使用do-while确保循环体至少运行一次。

Question

我正在为学校项目创建一个非常简单的数字猜谜游戏，并且在重复主菜单时遇到问题。我使用 do-while循环创建它，而我遇到的问题是菜单选择变量是 int ，所以当我（或用户）进入时从菜单中选择时，非int 意外输入主循环结束时的}while(condition)无法捕获它并且程序无限重复。相反，如果您在菜单选择中输入 invalid int ，程序将捕获它，显示“无效输入”消息，然后重复主菜单。

很难用书面形式解释我的意思，所以这里的源代码的相关行用星号表示。我保存为.cpp并使用g++ -ansi -pedantic -Wall -Werror在linux中进行编译。教师禁止在条件语句中进行硬编码，因此是全局常量。

#include <iostream>
#include <ctime>
#include <cstdlib>
using namespace std;
const int PLAY = 1, HIGH_SCORE = 2, EXIT = 3;
const char YES = 'y', NO = 'n';

int main()
{
// Randomly generated value
  int randomNumber;
// User input
  int userGuess, menuChoice;
  char repeat;
// Calculated value
  int numberOfGuesses;
// Place-holder values (to be replaced by calculated values)
  int score1 = 1000, score2 = 2000, score3 = 3000;

  cout << endl << endl;
  cout << "Greetings! This is a number guessing game where I think of" << endl
       << "a whole number between one and ten and you try to guess it!" << endl
       << "You can guess as many times as you like, so don't be afraid" << endl
       << "to use trial and error, but your score is based on the " << endl
       << "number of guesses you make (the lower the better) so don't " << endl
       << "guess too haphazardly. Remember, only guess whole numbers!" << endl
       << endl;

  do
  {
    cout << endl << "Main menu." << endl
         << "1. Play game" << endl
         << "2. Display high scores" << endl
         << "3. Exit game" << endl
         << "Please select an option: ";
    cin >> menuChoice;

    if (cin.fail()){     
      cout << "Please enter a valid choice" << endl;
      continue;
    } 
    cin.ignore();

    switch(menuChoice)
    {
      case PLAY:
      do
      {
        unsigned seed = time(0);
        srand(seed);
        randomNumber = 1 + rand() % 10;

        cout << endl << "Press enter when you're ready to begin!";
        cin.ignore();
        cout << "Ok I thought of one!" << endl << endl;

        numberOfGuesses = 0;

        do
        {
          numberOfGuesses++;

          cout << "Enter your guess: ";
          cin >> userGuess;
          cin.ignore();

// Check user's guess
          if (userGuess == randomNumber)
            cout << "Correct! That was impressive!" << endl << endl;
          else if (userGuess < randomNumber)
            cout << "Not quite, you guessed low." << endl << endl;
          else if (userGuess > randomNumber)
            cout << "Not quite, you guessed high." << endl << endl;
        }while (userGuess != randomNumber);

        cout << "Your score for this game was " << numberOfGuesses << endl;

// Determine if a high score was beaten
        if (numberOfGuesses <= score1)
        {
          score3 = score2;
          score2 = score1;
          score1 = numberOfGuesses;
          cout << "That's a new all time high score!" << endl;
        }
        else if (numberOfGuesses <= score2)
        {
          score3 = score2;
          score2 = numberOfGuesses;
          cout << "That's a new high score!" << endl;
        }
        else if (numberOfGuesses <= score3)
        {
          score3 = numberOfGuesses;
          cout << "That's a new high score!" << endl;
        }
        else
        {
          cout << endl; 
        }

        cout << "Would you like to play again? y/n: ";
        cin.get(repeat);
        cin.ignore();

        while (tolower(repeat) != YES && tolower(repeat) != NO)
        {
          cout << endl;
          cout << "Sorry, that is an invalid choice." << endl
               << "Please enter 'y' for yes or 'n' for no: ";
          cin.get(repeat);
          cin.ignore();
        }
      }while (tolower(repeat) == YES); 
        break;

      case HIGH_SCORE:
      cout << endl << "High Score 1: " << score1 << endl
           << "High Score 2: " << score2 << endl
           << "High Score 3: " << score3 << endl << endl;
      cout << "Press enter to continue. ";
      cin.ignore();
        break;

      case EXIT: 
      cout << endl << "Thanks for playing, I'll see you next time!" << endl << endl;
        break;

      default:
      cout << endl << "That is an invalid selection, please enter '1', '2' or '3'"
           << endl;
        break;
    } 
  }while (menuChoice != EXIT);

  return 0;
}

代码就当前答案编辑。

如果您需要更多信息，请告诉我，谢谢您的提前！

Answer 1

像这样使用cin.fail()（而不只是cin >> menuChoice;）（以this post为模型）：

cin >> menuChoice;
if (cin.fail()) {
  cout << "Please enter a valid choice" << endl;
  cin.clear();
  cin.ignore();
  continue;
}
//Remove the cin.ignore() at this place!

有关详细信息，请参阅this SO thread

Answer 2

使用do-while确保循环体至少运行一次。

通过使用do-while并在循环外提示用户，您可以假设用户想要玩一次游戏，但可能并非如此。

更清洁的方法IMO将使用while循环。显示菜单循环外和循环结束。用户可以选择立即退出。

 cout << "Greetings.....
 cout << menu
 // Get menuChoice input here.    
    while(menuChoice != EXIT){
        ...

         cout << menu //reprompt at end to continue or exit cleanly
         // Get menuChoice input here
    }

输入验证是使用do-while

的最佳时机

do{
   if(!cin){
      cout << "Invalid input"
      cin.clear()
      cin.ignore(numeric_limits<streamsize>::max(), '\n');
   }
}while(!(cin >> menuChoice)) // This gets console input. If fail, loop.

使用numeric_limits<streamsize>::max()完全清除缓冲。
使用cin.clear()重置cin上的失败标记，以便它不会总是假的。

cin.fail()没问题。但有些人会认为!cin更自然。

在do-while循环中使用输入验证的C ++错误

2 个答案:

使用do-while确保循环体至少运行一次。

输入验证是使用do-while