减去 std::strings 时出现编译器错误

Question

我是 C++ 的初学者，所以我只是在阅读文章和书籍时搞砸了一些东西。但我花了 20 分钟一遍又一遍地重新阅读这篇文章，我不知道它有什么问题。

#include <iostream>
#include <string>

using namespace std;

int main ()
{

    cout << "Hello there, this is your personal simple calculator.";
    cin.get();
    cout << "Type in what you want to do. (Addition, Subtraction, Multiplication, Division)"<< endl;
    string c;
    getline (cin, c);

    if (c == "Addition")
    {
        string a_1;
        string a_2;
        cout << "You chose addition. Press enter" << endl ;
        cin.get();
        cout << "Type in the first value: ";
        getline( cin, a_1);
        cout << "Type in the second value: ";
        getline (cin, a_2);
        cout << a_1 << " + " << a_2 << " = " << a_1 + a_2 << endl;
    }
    else
    {
        cout << "You spelled it wrong.";
        return 0;
    }
    if ( c == "Subtraction")
    {
        string s_1;
        string s_2;
        cout << "You chose subtraction. Press enter" << endl ;
        cin.get();
        cout << "Type in the first value: ";
        getline (cin, s_1);
        cout << "Type in the second value: ";
        getline (cin, s_2);
        cout << s_1 << " - " << s_2 << " = " << s_1 - s_2 << endl;
    }

}

我认为这是唯一的错误

42 83 C:\Users\Jason\Desktop\Lesson - Header Files\LH1.cpp [错误] no match for 'operator-' in 'first_argument - second_argument'

我不明白。加法符号有效，除减法之外的一切都有效。 所以我搞砸了别的东西

cout << first_argument << " - " << second_argument << " = " << first_argument - second_argument << endl;

但是减法部分工作正常。我不明白。请帮忙

Answer 1

string 可以处理文本。当您添加两个字符串时，它们被连接起来（"2"+"2"=="22"，而不是"4"）。字符串没有operator-。

要处理浮点数，请使用double。要处理整数，请使用int：

double d1, d2;
//some output
cin  >> d1;
//some output
cin  >> d2;
cout << d1 << " - " << d2 << " = " << (d1-d2) << '\n';

Answer 2

加法部分有效，因为字符串 + 字符串导致字符串字符串。它附加两个字符串并返回一个新字符串。

但是减去两个字符串并没有任何意义。

我相信您真正想要做的是将字符串转换为数字，然后减去数字。

为此，您需要使用以下内容：

double val_1, val_2;
cin >> val_1;
cin >> val_2;

cout << "result is " << (val_1 - val_2) << endl;

我将减法放在括号内，因为我相信<< aka "shift" 运算符与乘法处于同一级别，这意味着如果没有它们，它将尝试评估 ("result is "

由于我不确定运算符的优先级，我检查了http://en.cppreference.com/w/cpp/language/operator_precedence 并发现

Answer 3

#include <iostream>
#include <string>
#include <limits>

using namespace std;

int main()
{

    cout << "Hello there, this is your personal simple calculator.";
    cin.get();
    cout << "Type in what you want to do. (Addition, Subtraction, Multiplication, Division)" << endl;
    string c;
    getline(cin, c);

    if (c == "Addition")
    {
        int a_1;
        int a_2;
        cout << "You chose addition. Press enter" << endl;
        cin.get();
        cout << "Type in the first value: ";
        cin >> a_1;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        cout << "Type in the second value: ";
        cin >> a_2;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        cout << a_1 << " + " << a_2 << " = " << a_1 + a_2 << endl;
    }

    else if (c == "Subtraction")
    {
        int s_1;
        int s_2;
        cout << "You chose subtraction. Press enter" << endl;
        cin.get();
        cout << "Type in the first value: ";
        cin >> s_1;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        cout << "Type in the second value: ";
        cin >> s_2;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        cout << s_1 << " - " << s_2 << " = " << s_1 - s_2 << endl;
    }
    else
    {
        cout << "You spelled it wrong.";
        return 0;
    }
}

使用 if else 语句，否则您的代码将没有机会检查用户是否正在尝试减去。最好把 else 放在最后。 还将字符串更改为整数，因为字符串不能减去，因为它们不是数字。 最后一点，我使用 cin.clear() 和 cin.ignore() 来刷新 cin 缓冲区。