多條件if語句C++

Question

我對C++編程的概念非常陌生。我希望有一個多條件，如果使用||語句或者在一個語句中和和/或&/&。當我問我的大學教授這件事。她說這是可能的，然後侮辱了我對這個問題的有限知識。我有權訪問的所有示例都顯示多條語句，只有一條顯示||。它不顯示它們一起使用。我會學習如何讓線路工作。我會附上我的代碼。問題領域是編碼的最後一個位。

# 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

您的問題是&&的優先級高於||所以你需要括號。正如評論指出的，你還需要使用==，而不是分配（=）的：

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

Answer 2

您可以使用括號來指定這些布爾運算符的執行順序。你可能想評估||第一，所以你會使用：

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

的&&通常由默認的第一個評價，因爲它具有更高的優先級，讓你的代碼是相同的：

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

另外，如評論中所述，使用==進行比較，=進行分配。

Answer 3

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

您是所以接近有正確的答案。讓我給你兩個提示：

1. 的=運營商是不一樣的==操作。 =是賦值運算符。它評估其右側並將結果存儲在其左側命名的變量中。你想要==，等於運算符。它測試看它的右側和左側是否相等。

2. 使用括號(...)來強制執行您的評估順序意向。你明確的意思是說：「如果customerPackage是'a'或者是'A'，並且小時數足夠大，那麼......」。

試試這個行：

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

Answer 4

其他人已經幫你與你已經注意到這個問題。

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

如果你想所有這三個由else控制的那些語句，你需要將它們用括號來創建一個複合語句：我跟你顯然沒有注意到（還）一個單獨的問題開始：

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

因爲它現在代表，你的代碼是真的：

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

即對於excessCharged和的計算是否執行，而不管的if聲明中的條件是否爲真或假。

我還注意到，您的switch聲明並沒有真正大有作爲：

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: "; 

特別是，你究竟需要爲所有的情況下（除了默認）同樣的動作。您可以通過使用落空的情況下有點簡化這個：

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，你可能會考慮這樣的：

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; 
} 

就我個人而言，我會不同結構的東西了一下：首先得到一個有效的輸入，那麼獲得下：

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（你問other question）的新的問題，你需要一些調整。

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

    amountDue = packageB; 

    else 
    { 
     /* calculations */ 
    } 

是不正確的，這應該是

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

否則，當包= B和小時= 20，否則計算將在所有其他情況下完成的第一條語句只會被執行，像當包是A，或C.

希望這有助於！