Print a binary tree, python, in inorder

Question

Me and my friend are doing some school work with programming in Python 3.1 and are VERY stuck. We're programming a binary tree and it's working fine except when we want to print all the nodes in inorder in a way that would create a sentence (all the words in inorder just after one another in a row). We have been looking all over the internet for clues as to how to procede and we've been working with this little thing for like two hours. Any advice/help would be awesome.

Our program/Binary tree:

class Treenode:  
    def __init__(self, it = None, le = None, ri = None):  
        self.item = it  
        self.left = le  
        self.right = ri  

class Bintree:  
    def __init__(self):  
        self.item = None  
        self.left = None  
        self.right = None  

    def put(self, it = None):
        key = Treenode(it)
        if self.item == None:
            self.item = key
            return
        p = self.item
        while True:
            if key.item < p.item:
                if p.left == None:
                    p.left = key
                    return
                else:
                    p = p.left
            elif key.item > p.item:
                if p.right == None:
                    p.right = key
                    return
                else:
                    p = p.right
            else:
                return

    def exists(self, it):
        key = it
        p = self.item
        if p == key:
            return True
        while True:
            if key < p.item:
                if p.left == None:
                    return False
                else:
                    p = p.left
            elif key > p.item:
                if p.right == None:
                    return False
                else:
                    p = p.right
            else:
                return

    def isEmpty(self):
        if self.item == None:
            return True
        else:
            return False

def printtree (Treenode):
    if Treenode.left != None:
        printtree (Treenode.left)
    print (Treenode.item)
    if Treenode.right != None:
        printtree (Treenode.right)

We get a sort of print when we run the program which looks like this: "bintree.Treenode object at 0x02774CB0", which is not what we want.

We use the tree by running this:

import bintree

tree = bintree.Bintree()
print(tree.isEmpty())    # should give True
tree.put("solen")
print(tree.isEmpty())    # should give False
tree.put("gott")
tree.put("sin")
tree.put("hela")
tree.put("ban")
tree.put("upp")
tree.put("himlarunden")
tree.put("manen")
tree.put("seglar")
tree.put("som")
tree.put("en")
tree.put("svan")
tree.put("uti")
tree.put("midnattsstuden")

print(tree.exists("visa"))     # should give False
print(tree.exists("ban"))      # should give True
tree.printtree()               # print sorted

Also, the second last row gives us "None" instead of "True", which is wierd.

Answer 1

print(tree.exists("visa")) returns None, because in the last line of exists() there's return statement without any value (which defaults to None).

Also you shouldn't name a printtree argument Treenode since it's a name of an existing class and that might lead to confusion. It should look more like:

def printtree(tree_node):
    if tree_node.left is not None:
        printtree(tree_node.left)
    print(tree_node.item)
    if tree_node.right is not None:
        printtree(tree_node.right)

Another thing is calling printtree - it's a function, not Bintree method, so I suppose you should call it printtree(tree).

Answer 2

To print a binary tree, if you are printing a leaf you just print the value; otherwise, you print the left child then the right child.

def print_tree(tree):
    if tree:
        print tree.value
        print_tree(tree.left)
        print_tree(tree.right)

Answer 3

You are not specifying a starting case for printtree(). You're defining how to recurse through your tree correctly, but your call to printtree() has no node to start at. Try setting a default check to see if a parameter is passed in, and if one isn't start at the head node of the bintree.

The reason your second to last line is printing None is because, in your exists method, you just have a "return", rather than a "return True", for the case of finding a `p.item' that is equal to key.

Answer 4

One way to make testing easier is to use -assert()- instead of printing things and then referring back to your code.

tree = Bintree()
assert(tree.isEmpty())
tree.put("solen")
assert(not tree.isEmpty())
tree.put("gott")
tree.put("sin")
tree.put("hela")
tree.put("ban")

http://docs.python.org/reference/simple_stmts.html#the-assert-statement

It raises an error if its condition is not true. I know that doesn't fix your bug but making things less ambiguous always helps debugging.