使用cin.fail()防止无效输入

Question

我刚刚完成了这个大理石游戏程序的工作，它工作得很好，除了我需要做一件事来保护它免受无效输入。程序正确响应所有基于整数的无效输入，但如果输入非整数，则程序崩溃。我一直在使用cin.fail()函数，它起作用，但随后陷入循环。如果用户输入一个非整数，我希望程序告诉用户这是无效的输入，并提示用户再次输入他们的选择。

我添加了return 1;在无效的输入语句之后结束程序，而不是它崩溃，但这不是我想要做的。我希望它打印"无效输入"，然后重新提示用户再次输入他们的选择。

#include <iostream>
using namespace std;
int main()
{
    cout << "*** The Game of Nim ***nn" << endl;
    string player1, player2, currentPlayer;
    int marbles, selection;
    bool isDone = false;
    cout << "Enter a name for Player 1: ";
    cin >> player1;
    cout << "Enter a name for Player 2: ";
    cin >> player2;
    cout << "Enter how many marbles you would like to start with in the pile: ";
    cin >> marbles;
    cout << "nThere are " << marbles << " marbles in the pilen" << endl;
    currentPlayer = player1;
    do
    {
        bool turnDone = false;
        while (turnDone == false && marbles != 0)
        {
            if (currentPlayer == player1)
            {
                cout << currentPlayer << "n";
                cout << "How many marbles would you like to draw? : ";
                cin >> selection;
                if (cin.fail())
                {
                    cout << "nInvalid Inputn";
                    return 1;
                }

                if (selection < marbles / 2 + 1 && marbles > 1 && selection != 0)
                {
                    marbles = marbles - selection;
                    if (marbles == 1)
                    {
                        cout << "nUh Oh! There is only 1 marble remaining in the pilen" << endl;
                        currentPlayer = player2;
                        turnDone = true;
                        break;
                    }
                    cout << "nThere are " << marbles << " marbles remaining in the pilen" << endl;
                    currentPlayer = player2;
                    turnDone = true;
                }else if (marbles == 1)
                {
                    marbles = marbles - selection;
                    if (marbles <= 0)
                    {
                        cout << "nThere are 0 marbles remianing in the pilen";
                        cout << "You drew the last marble!" << endl;
                        cout << "You lose!nn" << endl;
                        cout << player2 << " winsnn" << endl;
                        turnDone = true;
                        return 0;
                    }
                }else if (selection == 0 || selection > marbles / 2 + 1)
                {
                    cout << "nYou must draw at least 1 marble, and no more than half the pilen" << endl;
                }
            }else if (currentPlayer == player2)
            {
                cout << currentPlayer << "n";
                cout << "How many marbles would you like to draw? : ";
                cin >> selection;
                if (selection < marbles / 2 +1 && marbles > 1 && selection != 0)
                {
                    marbles = marbles - selection;
                    if (marbles == 1)
                    {
                        cout << "nUh Oh! There is only 1 marble remaining in the pilen" << endl;
                        currentPlayer = player1;
                        turnDone = true;
                        break;
                    }
                    cout << "nThere are " << marbles << " marbles remaining in the pilen" << endl;
                    currentPlayer = player1;
                    turnDone = true;
                }else if (marbles == 1)
                {
                    marbles = marbles - selection;
                    if (marbles <= 0)
                    {
                        cout << "nThere are 0 marbles remianing in the pilen";
                        cout << "You drew the last marble!" << endl;
                        cout << "You lose!nn" << endl;
                        cout << player1 << " wins!nn" << endl;
                        turnDone = true;
                        return 0;
                    }
                }else if (selection == 0 || selection > marbles / 2 + 1)
                {
                    cout << "nYou must draw at least 1 marble, and no more than half the pilen" << endl;
                }
            }
        }
    }while (isDone == false);
    return 0;
}

Answer 1

下面是如何在程序中使用std::cin.fail():

int marbles;
bool check = false;
do {
    std::cout << "Enter how many marbles you would like to start with in the pile: ";
    std::cin >> marbles;
    if (std::cin.fail()) {
        std::cin.clear();
        std::cin.ignore();
    }
    else
        check = true;
}while(!check);

cin.fail()将返回一个布尔值，表示是否收到了预期的收入
如果没有，则必须使用clear()，它将失败值设置为false，然后忽略给定的输入
希望对你有所帮助

Answer 2

获取用户输入的字符串并检查其中是否有数字:

std::string input; 
std::cin >> input;
int marbles = std::atoi(input.c_str()); //Returns the first number met in the char table, if there is none, returns 0
if(marbles)
    std::cout << marbles;