printing binary tree to file

Question

I have a method that is supposed to print a binary tree to a file. This is it:

public void writeFile(Node mainNode)
{
    FileOutputStream outputStream = null;
    PrintWriter printWriter = null;

    try
    {

        outputStream = new FileOutputStream("BinaryTree.txt");
        printWriter = new PrintWriter(outputStream); 


        while(mainNode != null)
        {
             writeFile(mainNode.leftChild);
             printWriter.print(mainNode);
             writeFile(mainNode.rightChild); 

        }

        printWriter.close();

  }catch(IOException e)
  {
     System.out.println("An error occured");
      printWriter.close();
  }

}

The problem is that it seems to eternally loop as it's not finding the end of the tree. Is there anything I can try.

Here's the Node class too.

class Node
{
int id;
int grade;
String name;

Node leftChild;
Node rightChild;


Node(int id, int grade, String name )
{
    this.id = id;
    this.grade = grade;
    this.name = name;
}


public String toString()
{
    return name + " has a grade of " + grade + " and their ID is " + id;
}
}

Answer 1

I use an array to store nodes's data (Left - Node - Right) before this i write the data of the array to the file i want.

void LNR(TREE t, int a[], int &i){
   if(t != NULL)
   {
       LNR(t ->pLeft, a, i);
       a[i] = t ->data;
       i++;
       LNR(t ->pRight, a, i);
   }
}

void Output(const char *outfile, TREE t){
    int a[100];
    int i = 0;
    LNR(t, a, i);
    ofstream out;
    out.open(outfile);
    if (!out.is_open())
    {
       cout << "Unble to open the file.";
    }
    else
    {
       for (int j = 0; j < i; j++)
       {
           if (j < i)
               out << a[j] << " ";
           else
               out << a[j];
       }
    }
}

Answer 2

Here's the solution that worked for anyone who'd like to know.

public void writeFile(Node mainNode)
{
    FileOutputStream outputStream = null;
    PrintWriter printWriter = null;

    try
    {

        outputStream = new FileOutputStream("BinaryTree.txt");
        printWriter = new PrintWriter(outputStream); 

        write(mainNode, printWriter);

        printWriter.flush();

  }catch(IOException e)
  {
     System.out.println("An error occured");
      printWriter.close();
  }

}

 public void write(Node mainNode, PrintWriter w)
 {
     if(mainNode != null){
      write(mainNode.leftChild, w);
      w.print(mainNode);
      write(mainNode.rightChild, w); 
    }
 }

Answer 3

How do you expect this loop to end:

while(mainNode != null) {
    // never change mainNode
}

You need to pass your PrintWriter as an argument to your function, in order for all recursive calls to write (append) to the same file. Then provide a base case to stop:

public void writeFile(Node mainNode, PrintWriter w)
{
    if (mainNode == null)  // base case to stop recursion  
        return;
    top_call = false;  // Flag needed later
    if (w == null) {
        outputStream = new FileOutputStream("BinaryTree.txt");
        w = new PrintWriter(outputStream); 
        top_call = true;  // mark highest entry point to know when to close writer
    }
    writeFile(mainNode.leftChild, w);
    w.print(mainNode);
    writeFile(mainNode.rightChild, w);

    if (top_call)  // don't close writer in recursive calls
        w.close();
}

Answer 4

The entire writeFile method is wrong.

You have a loop over a single value without any go-to-next, so it'll never end.

It also calls itself recursively, trying to open the file again inside the recursive call. That's going to fail.

You have to split the method in two:

• First method open the file, calls second method, then closes file (using try-with-resources, please!).
• Second method does the three lines of call-self(left), write node, call-self(right).