无论我输入什么，我的“while”循环为什么执行？

Question

/********************************************************* 
** Purpose: Asks the user for cable package they bought ** 
** and the amount of hrs they used and //tells them  ** 
** their monthly bill         ** 
**********************************************************/           




#include <iostream> 
#include <iomanip> 

using namespace std; 

int main() 
{ 
    //Defining variables 
    double hours_over; //Amount of hrs the user went over their monthly allottment 
    double extra_pay; //Extra bill amount for going over monthly hrs allotted 
    double monthly_bill; //Monthly bill the user will pay 
    int hours;   // How many hours the user used during the month 
    char package;  //The package the user chose 

    //Getting the package the user bought 
    cout << "Your monthly subscription bill is based on your package."; 
    cout << "\n\nWhat package did you buy? Enter A, B or C: "; 
    cin >> package; 

    //Validating user input-must enter A, B or C 
    while (package != 'A' || package != 'B' || package != 'C') 
    { 
     cout << "\nPlease enter A, B or C(capitalized)."; 
     cout << "\n\nWhat package did you buy?: "; 
     cin >> package; 
    } 

    //Getting hours the user used during month 
    cout << "How many hours did you use?: "; 
    cin >> hours; 

    //Validating user input-hrs cant exceed 744 
    while (hours > 744) 
    { 
     cout << "I'm sorry but your monthly usage cannot exceed 744 hrs."; 
     cout << "\nPlease enter another number."; 
     cout << "How many hours did you use?: "; 
     cin >> hours; 
    } 

    //Fixing decimal place of answers 
    cout << setprecision(2) << fixed << showpoint << endl; 

    //Switch statement-go to the package the user bought 
    switch (package) 
    { 
     case 'A': 
      if (hours > 10) 
      { 
         hours_over=hours-10; 
         extra_pay=hours_over*(2.00); 
         monthly_bill=9.95+extra_pay; 

         cout << "Your monthly bill is: $" << monthly_bill << endl; 
      } 
      else 
      { 
       cout << "Your monthly bill is: $9.95"; 
      } 
      break; 

     case 'B': 
      if (hours > 20) 
      { 
         hours_over=hours-20; 
         extra_pay=hours_over; 
         monthly_bill=14.95+extra_pay; 

         cout << "Your monthly bill is: $" << monthly_bill << endl; 
      } 
      else 
      { 
       cout << "Your monthly bill is: $14.95"; 
      } 
      break; 

     case 'C': 
      cout << "Your monthly bill is: $19.95"; 
      break; 

     default: 
      break; 
    } 

cin.get(); 
return 0; 
} 

Answer 1

您的A，B或C的测试是错误的

while (package != 'A' || package != 'B' || package != 'C') 

应该

while (package != 'A' && package != 'B' && package != 'C') 

Answer 2

您应该检查cin是否能够传输所需类型的值ala if (cin >> my_int)，然后在错误输入后使用std::cin.clear()，然后再让它们重新输入该值。否则，像说一些文本输入不能转换为int的垃圾值会在错误状态下保留std::cin，甚至不会尝试下一个std::cin >> xxx。

Answer 3

考虑您的表达：

while (package != 'A' || package != 'B' || package != 'C') { 

让包有值 'A' 。

这个计算结果为

false || true || true 

这当然是正确的。

Answer 4

“while”只会在你想要的时候循环，但“if”总是会触发;你是这个意思吗？关于A，B或C的“if”总是会因为使用“||”意思是“或”来链接条件。对于变量的任何值，总是如此，它不是A，或者不是B，或者不是C！

Answer 5

首先，此代码：

//Validating user input-must enter A, B or C 
if (package != 'A' || package != 'B' || package != 'C') 
{ 
    cout << "\nPlease enter A, B or C(capitalized)."; 
    cout << "\n\nWhat package did you buy?: "; 
    cin >> package; 
} 

将无法​​正常工作，因为：（1）你是一个比较字符串（包）字符，和（2）你使用|| （或）而不是& &（和）。另外（3）你可能想要“而”而不是“如果”。

while循环对我来说工作得很好。

Answer 6

此行结果始终为true：

while (package != 'A' || package != 'B' || package != 'C') 

这也许应该是：

while (package != 'A' && package != 'B' && package != 'C') 

Answer 7

cin >> package; 
//Validating user input-must enter A, B or C 
while (package != 'A' || package != 'B' || package != 'C') 
{ 
    cout << "\nPlease enter A, B or C(capitalized)."; 
    cout << "\n\nWhat package did you buy?: "; 
    cin >> package; 
} 

如果该package值是内环路进入乙。它满足第一个条件package != 'A'并且因为它是一个或之后的操作（true || false || true导致true），循环进入。您应该改用&&。所以，改变

while (package != 'A' && package != 'B' && package != 'C') 
{ 
    // ..... 
}