如果我输入一个字母，我的程序怎么会卡在它的 while 循环中？

Question

对不起，如果我不够清楚或有任何错误，这是我第一次发帖。

我的代码在编译时运行没有错误，但是每当用户为 cin >> select; 键入字母（如 "a"）而不是必需的时，第一个 while 循环（在 int main 中）就会卡住循环1、2 或 3。

但是，当我输入 "4" 或任何其他随机数字字符串时，它运行良好，并按应有的方式转到我的错误消息。

为什么会这样，我该怎么做才能使其正常运行？ （运行错误消息以响应输入的字母，就好像它们是数字一样）。

我的代码：

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

void calculator();
void unavailableitem();

int main()
{
    string select;
    while (true)
    {
        cout << "\t[Main Menu]\n";
        cout << " 1. Calculator\n";
        cout << " 2. [unavailable]\n";
        cout << " 3. [unavailable]\n";
        cout << "\n Enter the number of your selection: ";
        cin >> select;

        if (select == "1")
        {
            cout << endl;
            calculator();
            break;
        }
        else if (select == "2")
        {
            unavailableitem();
            break;
        }
        else if (select == "3")
        {
            unavailableitem();
            break;
        }
        else
            cout << "\nInvalid response.\n";
    }
}

void unavailableitem()
{
    string react;
    cout << "\n \t [ITEM UNAVAILABLE]\n";
    while (true)
    {
        cout << "\nEnter 'menu' to return to main menu: ";
        cin >> react;

        if (react == "menu")
        {
            cout << endl;
            main();
            break;
        }
        else
            cout << "\nInvalid response.\n";
    }
}

void calculator()
{
    int choice;
    double num1;
    double num2;
    double answer;
    string choicesymbol;

    cout << "List of operations:\n";
    cout << " 1. Addition\n";
    cout << " 2. Subtraction\n";
    cout << " 3. Multiplication\n";
    cout << " 4. Division\n";
    cout << "Enter the number on the left to pick an operation: ";
    cin >> choice;
    cout << "\nEnter number 1: ";
    cin >> num1;
    cout << "\nEnter number 2: ";
    cin >> num2;

    if (choice == 1)
    {
        answer = num1 + num2;
        choicesymbol = " + ";
    }

    if (choice == 2)
    {
        answer = num1 - num2;
        choicesymbol = " - ";
    }

    if (choice == 3)
    {
        answer = num1 * num2;
        choicesymbol = " * ";
    }

    if (choice == 4)
    {
        answer = num1 / num2;
        choicesymbol = " / ";
    }

    cout << endl;
    cout << num1 << choicesymbol << num2 << " = " << answer;
}

新代码：

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

void calculator();
void unavailableitem();


int main()
{
    int select;
    while (true)
    {
        cout << "\t[Main Menu]\n";
        cout << " 1. Calculator\n";
        cout << " 2. [unavailable]\n";
        cout << " 3. [unavailable]\n";
        cout << "\n Enter the number of your selection: ";
        cin >> select;

        if(!(cin >> select))
        {
        cout << "Input must be an integer.\n";
        cin.clear();
        continue;
        }
        else if (select == 1)
        {
            cout << endl;
            calculator();
            break;
        }
        else if (select == 2)
        {
            unavailableitem();
            break;
        }
        else if (select == 3)
        {
            unavailableitem();
            break;
        }
    }
}



void unavailableitem()
{
    string react;
    cout << "\n \t [ITEM UNAVAILABLE]\n";
    while (true)
    {
        cout << "\nEnter 'menu' to return to main menu: ";
        cin >> react;

        if (react == "menu")
        {
            cout << endl;
            return;
            break;
        }
        else
            cout << "\nInvalid response.\n";
    }
}

void calculator()
{
    int choice;
    double num1;
    double num2;
    double answer;
    string choicesymbol;

    cout << "List of operations:\n";
    cout << " 1. Addition\n";
    cout << " 2. Subtraction\n";
    cout << " 3. Multiplication\n";
    cout << " 4. Division\n";
    cout << "Enter the number on the left to pick an operation: ";
    cin >> choice;
    cout << "\nEnter number 1: ";
    cin >> num1;
    cout << "\nEnter number 2: ";
    cin >> num2;

    if (choice == 1)
    {
        answer = num1 + num2;
        choicesymbol = " + ";
    }

    if (choice == 2)
    {
        answer = num1 - num2;
        choicesymbol = " - ";
    }

    if (choice == 3)
    {
        answer = num1 * num2;
        choicesymbol = " * ";
    }

    if (choice == 4)
    {
        answer = num1 / num2;
        choicesymbol = " / ";
    }

    cout << endl;
    cout << num1 << choicesymbol << num2 << " = " << answer;
}

Answer 1

我是一般编程的新手，但是玩你的代码和查找东西让我找到了某种解决方案。 cin.clear只是清除了输入的错误日志，相信还是保留了字母的值。

你应该在后面添加一个cin.ignore(#,'\n')（# 是一个非常非常大的数字）让它避开该行并直接跳过它。

找到了解决方案in another question，它解释了两个 cin 命令的使用。

Answer 2

Ad Ed Heal 提到，这里的问题是 cin 的故障位。当您执行cin >> choice 并且用户键入“a”时，转换为int 将失败。这将设置cin 的故障位，使所有未来的读取都失败，直到故障位被清除。因此，下次您到达cin >> choice 时，用户甚至都无法输入任何内容。

您可以使用cin.clear() 恢复正常工作状态。

要更稳健地执行此操作，您可以执行类似的操作

while(true)
{
    cout >> "Enter choice [1-4]: ";
    if(!(cin >> choice))
    {
        cout << "Input must be an integer.\n";
        cin.clear();
        continue;
    }
    do_stuff_with_choice();
 }