c++ - 如果我输入一个字母输入，为什么我的程序会卡在它的 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，但第一个 while 循环(在 int main 中)在用户键入一个字母(如 “a”)时卡在循环中cin >> select; 而不是所需的 1、2 或 3。

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

为什么会这样，我该怎么做才能让它正常运行？ (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

Ad Ed Heal 提到，这里的问题是cin 的故障位。当您执行 cin >> choice 时，用户键入“a”，然后转换为 int 失败。这设置了 cin 的 failbit，使得所有 future 的读取都失败，直到 failbit 被清除。因此，下次您到达 cin >> choice 时，用户甚至不会输入任何内容。

您可以使用 cin.clear() 恢复工作状态。

为了更稳健地做到这一点，你可以做类似的事情

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