In the previous Tree Meditation we pondered some preliminary ideas about treeness. Now let’s think about how to draw our trees. Before we start, make sure you paste this tree type code from the last installment into the Try OCaml REPL:


Last time we coded our little tree using the constructors Empty, Leaf, and Node defined in our tree type:


NOTE: if you’re using your own local OCaml REPL (ocaml or utop) make sure you terminate all of the entries here with double semicolon: ;; The Try Ocaml REPL seems to do this for you automatically.
To create a picture of the tree we’ll use Graphviz which is a suite of commandline tools that can be used to visualize graphs. Since our tree is a graph, we can use Graphviz to visualize it. Specifically, we’ll use a program called dot to create a visual representation of the graph.
This is what our tree should look like in the dot language:
graph tree {
{N1[label="1"]} {L2[shape=box,label="2"]}
{N1[label="1"]} {L3[shape=box,label="3"]}
}
We have a graph called tree. Inside of the tree block we define the connections between the nodes (or in our case between *Node*s and/or *Leaf*s, but no *Empty*s, of course, as they’re invisible).
Each node in the graph needs a unique identifier which is what N1,L2 and L3 are above. We also give each node a label which is what is shown in the graphical output of the tree. That’s the [label=“some label”] part of the node specifications above.
The ‘–’ means that there is a connection between two nodes.
It’s a fairly simple representation of our tree. We can generate the picture of the tree by running:
$ dot Tsvg o happy_tree.svg tree.dot
This will generate the file happy_tree.svg which we see here:
That’s nice and all, but what if our tree is much larger than this one and has many nodes and leaves? We’d like to be able to generate the tree.dot file automatically from our coded representation of our tree. To do that we need to talk a bit about tree traversal (visiting each node or leaf of our tree structure).
Let’s create a somewhat bigger tree to make our traversal discussion clearer:


Which looks like:
So why the odd order of the numbers in the tree above? One thing that’s not clear in our type definition from the previous article is that, by convention, an important property of binary trees is that child trees to the left of the current node should contain values that are less than the value held in the current node and child trees to the right of the current node should contain values that are greater than the value of the current node.
(As an aside, if we wanted to encode that requirement in our type definition we would need to be using a language which has dependent types )
Our chosen ordering above will become clearer as we talk about different types of traversal.
Tree Traversal
In order to generate a dot file that represents our tree we’ll need to traverse the nodes of our tree meaning we need to somehow visit all of the nodes of our tree structure.
There are two categories of tree traversal: Depth First or Breadth First (aka Level Order Traversal).
In all cases we start the tree traversal from the root of the tree. In our tree above, the root of the tree is the node that contains the value 3. (For whatever reason, in Computer Science trees are generally upsidedown with the root at the top and the leaves at the bottom.)
Depth First Traversal
There are three ways to do a depthfirst traversal of a tree:
1. Preorder Traversal
The steps of a Preorder traversal:
 Visit the current node and do something with the value found there.
 Traverse the left subtree.
 Traverse the right subtree.
An OCaml function to do a preorder traversal is defined as follows:


The recursive preorder_traverse function (note the rec in the function definition) takes a tree and uses pattern matching to determine what to do with each variant in the tree type. If t is an Empty we do nothing (empty parens () also known as unit in OCaml) . If It’s a Leaf we print it’s value. If it’s a Node we print it’s value and then call preorder_traverse on the left tree of the node and then call preorder_traverse on the right tree of the node.
Running this on our tree above we get (You’ve been typing the code into the online OCaml REPL right?) :
# preorder_traverse t ;;
3 1 0 2 5 4 6  : unit = ()
2. Inorder Traversal
The steps of an Inorder traversal:
 Traverse the left subtree.
 Visit current node and do something with value found there.
 Traverse the right subtree.
Here’s the OCaml function for an inorder traversal:


Running this on our tree we get:
# inorder_traverse t ;;
0 1 2 3 4 5 6  : unit = ()
Now you can get an idea of why we arranged the values in our tree as we did.
3. Postorder Traversal
Now you probably get the idea. For postorder we’re going to traverse the left subtree then traverse the right subtree and finally visit the current node and do something with it’s value:


And when we run this on our tree we get:
# postorder_traverse t ;;
0 2 1 4 6 5 3  : unit = ()
As an aside, instead of just printing the current value we’d probably want to make each of our traversal functions more general. This is functional programming, after all, and one of the great “wins” of functional programming is being able to pass functions to functions  function composition. We can generalize our traversal functions by passing in a function that will do something with the value:


