c++ - 如果我输入字母而不是数字，为什么会出现无限循环？

Question

我正在为家庭作业编写此代码(刚开始使用 C++，所以请放轻松)。我们今天刚刚开始了 while、do-while 和 for 循环。该程序运行良好，只是如果您在程序要求整数时输入一个字母，它将无限循环。到底是怎么回事？ (代码如下) ***编辑:为了澄清，循环的部分是:“您输入的数字是负数。请输入正数以继续。”但是用户没有机会输入另一个号码。它只是不断打印这个。

    #include <iostream>
using namespace std;

int main ( )
{
    //define variables
    int num1, num2, total;
    char answer1;

    do
    {
        //user enters a number
        cout << "\nPlease enter a positive number and press Enter: \n";
        cin >> num1;

        //check that the given num1 value is positive
        while (num1 < 0)
        {
            cout << "The number you entered is negative.\nPlease enter a positive number to continue.\n";
            cin >> num1;
        }

        cout << endl;

        //add the sum of 1 through num1 value
        num2 = 1;
        total = 0;
        while (num1 >= num2)
        {
            total = total + num2;
            num2 ++;
        }

        //tell the user the sum
        cout << "The total of all the integers\nfrom 1 to " << num1 << " is: \n";
        cout << total;

        //ask if the user wants to try again
        cout << "\n\nWould you like to try again with a new number?\nEnter y for yes or n for no.\n";
        cin >> answer1;
    } while (answer1 == 'y');   

    cout << endl;
    return 0;
}

Answer 1

basic_istream 就是这样工作的。在您的情况下，当 cin >> num1 输入错误时 - failbit 已设置且 cin 未清除。所以下次它会是同样的错误输入。要正确处理此问题，您可以添加检查输入是否正确，并在输入错误的情况下清除&ignore cin。例如:

    #include<limits>

    //user enters a number
    cout << "\nPlease enter a positive number and press Enter: \n";
    do {    
        while(!(cin >> num1)) {
            cout << "Incorrect input. Please try again.\n";
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
        }
        if(num1 < 0) cout << "The number you entered is negative. Please enter a positive number to continue.\n";
    } while(num1 < 0);