在if-statement中使用多个!=和||

Question

我在C++中还是个新手，我试图让我的代码在收到的输入(客户)不等于"r“、"R”、"b“或"B”时打印"Invalid customer type“。然后，它再次提示用户输入字母。使用我编写的代码"Invalid customer type“。即使当客户== "r“、"R”、"b“或"B”中的一个字母时，仍然打印出来。我曾尝试将客户转换为char而不是string，但这并没有太大的变化。

#include <iostream>
#include <string>
using namespace std;

int main ()
{
    string customer;
    do
    {
        cout << "Pleas Enter the Customer Type (R for Regular, B for Business): " << endl;
        cin >> customer;
        if ((customer != "r") || (customer != "R") || (customer != "b") || (customer != "B"))
        {
            cout << "Invalid customer type." << endl;
        }

    } while ((customer != "r") || (customer != "R") || (customer != "b") || (customer != "B"));
    return 0;
}

Answer 1

使用&&而不是||

#include <iostream>
#include <string>
using namespace std;

int main ()
{
    string customer;
    do
    {
        cout << "Pleas Enter the Customer Type (R for Regular, B for Business): " << endl;
        cin >> customer;
        if ((customer != "r") && (customer != "R") && (customer != "b") && (customer != "B"))
        {
            cout << "Invalid customer type." << endl;
        }

    } while ((customer != "r") && (customer != "R") && (customer != "b") && (customer != "B"));
    return 0;
}

为了简单起见，如果我们只考虑前两个条件

(customer != "r") || (customer != "R")

这些条件之一将始终为真，并且在使用逻辑OR时，如果任意子条件的计算结果为真，则整个条件的计算结果为真。

我们需要使用逻辑AND，这样如果任意子条件为false，则整个条件的计算结果为false。

这有点令人困惑，因为如果您的条件测试的是相等而不是不平等，则会使用||。

例如：

if ((customer == "r") || (customer == "R") || (customer == "b") || (customer == "B"))

您还可以重构您的代码，使其更具DRY色彩

int main ()
{
    string customer;
    while(true)
    {
        cout << "Pleas Enter the Customer Type (R for Regular, B for Business): " << endl;
        cin >> customer;
        if ((customer == "r") || (customer == "R") || (customer == "b") || (customer == "B"))
        {
            break;
        }

        cout << "Invalid customer type." << endl;
    } 
}

Answer 2

如果客户输入r、R、b或B，则它是有效客户的输入。这意味着有效客户的条件是：

((customer == "r") || (customer == "R") || (customer == "b") || (customer == "B"))

要检查客户的无效性，您必须采用上述条件的logical not，即：

!((customer == "r") || (customer == "R") || (customer == "b") || (customer == "B"))

这将会起作用。但是，您可以将De Morgan's Law应用于此表达式，这将导致：

((customer != "r") && (customer != "R") && (customer != "b") && (customer != "B"))