C++ adding a carriage return at beginning of string when reading file

Question

I have two questions:

1) Why is my code adding a carriage return at the beggining of the selected_line string?
2) Do you think the algorithm I'm using to return a random line from the file is good enough and won't cause any problems?

A sample file is:

line
number one
#
line number two

My code:

int main()
{
    srand(time(0));
    ifstream read("myfile.dat");
    string line;
    string selected_line;
    int nlines = 0;
    while(getline(read, line, '#')) {
        if((rand() % ++nlines) == 0)
            selected_line = line;
    }
    // this is adding a \n at the beginning of the string
    cout << selected_line << endl; 
}

EDIT: OK, what some of you suggested makes a lot of sense. The string is probably being read as "\nmystring". So I guess my question now is, how would i remove the first \n from the string?

Answer 1

What you probably want is something like this:

std::vector<std::string> allParagraphs;
std::string currentParagraph;

while (std::getline(read, line)) {        
    if (line == "#") { // modify this condition, if needed
        // paragraph ended, store to vector
        allParagraphs.push_back(currentParagraph);
        currentParagraph = "";
    else {
        // paragraph continues...
        if (!currentParagraph.empty()) {
            currentParagraph += "\n";
        }
        currentParagraph += line;
    }          
}

// store the last paragraph, as well
// (in case it was not terminated by #)
if (!currentParagraph.empty()) {
    allParagraphs.push_back(currentParagraph);
}

// this is not extremely random, but will get you started
size_t selectedIndex = rand() % allParagraphs.size();

std::string selectedParagraph = allParagraphs[selectedIndex];

For better randomness, you could opt for this instead:

size_t selectedIndex 
    = rand() / (double) (RAND_MAX + 1) * allParagraphs.size();

This is because the least significant bits returned by rand() tend to behave not so randomly at all.