Confused by how stringstream seems to work [duplicate] - c++

Question

This question already has answers here:
Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?
(5 answers)
Closed 6 months ago.
I'm new to c++. Currently I'm learning how to read and write to a file. I've created a file "nb.txt" with content like this:
1 2 3 4 5 6 7
2 3 4 5 6 7 9
I'm using a simple program to read this file, looping until reached EOF.
#include <iostream>
#include <fstream>
using namespace std;
int main() {
ifstream in("nb.txt");
while (in) {
int current;
in >> current;
cout << current << " ";
}
}
What I'm expecting is the program will output all of the values.
But what I'm really getting is this:
1 2 3 4 5 6 7 2 3 4 5 6 7 9 9
There's a multiple "9" in the output. I don't understand what's happening! Is it because of the while loop?
Can anyone help me to figure out why there is another "9"? Thanks!

Answer 1

The problem is that after you read the last value(which is 9 in this case) in is not yet set to end of file. So the program enters the while loop one more time, then reads in(which now sets it to end of file) and no changes are made to the variable current and it is printed with its current value(which is 9).
To solve this problem, you can do the following:
int main() {
ifstream in("nb.txt");
int current=0;
while (in >> current) { //note the in >> curent
cout << current << " ";
}
}
The output of the above program can be seen here:
1 2 3 4 5 6 7 2 3 4 5 6 7 9