在 C++ 中，为什么 ifstream getline 在我的 .txt 文件中返回每隔一个数字，而不是所有数字？

Question

When I run this code it doesn't print the contents of the .txt file which is numbers 1 to 100, it prints all of the even numbers up to 100 (eg 2 4 6 8 so on.) And I don't know why, it didn't before and I don't think I changed anything. I'm using xcode. Anybody got any ideas?

#include <stdio.h>     
#include <iostream>     
#include <cmath>        
#include <string>       
#include <sstream>      
#include <fstream>


using namespace std;

int main () {
    string line;
    int Points[100];
    ifstream myfile("StatNum.txt");
    if (myfile.is_open())
    {
        while ( getline (myfile,line) )
        {
            getline(myfile,line);
            stringstream(line) >> Points[100];  //uses stringstream to convert Myline (which is a string) into a number and put it into an index of Points
            cout << Points[100] << endl;
        }
        myfile.close();
    }

    else cout << "Unable to open file" << endl;

    return 0;
}

Answer 1

This happens because you call getline twice per iteration:

• First, you call it in the while header
• Then you call it inside the loop.

One invocation (the one in the while header) is sufficient, because the result is saved in the line variable, which the loop body is free to examine.

Removing the second invocation will fix the problem.

Answer 2

As @dasblinkenlight pointed out, you are calling std::getline() twice and that's the problem that you see.

The problem that you can't see is that you are writing data to Points[100] which is an invalid location, outside of the array's bounds. The 100 valid locations in your array are indexes 0 to 99, that is Points[0] , Points[1] , ..., Points[99] (because counting in C++ starts from 0, not 1).

Writing to Points[100] is Undefined Behavior which means that your program may crash, or worse: may not crash while corrupting its own data.

Since you're using C++ you have std::vector and other containers at your disposal, where you can easily store the numbers you read:

#include <vector>

// ...

vector<int> points;

while (getline(myfile, line))
{
    int temp;

    stringstream(line) >> temp;
    points.push_back(temp);
    cout << temp << endl;
}