C++ cout in while loop prints twice

Question

I adapted this code from Programming: Principles and Practice in C++ by Bjarne Stroustrup in an attempt to check that the input given was valid:

#include <iostream>
using namespace std;

int main()
{
    cout << "Please enter your first name and age: ";
    string first_name = "???";
    int age = -1;
    cin >> first_name >> age;

    while (!cin) {
        cin.clear();
        cout << "Sorry, can you enter that again? ";
        cin >> first_name >> age;
    }

    cout << "Hello, "<<first_name<<"! (age "<<age<<")\n";
    return 0;
}  

This works as expected, except that the words "Sorry, can you enter that again? " are being printed twice, once before the second input and once after.

Anyone know why this might be happening? Thanks.

Answer 1

It's not enough to clear the fail bit, you also have to clear any remaining characters in the input buffer. Furthermore, the code looks more convoluted than necessary. The following works:

std::string name;
int age;

std::cout << "Please enter your name & age: ";

while (not (std::cin >> name >> age)) {
    std::cout << "Sorry, can you enter that again? ";
    std::cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

std::cout << "Hello " << name << " (" << age << ")\n";

Oh yeah, and don't forget to flush the output stream, I don't think this will be done automatically for you.

That said, the standard input/output streams are, well, streams , so they aren't really suitable for interactive input anyway. You should use a proper library for that, such as readline or ncurses .

To illustrate why this is important, imagine the user enters “ foo42 ” and presses return. You'd expect the program to now say “Sorry, can you enter that again?” but it doesn't. Rather, it docilely waits for the user to enter a second token. That's certainly not what you'd expect from a usability perspective.

Answer 2

It depends on what you input!

• Input "Tim 29" : Works, "Hello Tim(29)".

• Input "Tim Tom 29" : Works, one repetition, "Hello Tom(29)".

• Input "Tim Tom Tina 29" : Works, two repetitions, "Hello Tina(29)".

I'm assuming that you're trying the third one. The first failure is Tom , who isn't a number. Resetting the error state is fine, but it doesn't get rid of Tom . We still need one more failed attempt just to consume him (failing on Tina , who at that point isn't a number). Only then are we ready for Tina .