文件輸入C++，其中行更改

Question

我需要創建一個基本的計算器，從文本文件中讀取。該文件的格式如下：

2 
add 5 3 
sub_prev 1 
1 
mul 4 5 

的2代表操作的次數，明顯加入裝置，5 + 6，計算器會保持以前的結果，這樣就可以+ - * /它。這意味着有2個操作：添加5 + 3，然後從前一個結果中減去1。等等。

我有文件打開罰款，但無法弄清楚如何讓它正確讀取。 理想情況下，它會讀取第一個整數，輸入一個讀取正確的下列行數的循環，然後執行操作。在for循環完成第一個操作數後，它仍然處於讀while循環的文件中，但會重新進入for循環並讀取和計算更多操作。

示例代碼

while(myfile) // while reading the file 
{ 
    int PR = 0; //initialize the previous result as 0 
    myfile >> no; // grabbing the number of operations 
    cout << no << endl; // testing to see if it got it correct 

    for (int i = 1; i <= no; i++) // for the number of operations 
    { 
     myfile >> no >> op >> x >> y; // read the lines 
     if (op == "add") //there would be an if for each operation 
     { 
      PR = x + y; 
     } 
    } 

    myfile >> no >> op >> x >> y; 
} 

的問題是，它從來沒有想進入for循環，它讀取操作的沒有（數量爲2，但它通過while循環繼續，而不是去到for循環

**更新的代碼**

while(myfile) // while reading the file 
{ 
    int PR = 0; //initialize the previous result as 0 
    myfile >> no; // grabbing the number of operations 


    for (int i = 1; i <= no; i++) // for the number of operations 
    { 
    myfile >> op; 
    if (op == "add") 
    { 
    myfile >> x >> y; 
    PR = x + y; 
    }  

    if (op == "sub_prev") 
    { 
     myfile >> x; 
     PR = PR - x; 
    } 

    if (op == "mul") 
    { 
     myfile >> x >> y; 
     PR = x * y; 
     //cout << x << y << PR << endl; //testing mul operation 
    } 

    } 

    cout << "The result of operation " << " is " << PR << endl; 


} 

它正確地計算，但輸出錯誤：

Enter file name: newfile.txt 
The result of operation is 7 
The result of operation is 20 
The result of operation is 20 

RUN SUCCESSFUL (total time: 3s) 

Answer 1

您正在修改no這裏

myfile >> no >> op >> x >> y; 

你不應該這樣做。我會建議像下面這樣的for循環體：

std::string operation; 
myfile >> operation; 
if (operation == "add") { 
    int a, b; 
    myfile >> a >> b; 
    PR = x + y; 
} else if (operation == "sub_prev") { 
    int value; 
    myfile >> value; 
    // do something 
} else { 
    // for other operations if added in future 
}