Then we could use this version of postorder_traverse to print the values in our tree:
# postorder_traverse (fun n > Printf.printf "%s " n) t;;
0 2 1 4 6 5 3  : unit = ()
Notice that none of these depth first traversals of the tree will work for us in creating the dot file for graphviz. We need some other way to traverse the tree…
We need something that gives us (not actual dot code, but you get the idea): 3 > 1 3 > 5 1 > 0 1 > 2 5 > 4 5 > 6
Which brings us to…
Breadth First (or Level Order) Traversal
A Level Order traversal means we’re traversing each level of the tree. A Level order traversal of our tree above would look like(line breaks added to emphasize the levels of the tree):
3
1 5
0 2 4 6
This still isn’t quite what we need yet, but it seems we’re getting closer.
Before we go on, let’s define a couple of helper functions that we’ll need to construct our dot file.
First off, we’ll need to construct a string that gets written to the dot file. It would be good to have a function that converts each of the 3 variants that can make up a tree type to a string in dot format that represents the node in the graph:


Notice that in this function definition we explicitly specified that the t argument passed to this function should be of type ‘a tree and the output is of type string  this is completely optional as the compiler can figure out the types of the arguments using type inference, but it can make the code easier to read. (Note: ^ is the string concatenation operator in OCaml)
If you pasted that function in the REPL you’ll see:
val to_dot_node : string tree > string = <fun>
Which means that this is a function that takes a string tree and returns a string.
We can try running this function on our tree:
# to_dot_node t ;;
 : string = "{N3[label=\"3\"]}"
Which is the label of the root of our tree (3).
Next, we’ll define a function that given two inputs of type ‘a tree will return an edge (a line between two nodes in the tree) in dot format:


Now we can go on to define our tree_to_dot function:


The first argument to tree_to_dot is acc which is specified as being a string. acc is our accumulator: we’ll use it to build up our dot file. Notice that the function is specified as being recursive ( the rec specifies that the function can call itself).
Our pattern match covers all of the potential cases we might encounter. The first one matches t with Empty and in that case returns our accumulator acc. The next match is against Leaf _ (underscore there meaning that the Leaf could contain any value). If t is a Leaf, then we’ll append the Leaf’s dot node format to the accumulator.
After this we do several matches where t is a Node variant. In order to create an edge we need to know about the children of the current Node. Pattern Matching allows us to do this, the first pattern match against Node is:
 Node( (Node (_,_,_) as n) , _, (Leaf _ as leaf))
 Node( Leaf _ as leaf, _, (Node (_,_,_) as n)) >
(edge t n) ^ (edge t leaf) ^(tree_to_dot acc n)
We’re considering two cases here:
 The case where the Node contains a Node as the left subtree and a Leaf in the right subtree.
 The case where the Node contains a Leaf as the left subtree and a Node in the right subtree.
If either of these cases match, we then create an edge from the current Node (which is t) to the next Node (which is n) and an edge from the current Node (t) to the Leaf (leaf). And we concatenate that with the recursive call to tree_to_dot acc n, meaning we’re passing the child node n on to tree_to_dot for further processing.
Next we match the two cases where there’s a single child Node and a single Empty node. If so we create an edge from the current Node (t) to the child Node and concatenat that with the call to tree_to_dot given the child Node n.
After that we match the two cases where one child of the Node is a Leaf and the other is Empty. If these cases match we only return an edge from the current Node t to the child Leaf. No recursive call to tree_to_dot in this case. Since there are no child *Node*s, there’s no reason to.
If both subtrees of the current Node t are *Leaf*s then the next pattern matches and we concatenate the two edges from the current Node t to each Leaf.
In the next case we check for both subtrees of the Node being Empty and if that case matches we just return the accumulator.
Finally, we cover the case where both subtrees of the current node are *Node*s. Notice that if this case matches we have two calls to tree_to_dot  one for the left subtree and the other for the right subtree.
Now we just need to do a bit of housekeeping to write out the complete dot file:


This function won’t work in the Try OCaml REPL since it writes to a file. But maybe by now you’ve been convinced to install OCaml. If so, here’s the whole program so you can compile it:


You can compile it with the following command:
$ ocamlopt o tree tree.ml