Question

当用户输入字母而不是数字时，我正在尝试显示“无效选项”。我尝试使用isalpha()函数，但我得到一个显示0 Invalid option!try again:的无限循环。输入字母时会显示输出中的0。当我实际输入数字0时，将显示消息并退出循环。

    #include <iostream>
    #include <cstdlib>
    #include <ctime>
    #include <cctype>
    using namespace std;

    int main() {
        // Vaules do not change
        const int minNum = 1;
        const int maxNum = 100;
        int userInput;

        // Changes every second
        srand(time(0));

        // Random number is stored
        const int answer = minNum + (rand() % maxNum);
        cout << answer << endl; // For testing purposes

        cout << "Guess a number 1-100: ";
        cin >> userInput;
        cout << endl;

        if(userInput == answer) {
          cout << "Correct!\n\n";
        }
        while(userInput != answer) {
          if(userInput < 1 || userInput > 100 || isalpha(userInput))  {
            cout << userInput << " Invalid option!\ntry again: ";
            cin >> userInput;
            cout << endl;
          }
          else if(userInput < answer) {
            cout << userInput << " is too low!\ntry again: ";
            cin >> userInput;
            cout << endl;
          }
          else if(userInput > answer) {
            cout << userInput << " is too high!\ntry again: ";
            cin >> userInput;
            cout << endl;
          }
        }
        cout << userInput << " is correct!\n\n";
        return 0;
    }

Answer 1

当您需要根据某些逻辑以不同方式处理用户输入时，最佳选择是：

阅读文字行。找出没有更多输入时该怎么做。
使用自定义逻辑处理每行文本。

在您的情况下，您可以使用：

#include <iostream>
#include <cstdlib>
#include <ctime>
#include <cctype>

using namespace std;

int main() {

   // Vaules do not change
   const int minNum = 1;
   const int maxNum = 100;
   int userInput;

   // Changes every second
   srand(time(0));

   // Random number is stored
   const int answer = minNum + (rand() % maxNum);
   cout << answer << endl; // For testing purposes

   std::string line;
   cout << "Guess a number 1-100: ";
   while ( getline(std:::cout, line ) )
   {
      // Deal with empty lines.
      if ( line.size() == 0 )
      {
         continue;
      }

      // If the first character is a letter ...
      if(isalpha(line[0]))  {
         cout << line << "\n Invalid option!\ntry again: ";
         continue;
      }

      // Extract the number from the line using a stringstream.
      // If there is a problem extracting the number ...
      std::istringstream str(line);
      if ( !(str >> userInput ) )
      {
         cout << line << "\n Invalid option!\ntry again: ";
         continue;
      }

      cout << endl;

      // Check the user input against the random answer.
      if(userInput == answer) {
         cout << "Correct!\n\n";
      }
      else if(userInput < 1 || userInput > 100 )  {
         cout << userInput << " Invalid option!\ntry again: ";
      }
      else if(userInput < answer) {
         cout << userInput << " is too low!\ntry again: ";
      }
      else if(userInput > answer) {
         cout << userInput << " is too high!\ntry again: ";
      }

      cout << "Guess a number 1-100: ";
   }
   cout << userInput << " is correct!\n\n";
   return 0;
}

如何检查输入是否是字母？

1 个答案: