How to detect empty lines while reading from istream object in C++?

Question

How can I detect if a line is empty?

I have:

1
2
3
4

5

I'm reading this with istream r so:

int n;
r >> n

I want to know when I reach the space between 4 and 5. I tried reading as char and using .peek() to detect \n but this detects the \n that goes after number 1 . The translation of the above input is: 1\n2\n3\n4\n\n5\n if I'm correct...

Since I'm going to manipulate the ints I rather read them as ints than using getline and then converting to int...

Answer 1

It could look like this:

#include <iostream>
#include <sstream>
using namespace std;

int main()
{
    istringstream is("1\n2\n3\n4\n\n5\n");
    string s;
    while (getline(is, s))
    {
        if (s.empty())
        {
            cout << "Empty line." << endl;
        }
        else
        {
            istringstream tmp(s);
            int n;
            tmp >> n;
            cout << n << ' ';
        }
    }
    cout << "Done." << endl;
    return 0;
}

output:

1 2 3 4 Empty line.
5 Done.

Hope this helps.

Answer 2

If you really don't want using getline, this code works.

#include <iostream>
using namespace std;


int main()
{
    int x;
    while (!cin.eof())
    {
        cin >> x;
        cout << "Number: " << x << endl;

        char c1 = cin.get();
        char c2 = cin.peek();

        if (c2 == '\n')
        {
            cout << "There is a line" << endl;
        }
    }
}

But be aware that this is not portable. When you using system that has different end lines characters than '\n' then would be problem. Consider reading whole lines and then extract data from it.