Why won't my cout statements print after opening a textfile?

Question

I am trying to write a program where I read a text file and then take each line in the textfile and store them in a string vector. I think I am able to open the textfile however I noticed that after I open the textfile anything after that point does not execute. For example I have a cout statement at the end of my main function that outputs when I enter the name of a file that doesn't exist. However if I type in a file name does exists I get no output from the last cout statement. Anyone know why this is? Thanks!


int main()
{
    vector<string>line;
    string fileName = "test.txt";
    ifstream myFile(fileName.c_str());
    int i = 0;
    int count = 0;
    vector<string>lines;
        cout << "test" << endl;



    if (myFile.is_open())
    {
            cout << "test2" << endl;    
            while (!myFile.eof())
            {
                getline(myFile, lines[i],'\n');
                i++;
            }
            myFile.close();

    }
        if (!myFile.is_open())
        {
            cout<< "File not open"<< endl;
        }
        myFile.close();

        cout << "Test3" <<endl;




        return 0;
}

`lines` is an empty vector. `lines[i]` is invalid for any value of `i`. Your program simply crashes. — Igor Tandetnik, Sep 01 '16 at 04:50
[why `while(!myFile.eof())` is wrong](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong) — Barmar, Sep 01 '16 at 04:51
Use a `std::string` to get line by line and push it into the vector. — Blacktempel, Sep 01 '16 at 04:54
Consider running your code through a debugger. A debugger would have shown that getline() was throwing an exception very quickly. — brader24, Sep 01 '16 at 11:59

score 4 · Answer 1 · edited May 23 '17 at 12:32

Try this:

string fileName = "test.txt";
ifstream myFile(fileName); // .c_str() not needed - ifstream can take an actual string
vector<string> lines;

string line; // temporary variable for std::getline
while (getline(myFile, line)) {
   lines.push_back(line); // use push_back to add new elements to the vector
}

As pointed out in the comments, the most likely reason that your program seems to "end" prematurely is that it's crashing. std::getline takes a reference-to-string as its second argument. In your code, your vector is empty; therefore lines[i] for any i returns a reference to invalid memory. When getline tries to access that memory, the program crashes.

If you want an exception thrown when you try to access an out-of-bounds index of a vector, use lines.at(i) instead of lines[i].

score 2 · Answer 2 · answered Sep 01 '16 at 05:17

You need to use push_back() because your initial vector is empty and, you can not use indexes on empty vector. If you do so, it will leads to undefined behavior.

std::ifstream input( "filename.ext" );
std::vector<std::string> lines;
for( std::string line; getline( input, line ); )
{
    lines.push_back(line); 
}

Why won't my cout statements print after opening a textfile?

2 Answers2