整数输入验证，怎么样？ [英] integer input validation, how?

问题描述

我有一个问题，应该是令人难以置信的简单的代码。我想要在1和3之间的一个整数与错误检查。它适用于检查过大或过小的数字，但是当输入字母/数字组合时，它会陷入无限循环。建议？

I'm having a problem with what should be incredibly simple code. I want to take in an integer between 1 and 3 with error checking. It works fine for checking for numbers that are too large or too small, but when a alpha/number combination is entered, it gets stuck in an infinite loop. Suggestions?

#include <iostream>
using namespace std;

int main(int argc, char *argv[]){
    int input;

    cout << "\nPlease enter a number from 1 to 3:" << endl;
    cout << "-> ";
    cin >> input;

    while(input< 1 || input> 3){
        cout << "\n---------------------------------------" << endl;
        cout << "\n[!] The number you entered was invalid." << endl;
        cout << "\nPlease re-enter a number from 1 to 3" << endl;
        cout << "-> ";
        cin >> input;
    }

    cout << "You chose " << input << endl;
}

推荐答案

问题是： / p>

The problem is that:

cin >> input;

当您尝试读取非数字值时，将导致设置错误位。之后，任何尝试使用运算符>> 的尝试被默认忽略。

Will cause the bad bit to be set when you try and read a non numeric value. After that happens any attempt to use the operator>> is silently ignored.

这是为了测试流是否处于良好状态，如果没有，则重置状态标志并尝试再次读取。但是请注意，错误的输入（导致问题）仍然在输入上，因此您需要确保将其丢弃。

So the way to correct for this is to test if the stream is in a good state and if not then reset the state flags and try and read again. But note that the bad input (that caused the problem) is still on the input so you need to make sure you throw it away as well.

if (cin >> input)
{
    // It worked (input is now in a good state)
}
else
{
    // input is in a bad state.
    // So first clear the state.
    cin.clear();

    // Now you must get rid of the bad input.
    // Personally I would just ignore the rest of the line
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    // now that you have reset the stream you can go back and try and read again.
}

为了防止它卡住（这是由设置坏位引起的）读入字符串，然后使用字符串流解析用户输入。我也喜欢这种方法（对于用户交互输入），因为它允许更容易地组合不同的阅读风格（即组合 operator>> 和 std :: getline（），因为你可以在stringstream上使用这些。）

To prevent it getting stuck (which is caused by the bad bit being set) read into a string then use a string stream to parse user input. I also prefer this method (for user interactive input) as it allows for easier combination of different styles of reading (ie combining operator>> and std::getline() as you can use these on the stringstream).

#include <iostream>
#include <sstream>
#include <string>
// using namespace std;
// Try to stop using this.
// For anything other than a toy program it becomes a problem.

int main(int argc, char *argv[])
{
    int          input;

    std::string  line;
    while(std::getline(std::cin, line))   // read a line at a time for parsing.
    {
        std::stringstream linestream(line);
        if (!linestream >> input)
        {
             // input was not a number
             // Error message and try again
             continue;
        }
        if ((input < 1) || (input > 3))
        {
             // Error out of range
             // Message and try again
             continue;
        }
        char errorTest;
        if (linestream >> errorTest)
        {
             // There was extra stuff on the same line.
             // ie sobody typed 2x<enter>
             // Error Message;
             continue;
        }

        // it worked perfectly.
        // The value is now in input.
        // So break out of the loop. 
        break;
    }
}

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