C++ if语句中的多个条件

Question

我对C++编程的概念很陌生.我希望有一个多条件if语句使用|| (或)和&&(和)在一个陈述中.当我问我的大学教授时.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了一个multi &&语句,只有一个显示了||.它没有显示它们一起使用.我想学习如何使线路正常工作.我会附上我的代码.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

你的问题是&&优先级高于||你需要的parens.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

其他人已经帮助你解决了你注意到的问题.我将从一个你显然没有注意到的单独问题开始(还):

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

如果您希望由这些语句控制所有这三个语句else,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,您的代码确实是:

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

即,无论声明中的条件是真还是假excessCharged,amountDue都执行和计算.if

我还要注意你的switch陈述并没有真正实现:

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,您对所有情况采取完全相同的操作(默认情况除外).您可以通过使用直通案例来简化这一点:

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,你可以考虑这样的事情:

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;
}

但就个人而言,我的结构有点不同:首先得到一个有效的输入,然后得到下一个:

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)

你已经非常接近得到正确答案了。让我给你两个提示：

1. 操作员=与操作员不一样==。=是赋值运算符。它计算其右侧并将结果存储在其左侧命名的变量中。您需要==，等于运算符。它测试其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图。你的意思显然是说“如果customerPackage是或者'a'它是'A'，并且也hoursUsed足够大，那么......”。

试试这一行：

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