C++ Integer Followed by char gets accepted as an input

Question

I am using a function to check that my input in an integer only:

    int input;
    while (true)
    {
        std::cin >> input;
        if (!std::cin)
        {
            std::cout << "Bad Format. Please Insert an integer! " << std::endl;
            std::cin.clear();
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
            continue;
        }
        else
            return input;
    }

However when inputing a integer followed by a char, eg. 3s, the integer gets accepted and the message gets printed.

How can I make sure that the input in such format does not get accepted, nor do input in form 4s 5, so when the integer appears after the space.

std::cin into a string and validate the string using regex or something. — Roy2511
– Roy2511, Commented May 27, 2020 at 7:44

SzymonO · Accepted Answer · 2020-05-27 07:46:55Z

5

That happens because chars in c++ are represented by their numeric value in the ascii table, you can try regular expressions like this:

#include <regex>
#include <string>
    std::string str;
    std::regex regex_int("-?[0-9]");
    while (true)
    {
        std::cin >> str;
        if (regex_match(str, regex_int))
        {  
           int num = std::stoi(str);
           //do sth
           break;
        }
    }

answered May 27, 2020 at 7:46

SzymonO

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2 Comments

Djboboch Over a year ago

Fixed the problem Thank you.

Jarod42 Over a year ago

"That happens because chars in c++ are represented by their numeric value" No, here, the issue is that std::cin >> input; stops once int has been read (so it remains extra char in std::cin).

Pete Becker · Accepted Answer · 2020-05-27 16:37:19Z

0

There's no need for a regular expression that duplicates the validation that std::stoi does. Just use std::stoi:

std::string input;
std::cin >> input;
std::size_t end;
int result = std::stoi(input, &end);
if (end != input.size())
    // error

answered May 27, 2020 at 16:37

Pete Becker

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