How to output an AVL Tree to a file in in-order traversal?

Question

I'm writing a program that let the user insert an AVL Tree by reading from a text file then I output it in in-order traversal to another text file, this is my program:

void in_order(AVLTree* root) 
{
    ofstream printToFile("output.txt");

    if (root == NULL)
        return;

    in_order(root->left);
    printToFile << root->data << " ";
    in_order(root->right);
}

int main() 
{
    AVLTree* root = NULL;
    ifstream readFromFile("input.txt");
    int node;

    while (!readFromFile.eof()) 
    {
        readFromFile >> node;
        root = insert(root, node);
    }
    in_order(root);
}

Here is the sample of the input file:

9 20 60 10 30 90 42

When I run the above program, the output file only has 1 number:

20

If I print it to the console screen instead of printing to a file, it's correct:

9 10 20 30 42 60 90

I don't know why when I print it to the console screen it's correct, but when I output to the file it only has 1 number. Can anyone help me with this? Thanks for your help!

Answer 1

The problem with your code may be due to the fact that in recursion you re-create ofstream printToFile("output.txt"); . ofstream buffers the output, and clears it (ie outputs to a file) at the destructor which happens not in the order as you output in a file

The solution to this problem can be to create this variable as a static variable, or to pass the stream as an argument or force to output the contents of the buffer to a file using flush (but not sure if this will help).

So it would looks like this

// static stream
void static_in_order(AVLTree* root) 
{
    static ofstream printToFile("output.txt");

    if (root == NULL)
        return;

    in_order(root->left);
    printToFile << root->data << " ";
    in_order(root->right);
}


//stream as argument
void arg_in_order(AVLTree* root, ofstream &printToFile) 
{
    if (root == NULL)
        return;

    in_order(root->left, printToFile);
    printToFile << root->data << " ";
    in_order(root->right, printToFile);
}

int main()
{
    //static solution 
    static_in_order(root);

    //arg solution
    ofstream printToFile("file.txt");
    arg_in_order(root, printToFile);
}

Answer 2

You should use std::ofstream ofs ("output.txt", std::ofstream::app);
If you don't add mode parameter it use std::ofstream::out default, that print your data to the begin of the file.

And don't forget call std::ofstream::close(); after complete output, otherwise may lost data in buffer.

Maybe a Better way:

void in_order(std::ofstream& ofs, AVLTree* root)
{
  ...
}

int main () 
{

  ...

  std::ofstream ofs ("test.txt");

  in_order(ofs, root);

  ofs.close();


}