C++ if 语句中的多个条件

Question

我对 C++ 编程的概念非常陌生。我想在一个语句中使用||（或）和&&（和）有一个多条件if语句。当我问我的大学教授时。她告诉这是可能的，然后侮辱了我在这个问题上的有限知识。我可以访问的所有示例都显示一个多 && 语句，只有一个显示 ||。它没有显示它们一起使用。我想学习如何让生产线正常工作。我将附上我拥有的代码。问题区域是编码的最后一点。

# include <iostream>
# include <cstring>

using namespace std;

main()
{
    
    const int maximumHours = 774;
    char customerPackage;
    double hoursUsed = 0,
           packageA = 9.95,
           packageB = 14.95,
           packageC = 19.95,
           overPackageA = 2.00,
           overPackageB = 1.00,
           overTime = 0,
           amountDue = 0,
           excessCharged = 0;
    
    cout << "Please enter the customer's package: ";
    cin >> customerPackage;
    
    switch (customerPackage)
    {
        case 'a' :
            cout << "Please enter the number of hours used: ";
                cin >> hoursUsed;
            break;
        
        case 'A' :
            cout << "Please enter the number of hours used: ";
                cin >> hoursUsed;
            break;
        
        case 'b' :
            cout << "Please enter the number of hours used: ";
                cin >> hoursUsed;
            break;
        
        case 'B' :
            cout << "Please enter the number of hours used: ";
                cin >> hoursUsed;
            break;
          
        case 'c' :
            cout << "Please enter the number of hours used: ";
                cin >> hoursUsed;
            break;
        
        case 'C' :
            cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
            break;        
        default: cout << "Error." 
            << " Please enter the customer's purchased package: ";
        cin >> customerPackage;
    }    
            
    if ( customerPackage ='a' || customerPackage ='A' && hoursUsed >= 10)           
        amountDue = packageA;
        else
            overTime = packageA - hoursUsed;
            excessCharged = overTime * overPackageA;
            amountDue = packageA + excessCharged;
}

Answer 1

您的问题是&& 的优先级高于||，因此您需要括号。如评论中所述，您还需要使用== 而不是赋值（=）：

if ( (customerPackage =='a' || customerPackage =='A') && hoursUsed >= 10)

Answer 2

其他人已经帮助您解决了您注意到的问题。我将从一个你显然还没有注意到的问题开始：

    else
        overTime = packageA - hoursUsed;
        excessCharged = overTime * overPackageA;
        amountDue = packageA + excessCharged;

如果您希望所有这三个语句都由else 控制，则需要将它们括在大括号中以创建复合语句：

else {
    overTime = packagA - hoursUsed;
    excessCharged = overTime * overPackageA;
    amountDue = packageA + excessCharged;
}

就目前而言，您的代码真的是：

    else
        overTime = packageA - hoursUsed;
    excessCharged = overTime * overPackageA;
    amountDue = packageA + excessCharged;

即，excessCharged 和 amountDue 的计算将执行，无论 if 语句中的条件是真还是假。

我还要注意，您的 switch 声明并没有真正完成太多：

switch (customerPackage)
{
    case 'a' :
        cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
        break;

    case 'A' :
        cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
        break;

    case 'b' :
        cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
        break;

    case 'B' :
        cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
        break;

    case 'c' :
        cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
        break;

    case 'C' :
        cout << "Please enter the number of hours used: ";
        cin >> hoursUsed;
        break;        
    default: cout << "Error." 
        << " Please enter the customer's purchased package: ";

特别是，您对所有情况（默认情况除外）采取完全相同的操作。您可以通过使用失败案例来简化这一点：

switch (customerPackage) {
    case 'a':
    case 'A':
    case 'b':
    case 'B':
    case 'c':
    case 'C':
            cout << "Please enter the number of hours used: ";
            cin >> hoursUsed;
            break;
    default:
         cout << "Error " /* ... */;
}

或者，您可以考虑类似：

static const char valid[] = "aAbBcC";

if (strchr(valid, userPackage)) {
    cout << "Please enter the number of hours used: ";
    cin >> hoursUsed;
}
else {
    std::cout << "Error: Please enter the customer's purchased package";
    std::cin >> userPackage;
}

不过，就我个人而言，我的结构有点不同：首先获取一个有效输入，然后获取下一个：

do { 
    std::cout << "Please enter the customer's purchased package (a, b, or c): ";
    std::cin >> userPackage;
} while (!strchr(valid, userPackage));

std::cout << "Please enter the number of hours used: ";
std::cin >> hoursUsed;

if (tolower(customerPackage == 'a') && hoursUsed >= 10)
// ...

Answer 3

if ( customerPackage ='a' || customerPackage ='A' && hoursUsed >= 10)

您如此接近于正确答案。让我给你两个提示：

1. = 运算符与 == 运算符不同。 = 是赋值运算符。它评估其右侧并将结果存储在其左侧命名的变量中。你想要==，相等运算符。它测试它的右侧和左侧是否相等。

2. 使用括号( ... ) 强制执行您的评估顺序意图。你的意思显然是说“如果customerPackage 是'a' 或者它是'A'，并且hoursUsed 足够大，那么......”。

试试这条线：

if ( (customerPackage == 'a' || customerPackage == 'A') && hoursUsed >= 10)

Answer 4

您可以使用括号指定布尔运算符的执行顺序。你可能想先评估||，所以你会使用：

if ((customerPackage == 'a' || customerPackage == 'A') && hoursUsed >= 10) 

&& 通常默认首先被评估，因为它具有更高的优先级，所以你的代码相当于这样：

if (customerPackage == 'a' || (customerPackage == 'A' && hoursUsed >= 10))

此外，如 cmets 中所述，使用 == 进行比较，使用 = 进行分配。

Answer 5

对于您遇到的新问题（在您提出的other question 中），您需要进行一些重组。

if ( (customerPackage == 'b' || customerPackage == 'B') && hoursUsed <= 20)

    amountDue = packageB;

    else
    {
        /* calculations */
    }

不正确，应该是

if ( customerPackage == 'b' || customerPackage == 'B')
{
   if (hoursUsed <= 20)
   {
      amountDue = packageB;
   }
   else
   {
        /* calculations */
   }
}

否则第一条语句只会在 package=B AND hour=20 时执行，否则将在所有其他情况下进行计算，例如 package 为 A 或 C 时。

希望这会有所帮助！