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c++ - C++ 嵌套 if 语句，基本货币兑换

问题描述

我的 if 语句有一个简单的 C++ 问题，程序将值作为初始 if 参数运行，而不是执行 else if。我还尝试编写代码，以便如果函数的结果等于 1，它将输入语法正确的语句，除了使用另一个 if 语句之外，还有更好的方法吗？

int main()
{
cout << "Please type out amount and currency type to convert to dollars.\n";
double val1;
string currency;
double result = 0;
cin >> val1 >> currency;
if (currency == "yen" || "y")
{
    result = val1/100;
    if (result == 1) {
        cout << "You have " << val1 << "yen which equals \n"
            << result << ", dollar.";
    }
    else
    cout << "You have " << val1 << "yen which equals \n"
        << result << ", dollars.";
}
else if (currency == "euros" || "e")
{
    result = val1*1.25;
    if (result == 1) {
        cout << "You have " << val1 << "euros which equals \n"
            << result << ", dollar.";
    }
    else
        cout << "You have " << val1 << "euros which equals \n"
        << result << ", dollars.";
}
else if (currency == "pounds" || "p")
{
    {
        result = val1 *1.2;
        if (result == 1) {
            cout << "You have " << val1 << "pounds which equals \n"
                << result << ", dollar.";
        }
        else
            cout << "You have " << val1 << "pounds which equals \n"
            << result << ", dollars.";
    }
}
else
    cout << "I didn't understand, please try again.\n";
}

标签： c++if-statement

解决方案

当您输入 时if (currency == "yen" || "y")，您可能希望它的意思是“如果currency等于"yen"或等于"y"，做某事”。

但是，这不是您的 if 语句实际执行的操作。if 语句

if (currency == "yen" || "y")

首先测试字符串currency是否等于"yen"，如果不等于，它会自行评估"y"并检查它是否等于真。

在这种情况下，“y”将被截断为 a bool，这意味着它将评估为true（因为"y"不等于0）。结果，您的第一个 if 语句将始终评估为真！

要解决此问题，请将您的 if 语句替换为 if (currency == "yen" || currency == "y").

对于第二个问题，您可以使用条件运算符，其工作原理如下：

cout << "You have " << (val1 == 1 ? "thing" : "things") << endl;

这种情况下的条件是(val1 == 1 ? "thing" : "things")。它检查 if ，如果是val1 == 1则返回，否则返回。"thing"true"things"

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