C ++ while switch case在默认值内创建一个循环

Question

我在使用 while switch case 时遇到问题，我有这样的代码 我想看看用户写的不是 1,2,3,4,5，然后我想只打印“请输入选项”并要求输入新的输入，这意味着我没有想回到选择菜单，我知道它看起来很简单，我尝试使用 bool 并在 while 内部使用，但我陷入了无限循环。

   cout << "Welcome to program." << endl;
  while (true) 
    
      cout << "Please make a choice from the following menu: " << endl;
      cout << " Press 1 for this.... ," << endl;
      cout << "Press 2 for this.... . ," << endl;
      cout << "Press 3 for this.... . ," << endl;
      cout << "Press 4 for this.... . ," << endl;
      cout << "Press 5 for this.... ." << endl;
      cout << endl;
      cin >> num;
      cout << endl;
        switch (num)
     {
      case 1: 
        
          cout << "Success!" << endl;
          break;
      case 2 : 
          cout << "You presed 2 " << endl;
          break;

      case 3 : 
          cout << "You pressed 3 " ;
          break;

      case 4: 
          cout << "You pressed 4 " ;
          cout << endl;
          break;
      case 5 : 
           cout << "You pressed 5 " ;
          
         
      default:
             cout << "Please enter a valid option!" << endl; // only print this while user write other than cases
             cin >> num;
           
             cout << endl;

Answer 1

while (true) 
  

我可以看到您在 while 循环之后没有使用括号，所以您希望它如何正确循环。如果没有括号，它只会执行下一行代码。所以，把你的代码封装在while(true)

除此之外，您永远不会脱离 while 循环，因此您会被困在 while 循环中。您在 while 循环中没有任何条件可以随时使您跳出循环。 switch-case 块中的breaks 只会让你脱离 switch-case 块，但你仍然会留在 while 循环中。所以，要想跳出循环，就必须在switch块外跳出循环。

您可能会维护一个标志，指示选项 1、2、3、4、5 是否执行，然后您可以跳出循环。或者您可以考虑适合您的用例的逻辑。我只是指出了你哪里出错了。

正确代码 -

cout << "Welcome to program." << endl;
int num=3;
while (true) {
  if(num>=1&&num<=5){
  cout << "Please make a choice from the following menu: " << endl;
  cout << " Press 1 for this.... ," << endl;
  cout << "Press 2 for this.... . ," << endl;
  cout << "Press 3 for this.... . ," << endl;
  cout << "Press 4 for this.... . ," << endl;
  cout << "Press 5 for this.... ." << endl;
  cout << endl;
    }
  cin >> num;
  cout << endl;
    switch (num)
 {
  case 1:         
      cout << "Success!" << endl;
      break;
  case 2 : 
      cout << "You presed 2 " << endl;
      break;

  case 3 : 
      cout << "You pressed 3 " ;
      break;

  case 4: 
      cout << "You pressed 4 " ;
      cout << endl;
      break;
  case 5 : 
       cout << "You pressed 5 " ; 
       break;                  
  default:
         cout << "Please enter a valid option!" << endl; // only print this while user write other than cases
        continue;
     }
}

上面的代码会一直循环，直到用户输入了一个无效的选项，并且只有在用户输入了一个正确的选项后才退出循环。

Answer 2

任何你不想重复的东西，不要循环。就是这么简单。在循环外显示完整菜单和第一个提示。您的默认案例仅会在输入错误号码时打印其消息。我还缩小了开关，因为所有额外的输入都是不必要的。 [[fallthrough]] 是一个很好的工具，但它需要 C++17。

如果你想要的只是一个有效的输入，你真的不应该为while (true) 无限循环而烦恼。只要您的输入无效，就进行迭代。

#include <iostream>

int main() {
  int num = 0;

  std::cout << "Welcome to the program.\n";
  std::cout << "Please make a choice:\n"
            << "1.\n2.\n3.\n4.\n5.\n\n"
            << "Please enter option: ";

  while (num < 1 || num > 5) {
    std::cin >> num;

    switch (num) {
      case 1:
        std::cout << "Success!\n";
        break;
      case 2:
        [[fallthrough]];  // C++17 needed for this, otherwise leave empty
      case 3:
        [[fallthrough]];
      case 4:
        [[fallthrough]];
      case 5:
        std::cout << "You pressed " << num << ".\n";
        break;
      default:
        std::cerr << "Invalid entry!\nPlease enter a **valid** option: ";
    }
  }
}