如何在do ... while循环中验证输入，然后在必要时重新提问？

问题描述

到目前为止，我已经尝试做一个do ... while循环，它询问两个问题。一个是多少英里，第二个是包装重量。如果英里数等于或小于0，则应输出错误，然后重新提问。验证后，它应该转到具有相同要求的权重问题，并且如果权重无效，则应该重复权重问题，因为里程问题已经有效。

这是我到目前为止的代码：

 apiInterface = ApiClient.getClient().create(ApiInterface.class);
apiInterface.getTopRatedMovie(API_KEY)
                .interval(3,TimeUnit.SECONDS)
                .flatMap(n -> apiInterface.getTopRatedMovie(API_KEY))
                .repeat()
                .subscribe(new Observer<MovieResopnse>() {
                    @Override
                    public void onSubscribe(@NonNull Disposable d) {
                        compositeDisposable.add(d);
                    }

                    @Override
                    public void onNext(@NonNull MovieResopnse movieResopnse) {
                        movies = movieResopnse.getResults();
                        Log.d(TAG,"onResponse: Number of movies received: " + movies.size());

                        tv_count.setText("Movie: " + count);
                        count++;
                        Picasso.get()
                                .load(IMAGE_URL + movies.get(item).getPosterPath())
                                .into(image);

                        title.setText(movies.get(item).getOriginalTitle());
                        subtitle.setText(movies.get(item).getTitle());
                        description.setText(movies.get(item).getOverview());
                        rating.setText("" + movies.get(item).getVoteAverage());
                        release_date.setText("Release: " + movies.get(item).getReleaseDate());
                    }

                    @Override
                    public void onError(@NonNull Throwable e) {
                        Log.d(TAG,"onError: " + e.getMessage());
                        Toast.makeText(MainActivity.this,"Error: " + e.getMessage(),Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public void onComplete() {

                    }
                });

解决方法

您可以使用多个while循环，每个输入一个。

int miles = 0;
double weight = 0.0;
bool correct = false;

while (!correct)
{
    cout << "Enter the number of miles as a whole number: " << std::endl;
    bool success = cin >> miles;
    if (!success || miles < 0) {
        cout << "Invalid miles value -- must be nonnegative." << std::endl;
    }
    else {
        correct = true;
    }
}

correct = false;

while (!correct)
{
    cout << "Enter the weight in pounds: " << std::endl;
    bool success = cin >> weight;
    if (!success || weight < 0 || weight > 10) {
        cout << "Invalid weight value -- must be between 0 and 10." << std::endl;
    }
    else {
        correct = true;
    }
}

// do calculation

cout << "\nGood-bye!\n";

return 0;

,

您需要其他循环才能从cin读取，直到输入有效为止。并且您需要确保cin甚至在读取值之前都能成功读取，然后再对该值执行范围检查。

我建议在单独的函数中进行读取+验证，例如：

#include <iostream>
#include <limits>

template<typename T>
T prompt(const char *msg,T maxValue = std::numeric_limits<T>::max())
{
    T value;

    do
    {
        cout << msg << ": ";

        if (!(cin >> value))
        {
            cout << "\n\tError: Invalid input!\n" << endl;
            cin.clear();
            cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
        }
        else if (value < 0)
        {
            cout << "\n\tError: Value must be greater than 0!\n" << endl;
        }
        else if (value >= maxValue)
        {
            cout << "\n\tError: Value must be less than " << maxValue << "!\n" << endl;
        }
        else
            break;
    }
    while (true);

    return value;
}

...

int miles;
double weight;
int choice;

do
{
    miles = prompt<int>("Enter the number of miles as a whole number");
    weight = prompt<double>("Enter the weight of the package in pounds",10.0);
    choice = prompt<int>("Enter 1 to continue or 0 to quit",2);
    cout << endl;
}
while (choice != 0); 

cout << "\nGood-bye!\n";