c++ - 如何将输入限制为仅数字

Question

我最近创建了一个程序，可以根据用户输入创建一道数学题。通过输入 1-4，程序可能会产生问题，或者用户可以通过输入 5 来退出。我遇到的唯一问题是，当我输入一个字符时，程序会进入无限循环。我可以使用什么函数来检查输入是否不是数字以便显示错误消息？

---

//CIS180 Assignment #4
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
using namespace std;

int main()
{
    //Declare variables.
    int num1, num2, menuNum;
    int addInput, subInput, multInput, divInput;
    int addAnswer, subAnswer, multAnswer, divAnswer;
    int addSolution, subSolution, multSolution, divSolution;
    srand(time(0));
    //Display menu.
    cout << "Menu" << endl;
    cout << "1. Addition problem" << endl;
    cout << "2. Subtraction problem" << endl;
    cout << "3. Multiplication problem" << endl;
    cout << "4. Division problem" << endl;
    cout << "5. Quit this program" << endl << endl;
    cout << "Enter your choice (1-5): " << endl;
    cin >> menuNum;
    //Loop that will provide math problems when user inputs number.
    while(menuNum != 5)
    {
        //Check if the input is valid.
        while((menuNum < 1) || (menuNum >5))
        {
            cout << "The valid choices are 1, 2, 3 , 4, and 5. Please choose: " << endl;
            cin >> menuNum;
        }
        //Generate two random numbers for addition and display output.
        if(menuNum == 1)
        {
            num1 = rand()%500 + 1;
            num2 = rand()%500 + 1;
            addSolution = num1 + num2;
            cout << setw(5) << right << num1 << endl;
            cout << setw(2) << left << "+" << setw(3) << right << num2 << endl;
            cout << setw(5) << fixed << "-----" << endl;
            cin >> addAnswer;
            //Check if the addition answer input is correct.
            if(addAnswer != addSolution)
                cout << "Sorry, the correct answer is " << addSolution << "." << endl;
            else if(addAnswer == addSolution)
                cout << "Congratulations! That's right." << endl << endl;
        }
        .
            .
            .

Answer 1

首先，您应该检测您的输入尝试是否成功:始终检查之后读取是否成功。接下来，当您确定无法读取某个值时，您需要使用 clear() 将流重置为良好状态。并且您需要摆脱任何不良字符，例如，使用 ignore() .鉴于字符通常是输入的，即用户必须在使用字符之前按回车键，因此通常可以合理地获取整行。例如:

for (choice = -1; !(1 <= choice && choice <= 5); ) {
    if (!(std::cin >> choice)) {
         std::cout << "invalid character was added (ignoring the line)\n";
         std::cin.clear();
         std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
}

std::numeric_limits<std::streamsize>::max()的使用是获得魔数(Magic Number)的方法ignore()尽可能多的字符，直到找到具有第二个参数值的字符。