# Expression Trees¶

In this section, we discuss some ways that we can perform advanced manipulation of expressions.

Before we can do this, we need to understand how expressions are represented in Diofant. A mathematical expression is represented as a tree. Let us take the expression $$x y + x^2$$. We can see what this expression looks like internally by using repr()

>>> repr(x*y + x**2)


The easiest way to tear this apart is to look at a diagram of the expression tree:

First, let’s look at the leaves of this tree. We got here instances of the Symbol class and the Diofant version of integers, instance of the Integer class, even technically we input integer literal 2.

What about x*y? As we might expect, this is the multiplication of x and y. The Diofant class for multiplication is Mul.

>>> repr(x*y)
"Mul(Symbol('x'), Symbol('y'))"


Thus, we could have created the same object by writing

>>> Mul(x, y)
x⋅y


When we write x**2, this creates a Pow class instance.

>>> repr(x**2)
"Pow(Symbol('x'), Integer(2))"


We could have created the same object by calling

>>> Pow(x, 2)
2
x


Now we get to our final expression, x*y + x**2. This is the addition of our last two objects. The Diofant class for addition is Add, so, as you might expect, to create this object, we could use

>>> Add(Pow(x, 2), Mul(x, y))
2
x  + x⋅y
>>> x*y + x**2
2
x  + x⋅y


Note

You may have noticed that the order we entered our expression and the order that it came out from printers like repr() or in the graph were different. This because in Diofant, the arguments of Add and the commutative arguments of Mul are stored in an arbitrary (but consistent!) order, which is independent of the order inputted.

There is no subtraction class in Diofant. x - y is represented as x + (-1)*y

>>> repr(x - y)


Similarly to subtraction, there is no division class.

>>> repr(x/y)
"Mul(Symbol('x'), Pow(Symbol('y'), Integer(-1)))"


We see that x/y is represented as x*y**(-1).

But what about x/2? Following the pattern of the previous example, we might expect to see Mul(x, Pow(Integer(2), -1)). But instead, we have

>>> repr(x/2)
"Mul(Rational(1, 2), Symbol('x'))"


Rational numbers are always combined into a single term in a multiplication, so that when we divide by 2, it is represented as multiplying by 1/2.

## Walking the Tree¶

Let’s look at how to dig our way through an expression tree. For this every object in Diofant has a very generic interface — two important attributes, func, and args.

The head of the object is encoded in the func attribute. Usually it is the same as the class of the object, but not always.

>>> expr = 2 + x*y
>>> expr
x⋅y + 2
>>> expr.func
>>> type(expr)


The class of an object need not be the same as the one used to create it.

>>> Add(x, x)
2⋅x
>>> _.func
<class 'diofant.core.mul.Mul'>


Note

Diofant classes make heavy use of the __new__() class constructor, which, unlike __init__(), allows a different class to be returned from the constructor.

The children of a node in the tree are held in the args attribute.

>>> expr.args
(2, x⋅y)


From this, we can see expr can be completely reconstructed from its func and its args.

>>> expr.func(*expr.args)
x⋅y + 2


Note

Every well-formed Diofant expression must either have empty args or satisfy invariant

>>> expr == expr.func(*expr.args)
True


In Diofant, empty args signal that we have hit a leaf of the expression tree.

>>> x.args
()
>>> Integer(2).args
()


This interface allows us to write simple algorithms that walk expression trees.

>>> def pre(expr):
...     yield expr
...     for arg in expr.args:
...         for subtree in pre(arg):
...             yield subtree


See how nice it is that empty tuple signals leaves in the expression tree. We don’t even have to write a base case for our recursion — it is handled automatically by the for loop.

Let’s test this by printing all nodes of the expression at each level.

>>> expr = x*y + 2
>>> for term in pre(expr):
...     print(term)
x*y + 2
2
x*y
x
y


Can you guess why we called our function pre? We just wrote a pre-order traversal function for our expression tree. See if you can write a post-order traversal function.

Such traversals are so common in Diofant that the generator functions preorder_traversal() and postorder_traversal() are provided to make such traversals easy. We could have also written our test as

>>> for arg in preorder_traversal(expr):
...     print(arg)
x*y + 2
2
x*y
x
y