为什么此代码段无限循环？ [英] Why is this code snippet infinitely looping?

问题描述

我对编程还很陌生，我正尝试从 cin 过滤输入（没有特殊原因；这是我所知道的唯一输入方法）现在），这样一来只能输入1-10并获得肯定响应。这是我的代码：

I'm fairly new to programming, and I'm trying to filter inputs from cin (for no particular reason; It is the only input method I know of right now) such that one can only enter 1-10 and get the "positive" response. Here is my code:

#pragma region Check cin is a valid input (an integer between 1 and 10, not a string or number out of range)
int input;
bool valid2=false;
    while (!valid2)
    {
        cout << "Enter an integer (1-10): ";
        cin >> input;
        if (!cin || input > 10 || input < 1)
        {
            cout << "Error: Invalid. Try again." << endl << endl;
            cin.ignore(numeric_limits<streamsize>::max());
            cin.clear();
        }
        else
        {
            valid2 = true;
        }
    }
#pragma endregion

忽略 pragma ，对于每个小实验，我都有一个带有块的测试脚本，以节省时间，这就是当它们不再有效时我将它们折叠的方法。

Ignore the "pragma", I have one test script with blocks for each little experiment to save me time and this is just how I collapse them when they're no longer valid.

我的问题（除其他外）是，当我输入 f 之类的非整数时，它会无限循环错误：无效。请重试 ，我不知道为什么。我以为也许是由于定义了 input 之后，它又回到了 cin 之后。但是我检查了一个孤立的示例，在继续之前， cin 让我专门重新定义了输入。

My problem (among others) is that when I enter a non-integer such as f, it loops infinitely with "Error: Invalid. Try Again", and I can't figure out why. I thought maybe since input became defined, it was running past cin when it comes back around. But I checked an isolated example, and cin let me specifically redefine input before continuing.

侧边栏：说到 pragma ，有人在c ++中有更好的方法吗？除了创建单个方法之外，因为我的大多数测试都可以在 main 内部运行。

Sidebar: Speaking of pragma, does anyone have a better method for this in c++? Aside from making individual methods, because most of my tests can just run inside main.

编辑：
我已经切换了 cin.ignore（...）和 cin.clear（）现在程序可以正确过滤输入了；但是现在，它根本不会循环播放。给我后，错误：无效。请重试。 ，它挂在应再次要求我输入整数的位置。

I've switched cin.ignore(...) and cin.clear() and now the program can properly filter the inputs; however now, it doesn't loop back at all. After giving me "Error: Invalid. Try again.", it hangs where it should ask me again to enter the integer.

推荐答案

您的原始代码有两个问题（至少就我发现的问题而言-不保证不再存在）：

Your original code has TWO problems (at least as far as I've spotted - not guaranteeing there aren't any more):

1. cin.clear（）和 cin的顺序。 ignore（）- cin.clear（）应该在 cin.ignore（）之前，或 cin.ignore（）无效。


2. cin.ignore（numeric_limits< streamsize> :: max（））; 一直读取到分隔符为止，默认情况下，该分隔符为 EOF -换句话说，无论缓冲区中输入了什么或在缓冲区中输入了什么，它都会继续接受它。

1. The order of cin.clear() and cin.ignore() - cin.clear() should be before cin.ignore(), or cin.ignore() will do nothing.
2. The cin.ignore(numeric_limits<streamsize>::max()); is reading until the delimiter, which defaults to EOF - in other words, no matter what input is in the buffer, or entered after, it keeps on accepting it.

要解决这两个问题，请使用以下方法：

To fix both problems use this:

  cin.clear();
  cin.ignore(numeric_limits<streamsize>::max(), '\n');

这将一直读到当前行的末尾，这可能与您所读内容的相似想。

that will read until the end of the current line, which is probably more along the lines of what you want.

哦，然后更改：

    cin >> input;
    if (!cin || input > 10 || input < 1)

到：

    if (!(cin >> input) || input > 10 || input < 1)

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