Check if input is integer

问题

In order to learn C++, I'm translating a program I wrote in Python.

I wrote this

n = 0
while n < 2:
    try:
        n = int(raw_input('Please insert an integer bigger than 1: '))
    except ValueError:
        print 'ERROR!'

in order to get an integer bigger than 1 from the user.

This is what I wrote in C++ for the moment:

int n = 0;
while (n < 2) {
    cout << "Please insert an integer bigger than 1: ";
    cin >> n;
}

I took a look at try-catch and it seems pretty straight forward. My concern is about how to check the input is an integer. I read about cin.fail() but I couldn't find any official document and I didn't really get how it works.

So, how can I check if the input is integer?

More in general, how can I check if the input is "anything"?

回答1:

For a situation like this, you'd probably want to read the input as a string, then inspect the string (e.g., "contains only digits, up to a maximum of N digits"). If and only if it passes inspection, parse an int out of it.

It's also possible to combine the inspection and conversion--for example, Boost lexical_cast<int>(your_string) will attempt to parse an int out of the string, and throw an exception if it can't convert the whole thing to an int.

回答2:

Your Python can code be translated more directly if you use C++11's std::stoi combined with std::getline to read a whole line of input. This is much easier than struggling with standard I/O error handling, which arguably does not have a very user-friendly interface.

std::stoi throws std::invalid_argument if the input could not be correctly parsed as an integer number, and std::out_of_range if the number is too small or too big to fit in an int.

#include <iostream>
#include <string>

int main() {
    int n = 0;
    while (n < 2) {
        std::cout << "Please insert an integer bigger than 1: ";
        std::string input;
        std::getline(std::cin, input);
        try {
            n = std::stoi(input);
        } catch (std::exception const&) {
            std::cerr << "ERROR!\n";
        }
    }
}

If you want to make the code even more similar to its Python equivalent, then you can encapsulate the input in a function:

#include <iostream>
#include <string>

int raw_input(std::string const& message)
{
    std::cout << message;
    std::string input;
    std::getline(std::cin, input);
    return std::stoi(input);
}

int main() {
    int n = 0;
    while (n < 2) {
        try {
            n = raw_input("Please insert an integer bigger than 1: ");
        } catch (std::exception const&) {
            std::cout << "ERROR!\n";
        }
    }
}

来源：https://stackoverflow.com/questions/35299101/check-if-input-is-integer

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python

c++

cin