How do i check if input is an int in C++?

Question

If the input is an integer, I want to set it equal to an integer variable.

If the input is a string, I will want to set it to a string variable, and later check if the string is "quit".

I don't know how to check it. I've looked for a built in function and found nothing.

while (true) {

    int numberEntered;
    string stringEntered; 

    cout << "enter a number to see if it is greater than 5: \n or enter \'quit\' to exit the program";

    //I don't know what to do below here
    cin >> ;

    if (stringEntered == "quit") {
        break;
    }

    if (numberEntered > 5) {
        cout << "that number is greater than 5" << endl;
    }
    else {
        cout << "not greater than 5" << endl;
    }
}

What do you want to happen if the person types "52hello" or "5 quit" or "quite a long sentence" ? — M.M, May 07 '14 at 02:09
@Matt McNabb I was going to dela with that later. I want a message to appear that says 'try again'. — george, May 07 '14 at 02:11

score 2 · Answer 1 · answered May 07 '14 at 02:02

2

cin >> numberEntered;
if (!cin.fail())
{
   ...

It may be more idiomatic to use:

if (cin >> numberEntered)

answered May 07 '14 at 02:02

David S.

518
3
5

1

Expanding on this; if it turns out that `cin >> numberEntered` failed, then you can go `cin.clear(); cin >> stringEntered;` to get the next word, or `cin.clear(); getline(cin, stringEntered)` to get the remainder of the line. – M.M May 07 '14 at 02:07

score 0 · Accepted Answer · answered May 07 '14 at 02:26

David S.'s answer is good. If you want to tidily handle garbage being entered after the line, here is another option (this is more complicated for your situation, but if you later want to expand your program to handle a lot of different input, then this way may come out to be simpler).

while( true )
{
    string stringEntered;
    cout << "enter a number to see if it is greater than 5: \n or enter \'quit\' to exit the program: " << flush;

// read the whole line, this ensures no garbage remains in the input stream
    getline(cin, stringEntered);

    if ( stringEntered == "quit" )
        break;

// this checks that a number was entered and nothing else
    istringstream iss(stringEntered);
    int numberEntered;
    char ch;
    if ( !(iss >> numberEntered) || (iss >> ch) )
    {
        cout << "please try again. ";
        continue;
    }

// process the number
    cout << "that number is " << (numberEntered > 5 ? "" : "not ")
            << "greater than 5." << endl;
}

You may need #include <sstream>.

superb. thanks. the comments help me understand. is flush important? the program works fine without it. — george, May 07 '14 at 02:55
It depends on your compiler whether or not it works without `flush` — M.M, May 07 '14 at 03:17
@MattMcNabb: No, it doesn't. It depends on whether you [untied](http://stackoverflow.com/questions/14052627/why-do-we-need-to-tie-cin-and-cout) `std::cin` and `std::cout`. At startup, they are tied and then flushing is automatic. — MSalters, May 07 '14 at 09:20

How do i check if input is an int in C++?

2 Answers2