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;
}
}

user2390182 · Accepted Answer · 2016-03-26 23:12:56Z

1

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();
}

edited Mar 26, 2016 at 23:12

answered Mar 26, 2016 at 22:59

user2390182

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8 Comments

hyperneutrino Over a year ago

Exactly. You might want to elaborate a bit more, though. I didn't see that at first! Good thinking.

Allan Over a year ago

Well, when the mainNode returns null then it should break out. Shouldn't it?

user2390182 Over a year ago

@Allan Well, the mainNode is not null when you enter the loop and it then never changes. The recursive calls inside are just repeated over and over even though somewhere in these recursive calls, there might be a null mainNode

Allan Over a year ago

How do I know when the binary tree ends then?

António Ribeiro Over a year ago

@Allan, since you're using recursion, by principle it will finish, i.e. start going back, as soon as mainNode is null. This behavior is applicable on both sides of the tree.

|

Andreas · Accepted Answer · 2016-03-26 23:07:57Z

1

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).

answered Mar 26, 2016 at 23:07

Andreas

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Comments

Allan · Accepted Answer · 2016-03-27 00:01:30Z

0

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); 
    }
 }

answered Mar 27, 2016 at 0:01

Allan

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Comments

Lê Đăng Dũng · Accepted Answer · 2019-05-25 04:27:23Z

0

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];
       }
    }
}

answered May 25, 2019 at 4:27

Lê Đăng Dũng

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