Visualizing binary trees with Graphviz

November 23rd, 2009 at 6:08 am

When implementing binary trees of some kind, one of the first utilities one writes is a visualization function that given a tree prints it to the screen.

The basic printing of a binary tree is almost always a variation of:


That is, 6 is the left (first) child of 15, 18 is its right child. 6 has ho children, 18 has only a left child (- are NULL nodes). Given a typical tree node declaration:

typedef struct bst_node_t bst_node;

struct bst_node_t
    int key;
    bst_node* left;
    bst_node* right;

The printing code is easy to write:

/* Auxiliary for bst_print_ascii
void print_offset(FILE* stream, int offset)
    int i;
    for (i = 0; i < offset; ++i)
        fprintf(stream, " ");

/* Prints the BST horizontally as ASCII
void bst_print_ascii(bst_node* tree, FILE* stream)
    static int offset = 0;

    print_offset(stream, offset);

    if (tree == NULL)
        fprintf(stream, "-\n");
    fprintf(stream, "%d\n", tree->key);

    offset += 3;
    bst_print_ascii(tree->left, stream);
    bst_print_ascii(tree->right, stream);
    offset -= 3;

The problem with this representation is that it isn’t particularly helpful, because (especially for larger trees) it’s quite difficult to understand. Printing trees properly in ASCII, level by level is a much more difficult job.

But there’s a better way!

GraphvizGraph Visualization Software – is a language (called DOT) and a set of tools for automatically generating visualizations of graphs. Graphviz is used heavily in academy to supply publication-quality visualizations for papers. It’s also used by the Doxygen documentation tool for generating class hierarchies.

The power of Graphviz is in its powerful layout algorithms. You provide a textual description of the graph – which edges are there, what is connected to what, and so on, and Graphviz automagically lays out the graph in a visually pleasant way. The DOT language is a great example of a "mini-language" or an external DSL, and is very easy to use.

It isn’t very difficult to craft the C code that auto-generates the DOT file for a given binary tree:

void bst_print_dot_null(int key, int nullcount, FILE* stream)
    fprintf(stream, "    null%d [shape=point];\n", nullcount);
    fprintf(stream, "    %d -> null%d;\n", key, nullcount);

void bst_print_dot_aux(bst_node* node, FILE* stream)
    static int nullcount = 0;

    if (node->left)
        fprintf(stream, "    %d -> %d;\n", node->key, node->left->key);
        bst_print_dot_aux(node->left, stream);
        bst_print_dot_null(node->key, nullcount++, stream);

    if (node->right)
        fprintf(stream, "    %d -> %d;\n", node->key, node->right->key);
        bst_print_dot_aux(node->right, stream);
        bst_print_dot_null(node->key, nullcount++, stream);

void bst_print_dot(bst_node* tree, FILE* stream)
    fprintf(stream, "digraph BST {\n");
    fprintf(stream, "    node [fontname=\"Arial\"];\n");

    if (!tree)
        fprintf(stream, "\n");
    else if (!tree->right && !tree->left)
        fprintf(stream, "    %d;\n", tree->key);
        bst_print_dot_aux(tree, stream);

    fprintf(stream, "}\n");

For the tree shown in ASCII in the beginning of this post, the generated DOT file is:

digraph BST {
    node [fontname="Arial"];
    15 -> 6;
    null0 [shape=point];
    6 -> null0;
    null1 [shape=point];
    6 -> null1;
    15 -> 18;
    18 -> 17;
    null2 [shape=point];
    17 -> null2;
    null3 [shape=point];
    17 -> null3;
    null4 [shape=point];
    18 -> null4;

And here is the result (running the dot tool with PNG output):

Much nicer, isn’t?

Graphviz is a tool for drawing graphs, not trees, so there’s some tiny tweaking needed for trees. Particularly, to differentiate left from right pointers, I always draw both. The NULL children are drawn as empty dots. There are alternative ideas for drawing trees with Graphviz, but this one is IMHO both easy to implement and looks most familiar.

Related posts:

  1. Abstract vs. Concrete Syntax Trees
  2. SICP section 2.3.4
  3. nice solution found to that binary stream problem
  4. Perl’s “guess if file is text or binary” implemented in Python
  5. SICP section 3.3.3

