循环不终止时的C ++输入验证

Question

我是C ++计算机科学入门课程的学生，这是我第一次在这里发布。 我们刚刚了解了while循环，尽管该赋值不需要它，但我正在尝试对该赋值进行输入验证。 该程序旨在读取数字列表，并找出该列表中前8位和后8位的位置。 因此，如果我有四个数字（1、8、42、8）的列表，则前8个位置是2和4。该集合的大小由用户确定。

我试图进行一个while循环测试，以确保用户输入的内容实际上是一个数字，但是当我尝试输入类似“”的内容时。 或“ a”循环无限进行且不会终止。 我找不到错误，据我所知，我使用的语法与教科书中的语法完全相同。 有人可以告诉我我的while循环怎么了吗？

int numbers,            //How large the set will be
    num,                //What the user enters for each number
    first8position = 0, //The first position in the set that has an 8
    last8position = 0;  //The last position in the set that has an 8

//Prompt the user to get set size
cout << "How many numbers will be entered?  ";
cin >> numbers;

//Loop to get all the numbers of the set and figure out
//which position the first and last 8 are in
for (int position = 1; position <= numbers; position++)
{
    cout << "Enter num:  ";
    cin >> num;

    //If num isn't a digit, prompt the user to enter a digit
    while (!isdigit(num))
    {
        cout << "Please enter a decimal number:  ";
        cin >> num;
    }

    //If num is 8, and first8position still isn't filled,
    //set first8position to the current position.
    //Otherwise, set last8position to the current position.
    if (num == 8)
    {
        if (first8position == 0)
            first8position = position;
        else
            last8position = position;
    }


}

//If the set had an 8, print what its position was
if (first8position != 0)
    cout << "The first 8 was in position " << first8position << endl;

//If there was more than one 8, print the last 8 position.
//Otherwise, the first and last 8 position are the same.
if (last8position != 0)
    cout << "The last 8 was in position " << last8position << endl;
else
    cout << "The last 8 was in position " << first8position << endl;

//If there were no 8s, say so.
if (first8position == 0)
    cout << "Sorry, no eights were entered.";

return 0;

}

Answer 1

导致无限循环的有两个问题：

首先，使用cin >> num ，您尝试读取一个整数值。 如果用户输入的东西像a或. ，它不能是整数值的开头，不会读取任何内容，并且a或. 保留在输入缓冲区中； 因此，每个随后的cin >> num都将立即失败（因为a或.仍在输入缓冲区中并将保持在那里，所以没有给用户输入任何内容的机会）。 因此，在这种情况下，您将不得不使用cin这些字符，例如，通过使用cin.ignore ，并且还必须重置在这种情况下设置的failbit 。

其次，请注意isdigit(int c)检查ASCII值c是否为数字，即c >= 48 && c <= 57 。 因此，直到用户输入48到57之间的数字，您的check isdigit(num)才会失败。

请参阅以下代码，演示如何处理输入失败。 希望能帮助到你。

int main() {

    int num;
    cin >> num;
    while (!cin.eof() && cin.fail()) {  // failure when extracting an integral value?
        cout << "not an integral value." << endl;

        // clear failbit
        cin.clear();

        // remove characters that are still in the input buffer (until next end of line)
        cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

        // next try to read in an integer
        cin >> num;
    }
    if (!cin.eof()) {
        cout << "juu:" << num << endl;
    }
}