div_prev")
        {
            myfile >> z;
            PreviousResult /= z;
        }
    }
    result = PreviousResult;
    //Print the results.
    cout << "The result of calculation " << operation <<" is: " << result <<  endl;
    //The count is incremented by 1
    operation ++;
}
  return 0;
  }

int PreviousResult = 0;

PreviousResult = 0;

while(!myfile.eof())

while(myfile >> numCommands)

PreviousResult = 0;