多条件if语句C++

Question

我对C++编程的概念非常陌生。我希望有一个多条件，如果使用||语句或者在一个语句中和和/或&/&。当我问我的大学教授这件事。她说这是可能的，然后侮辱了我对这个问题的有限知识。我有权访问的所有示例都显示多条语句，只有一条显示||。它不显示它们一起使用。我会学习如何让线路工作。我会附上我的代码。问题领域是编码的最后一个位。

# 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||所以你需要括号。正如评论指出的，你还需要使用==，而不是分配（=）的：

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

Answer 2

您可以使用括号来指定这些布尔运算符的执行顺序。你可能想评估||第一，所以你会使用：

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

的&&通常由默认的第一个评价，因为它具有更高的优先级，让你的代码是相同的：

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

另外，如评论中所述，使用==进行比较，=进行分配。

Answer 3

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

您是所以接近有正确的答案。让我给你两个提示：

1. 的=运营商是不一样的==操作。 =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==，等于运算符。它测试看它的右侧和左侧是否相等。

2. 使用括号(...)来强制执行您的评估顺序意向。你明确的意思是说：“如果customerPackage是'a'或者是'A'，并且小时数足够大，那么......”。

试试这个行：

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

Answer 4

其他人已经帮你与你已经注意到这个问题。

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

如果你想所有这三个由else控制的那些语句，你需要将它们用括号来创建一个复合语句：我跟你显然没有注意到（还）一个单独的问题开始：

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

因为它现在代表，你的代码是真的：

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

即对于excessCharged和的计算是否执行，而不管的if声明中的条件是否为真或假。

我还注意到，您的switch声明并没有真正大有作为：

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，你究竟需要为所有的情况下（除了默认）同样的动作。您可以通过使用落空的情况下有点简化这个：

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，你可能会考虑这样的：

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; 
} 

就我个人而言，我会不同结构的东西了一下：首先得到一个有效的输入，那么获得下：

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 5

有了你有（你问other question）的新的问题，你需要一些调整。

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

    amountDue = packageB; 

    else 
    { 
     /* calculations */ 
    } 

是不正确的，这应该是

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

否则，当包= B和小时= 20，否则计算将在所有其他情况下完成的第一条语句只会被执行，像当包是A，或C.

希望这有助于！