在 C++ 中读取文件时获取最后一个值打印两次

Question

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