5 Responses to “Visualizing binary trees with Graphviz”

  1. Tom LynnNo Gravatar Says:

    Here’s a translation into Python:

    def bst_to_dot(tree, stream,
                   get_key=lambda node: node.key,
                   get_left=lambda node: node.left,
                   get_right=lambda node: node.right,
        """Prints a GraphViz .dot file of tree to stream.
           get_left(node) and get_right(node) should return the appropriate
           child of node, or None if there is no such child.
           get_key(node) should return a str()-able object to use as both the
           node name and its displayed value (FIXME: this is not a great idea).
        def escape(text):
            return '"%s"' % text  # TODO: escaping
        nullcount = [0]
        def null_to_dot(key):
            print >>stream, "    null%d [shape=point];" % nullcount[0]
            print >>stream, "    %s -> null%d;" % (escape(key), nullcount[0])
            nullcount[0] += 1
        def node_to_dot(node):
            if get_left(node) is not None:
                print >>stream, "    %s -> %s;" % (
                    escape(get_key(node)), escape(get_key(get_left(node))))
            if get_right(node) is not None:
                print >>stream, "    %s -> %s;" % (
                    escape(get_key(node)), escape(get_key(get_right(node))))
        print >>stream, "digraph BST {"
        print >>stream, "    node [%s];" % nodestyle
        if tree is None:
            print >>stream
        elif get_right(tree) is None and get_left(tree) is None:
            print >>stream, "    %s;" % escape(get_key(tree))
        print >>stream, "}"
    def bst_to_dot_string(tree, *args, **kwargs):
        import StringIO
        stream = StringIO.StringIO()
        bst_to_dot(tree, stream, *args, **kwargs)
        return stream.getvalue()
    def save_bst(tree, filename, *args, **kwargs):
        'Saves a PNG of tree to filename by executing "dot -Tpng -o<filename>".'
        import subprocess
        p = subprocess.Popen(["dot", "-Tpng", "-o"+filename],
        p.stdin.write(bst_to_dot_string(tree, *args, **kwargs))
        if p.wait() != 0:
            raise RuntimeError('Unexpected return code: %d' % p.returncode)
    def test():
        import sys
        class Node(object):
            def __init__(self, key, left=None, right=None):
                self.key, self.left, self.right = key, left, right
        tree = Node(15, Node(6), Node(18, Node(17)))
            filename = sys.argv[1]
        except IndexError:
            sys.exit('usage: %s FILENAME\n' % sys.argv[0] +
                     'Test this code - save a test PNG to FILENAME')
        save_bst(tree, filename)
    if __name__=='__main__':
  2. Zach "theY4Kman" KanzlerNo Gravatar Says:

    Thank you for this post; it’s helped me immensely in debugging my binary heap implementation. I added one additional detail to help me, which was to add labels to each node. This can be done by adding one line –

    void bst_print_dot_aux(bst_node* node, FILE* stream)
        static int nullcount = 0;
        fprintf(stream, "    %d [label=\"%s\"];\n", node->key, "node label");
  3. j0braNo Gravatar Says:

    Thanks for the post,
    I’ve implemented it in Javascript:
    (no great code but does his job)

  4. Ron FredericksNo Gravatar Says:

    Thank you for your post. I too have developed a python code implementation to visualize trees. My version uses pydot with graphviz along with TK graphics for slide show demonstrations of search algorithms. Also, I present the code and script to generate mpeg4 videos from the png images.

    Here is the link to the code and sample videos it produces:

  5. pepperpotNo Gravatar Says:

    Thanks for this post, really useful. I modified your code to use in my C++ implementation of a binary search tree. Here’s the code:

    #ifndef TREE_H_
    #define TREE_H_
    #include <string>
    #include <sstream>
    template <class T>
    class TreeNode
      TreeNode(T item): item(item), left(NULL), right(NULL){ CreateLabel(); }
      void insert(T item);
      void TreeNode<T>::CreateLabel();
      void TreeNode<T>::PrintDot(std::ofstream& dotfile);
      void TreeNode<T>::PrintDotAux(std::ofstream& dotfile);
      void TreeNode<T>::PrintDotNull(std::string label, int nullcount, std::ofstream& dotfile);
      TreeNode<T> * left;
      TreeNode<T> * right;
      T item;
      std::string label;
    template<class T>
    void TreeNode<T>::insert(T val)
      if(val > item)
          right = new TreeNode(val);
      if(val < item)
          left = new TreeNode(val);
    template<class T>
    void TreeNode<T>::CreateLabel()
      std::ostringstream ostr;
      ostr << "node" << item;
      label = ostr.str();
    template <class T>
    void TreeNode<T>::PrintDotNull(std::string label, int nullcount, std::ofstream& dotfile)
      std::ostringstream ostr;
      ostr << nullcount;
      dotfile << "    null" << ostr.str() << " [shape=point];\n";
      dotfile << "    " << label << " -> null" << ostr.str() << ";\n";
    template <class T>
    void TreeNode<T>::PrintDotAux(std::ofstream& dotfile)
      static int nullcount = 0;
        dotfile << "    " << label << " -> " << left->label << ";\n";
        PrintDotNull(label, nullcount++, dotfile);
      if (right)
        dotfile << "    " << label << " -> " << right->label << ";\n";
        PrintDotNull(label, nullcount++, dotfile);
    template <class T>
    void TreeNode<T>::PrintDot(std::ofstream& dotfile)
      dotfile << "digraph BST {\n";
      dotfile << "    node [fontname=\"Arial\"];\n";
      if (!right && !left)
        dotfile << "    " << label << ";\n";
      dotfile << "}\n";

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