else if 循环 qudry

Question

我处于项目的第二阶段，我需要将我的程序扩展为菜单驱动的应用程序，以查询我在 .txt 文件中拥有的数据库。所以，我的麻烦是我不能让我的循环是永久的。它总是在从一个选项初始化到下一个选项时终止。这是我的 int main 代码的 sn-p：

    int main ()
{
        char Q,q;
        char S,s;
        char task;
        string pathname;
        string z;
        int count=0;
        cout << "Welcome to Jason Rodriguez's Library Database." << endl;
        cout << "Please enter the name of the backup file: ";
        cin >> pathname;
        ifstream inFile(pathname.c_str());
        while(!inFile.eof())
        {
            getline(inFile,z);
            count++;
        }

    while (task != 'Q' || task != 'q') {

        cout << count << " records loaded successfully." << endl;
        cout << "Enter Q to (Q)uit, Search (A)uthor, Search (T)itle, (S)how All: ";
        cin >> task;
        if ((task == 'Q')||(task =='q'))
        {
            cout << "Program will now terminate";
            break;
        }
        else if ((task == 'S')||(task =='s'))
        {
            showAll (loadData (pathname));
            cout << endl;
            cout << "Enter Q to (Q)uit, Search (A)uthor, Search (T)itle, (S)how All: ";
            cin >> task;
        }

    }
}

我需要在这两个之上的循环中再添加两个选项，但我认为我应该首先让前两个正常工作。之后其他两个应该是即插即用。基本上我想做的是说如果用户输入 Q 或 q，则终止程序。否则，如果用户点击 S 或 s，则激活 showall 功能，然后返回原始查询。但它不起作用。欢迎并感谢您提供帮助。

Answer 1

菜单几乎总是需要循环 - 尤其是需要用户输入正确选择输入的菜单。在这种情况下，最适用的是 while 循环 - 但本质上，可以使用任何其他循环变体。

更新：

int main ()
{
    char task;//this is the only char needed. Your other chars were redundant
    string pathname;
    string temp;//I changed z to temp to better reflect its purpose
    int count=0;

    cout << "Welcome to Jason Rodriguez's Library Database." << endl;
    cout << "Please enter the name of the backup file: ";
    cin >> pathname;

    ifstream inFile(pathname.c_str());//this is potentially a problem in that you aren't verifying that the pathname is a valid one

    //you did not check to see that your file was open, otherwise there is no way to tell that you successfully opened the file
    if (inFile.is_open()) {
        //while(!inFile.eof()) is a character by character read and comparison
        //made your life easier by shortening it down to this - which ensures
        //that a line is read. (Much faster and more readable)
        while(getline(inFile,temp)) 
        {
            count++;
        }
        inFile.close();//always close a file after you've used it

        //At this point the entire file has been read. So, this is where this message SHOULD be 
        cout << count << " records loaded successfully." << endl;
    }
    else {
        //if there was an error opening the file (i.e. wrong path, or it simply does not exist), this will be displayed
        cout << "There was a problem opening your file" << endl;
        exit(0);//and the program will terminate
    }

    while (task != 'Q' || task != 'q') {
        cout << "Enter Q to (Q)uit, Search (A)uthor, Search (T)itle, (S)how All: ";
        cin >> task;
        if ((task == 'Q')||(task =='q'))
        {
            cout << "Program will now terminate";
            break;
        }
        else if ((task == 'S')||(task =='s'))
        {
            string author;
            //showAll (loadData (pathname));
            cout << endl;
            cout << "Search an Author" << endl;
            cin >> author;//get the author name to search from the user

            //write code to search an author here
        }
    }
}

您发布的代码存在许多问题，为简洁起见，我将放弃这些问题。因此，请注意以下几点：

您的代码在每个选项中都打印了相同的消息（退出除外）。当然，它似乎不起作用。每个选项都是不同的任务。打印每个任务的作用（类似于我所做的）。

1. 您希望在文件中搜索作者，但尚未存储。寻找一种让你的导师满意的存储方式。

2. 考虑到您的代码越来越复杂，在这种情况下，您最好使用switch。

3. 尝试将每个任务分解为函数，并调用它们以使您的主函数可读。事实上，让你的 main 函数尽可能小是一种很好的编程习惯。

而且，正如 juanchopanza 非常正确地指出的那样：C++ 存在一些基本问题。尝试做更多的练习，并从一本好的 C++ 书籍中做更多的例子。