Error Vector subscript out of range

Question

I believe my error is within my writeline function, when I attempt to write the contents of the vector to the new file using a while loop.

//Read from txt file, write to new text file

#include<iostream>
#include<fstream>
#include<vector>
#include<string>
#include<algorithm>
using namespace std;

void readline();
void sortline(vector<string>& sortthis);
void writeline(vector<string>& list);

int main()
{
    readline();
    system("pause");
    return 0;
};

void readline()
{
    string line;
    vector<string> lines;
    ifstream myfile("classes.txt");
    if (myfile.is_open())
    {
        while (myfile.good())
        {
            getline(myfile, line);
            lines.push_back(line);
        };
        myfile.close();
    }
    cout << "readline() has run" << endl;
    sortline(lines);
    writeline(lines);
};

void  sortline(vector<string>& sortthis)
{
    sort(sortthis.begin(), sortthis.end());
};

void  writeline(vector<string>& list)
{
    ofstream myfile2("new.txt");
    if (myfile2.is_open())
    {
        int i = 0;
        while(i !=list.size()-1)
        {
            myfile2 << list[i] << endl;
            i++;
        };
        myfile2.close();
    };
     cout << "writeline() has run" << endl;
};

this is a project from a semester ago that i'm revisiting. I wrote the program on my mac, now i'm trying to run it on my windows comp with visual studio. I'll describe what I'm attempting to do, I apologize if my choice of words is terrible in advance. anywhere I put a * is where I'm not sure what is happening, but I'll take a stab at it.. any explanations of my code is very appreciated!!

my readline() function does the following: creates a string called line, creates a vector of string type called lines, **input the file classes.txt and establish myfile as it's object, then open myfile for writing, **use the while loop to write the lines from the txt into the myfile object, then close myfile, print out a statement to let the user know readline() has run, then **pass the vector called lines into the sortline function, and then pass lines into the writeline function.

** sortline takes in a vector of strings as its arg, and assigns it the object sortthis?? then I'm not sure what happens, but it looks like i applied a sorting algorithm, anybody have any thoughts?

and finally we get to my writeline function which takes in a vector of strings as its arg and assigns them the name lines (is that correct?) i then want to establish a new out file stream to a new textfile called "new.txt" with an object name myfile2, if myfile2 is open, then i want to write all the lines from the vector of strings(which contain the contents of the original text file) into myfile2, which will write them to the new.txt file, then close myfile2, print a message stating the function has run, and that is all.

Answer 1

The way you loop through list in writeline is not safe. You should use a for loop or a while loop with iterator. As it is, your code probably doesn't do what you intend it to do even if there are several elements in list. Consider the following:

std::vector<std::string> vLines;
vLines.push_back("Hello");
vLines.push_back("File");
vLines.push_back("World");
std::ofstream of("file.txt");
int i = 0;
while (i != vLines.size() - 1)
{
    of << vLines[i] << std::endl;
    ++i;
}

Even with several elements in vLines, this will only actually print output 2 elements into of.
i will be 0 which is not 2, so "Hello" will be output to of.
i will be 1 which is not 2, so "File" will be output to of.
i is now 2, which is equal to 2, so "World" will not be output to of.

That's with elements. If there are 0 elements in vLines, you will be indexing out of bounds (which I suspect is what you are doing, hence your error):

std::vector<std::string> vLines;
std::ofstream of("file.txt");
int i = 0;
while (i != vLines.size() - 1)
{
    of << vLines[i] << std::endl;
    ++i;
}

i will be 0, which is not equal to -1, so the code will run and try to output vLines[0] to of, but there is no vLines[0]! I suspect this is what you are experiencing.

This will go away if you use a proper range-based loop instead (credit to @WhozCraig for C++11 solution):

for (auto const& s : vLines)
    of << s;

Or if you don't have C++11 you can still mimic a proper range-based loop with the following:

for (int i = 0; i < vLines.size(); ++i)
    of << vLines[i] << std::endl;

Or an iterator:

for (auto it = vLines.begin(); it != vLines.end(); ++it)
    of << *it << std::endl;

You will now output all elements in your std::vector to your std::ofstream as well as properly handle situations where there are no elements.