I can only access elements of a vector while inside a for loop

Question

I have a function with a vector that is passed in by reference. I am able to use the vector inside a for loop to write its contents to a file. However, if I try to access any index of the vector outside of the loop, is gives me this error:

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 10) >= this->size() (which is 0)
Aborted (core dumped)

Here is my code:

void writeFileVector(vector<string> &wordVector, char *argv[]) {
    ofstream newFileName;
    stringstream ss;

    ss << argv[1];
    string argumentString = ss.str();               // This code is just for creating the name
    ss.str(""); // Clear the stringstream           // of the file
    ss << argumentString << "_vector.txt";          //
    string fileName = ss.str();                     //
    newFileName.open(fileName.c_str());             //

    cout << wordVector.at(0) << endl;               // THIS GIVES ME THE ERROR
    
    for (int i = 0; i < wordVector.size(); i++) {
        cout << wordVector.at(0) << endl;           // This would not give an error, but it also
                                                    // doesn't output anything to the terminal.

        newFileName << wordVector.at(i) << endl;    // This is okay
    }

    newFileName.close();
}

Answer 1

I can only access elements of a vector while inside a for loop

No, you can access elements of a vector anywhere where the vector is is scope. But you need to ensure you're accessing actual elements, not made-up ones :-)

With regard to the (slightly modified) code:

//cout << wordVector.at(0) << endl;             // THIS GIVES ME THE ERROR
for (int i = 0; i < wordVector.size(); i++) {
    cout << wordVector.at(0) << endl;           // THIS DOESN'T
}

When the vector is empty, the cout inside the loop is never executed, because the loop body doesn't run. That's because i < wordVector.size() starts out as false because both i and wordVector.size() are zero.

If you were to modify the loop so that the body did run, you would see it also fails within the loop:

//cout << wordVector.at(0) << endl;             // THIS GIVES ME THE ERROR
for (int i = -1; i < wordVector.size(); i++) {
    cout << wordVector.at(0) << endl;           // SO DOES THIS
}