Question

我仍然是C ++的新手，当收到的输入（客户）不等于“ r”，“ R”，“ b”或“ B”时，我试图使代码打印为“无效的客户类型”。然后，它再次提示用户输入字母。使用我编写的代码“无效的客户类型”。即使客户==字母“ r”，“ R”，“ b”或“ B”之一时，仍会打印出。我尝试使客户成为char而不是string，但这并没有太大变化。

#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&&代替||

#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;
}

为简单起见，如果我们仅采用前两个条件

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

这些条件之一将始终为true，并且在使用逻辑或时，如果任何子条件的值为true，则整个条件的值为true。

我们需要使用逻辑AND，以便如果任何子条件为假，则整个条件的值为假。

这有点令人困惑，因为如果您的条件是测试相等性而不是不平等，您将使用||。

例如

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

您还可以将代码重构得更多DRY

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将是：

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

要检查客户的无效性，您必须采取上述条件的逻辑非：

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

这将起作用。但是，您可以将De Morgan's Law应用于此表达式，结果将是：

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

使用多个！=和||在if陈述中

2 个答案: