# CS 440 Homework 1 OCaml Continued solution

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## Programming Languages and Translators1 Tail Recursion

NOTE: All functions in this section must be tail-recursive. Make sure you understand what
that means.
Write a tail-recursive function fact : int -> int that calculates the factorial of an argument n
(that is, n!). Remember that
n! = n · (n − 1) · (n − 2) · … · 1
0! = 1
1! = 1
2! = 2
. . .

## Task 1.2 (Programming, 7 points).

Write a tail-recursive function split : ’a list -> ’a list * ’a list that takes a list and returns
a pair of lists such that for all l, (fst (split l)) @ (snd (split l)) is a permutation of l. That is,
it splits a list into two lists and returns a pair of the two lists. The lengths of the two lists should
differ by at most one.
For example, split [1;2;3;4;5] can return ([1;2;3], [4;5]) or ([1;3;5], [2; 4]) or
([5;3], [4;2;1]
but not ([], [1;2;3;4;5]) or ([1;2;3], [3;4;5]) or ([1], [5]).
1 CS 440

## Bonus Task 1.3 (Programming, 2 points).

Write a tail-recursive version of List.fold_right without using List.fold_left or List.rev (or
re-implementing either of these; your function should make just one pass over the list). Other than tail
recursion, your function should behave identically to List.fold_right.
Hint: Try changing the function you pass to the recursive application.
Note: This, like other bonus tasks we’ll occasionally include in assignments, is quite difficult and is
worth a small number of bonus points. Try it for an extra challenge if you want (probably after completing
the rest of the assignment).

## 2 Trees

Higher-order functions aren’t just for lists! Recall the algebraic data type of binary trees from lecture:
type ’a tree = Leaf | Node of ’a * ’a tree * ’a tree
In this section, you’ll implement and use higher-order functions over trees.
As an example, we implemented the function tree_fold : ’a tree -> ’b -> (’a -> ’b -> ’b -> ’b)
that folds over trees like List.fold_right folds over lists. For example,
tree_fold (Node (v1, Node (v2, Leaf, Leaf), Leaf)) u f
is f v1 (f v2 u u) u. Note that our implementation isn’t tail-recursive. The Cornell book (linked from
the course website) gives a tail-recursive version.

## Task 2.1 (Programming, 5 points).

Implement the function tree_map : ’a tree -> (’a -> ’b) -> ’b tree that returns a tree with
the same structure as the original tree, but with the given function applied to every node. Use tree_fold.
Do not use recursion.
There are various ways of traversing a tree to flatten it. Consider the tree below.
1
2 3
4 5
An in-order traversal goes down the left subtree of a node first, then visits the node itself, then the right
subtree. A in-order traversal of the above tree would visit nodes in the order 4, 2, 5, 1, 3. You can think of
this as basically visiting the nodes left-to-right as they’re drawn on the page.
A pre-order traversal visits the node itself, then the left subtree, then the right subtree. A pre-order
traversal of the above tree visits the nodes in the order 1, 2, 4, 5, 3.

## Task 2.2 (Programming, 5 points).

Implement the function inorder : ’a tree -> ’a list that lists the nodes of a tree in the order
given by an in-order traversal. (So, inorder applied to the above tree would give [4;2;5;1;3].) Use
tree_fold. Do not use recursion.
Implement the function preorder : ’a tree -> ’a list that lists the nodes of a tree in the order
given by an pre-order traversal. (So, preorder applied to the above tree would give [1;2;4;5;3].) Use
tree_fold. Do not use recursion.
2 CS 440

## 3 Arithmetic Expressions

In this section, we’ll work with a datatype that represents arithmetic expressions of a single variable.
This is actually defined as two data types:
(* Binary operators. *)
type binop = Add | Sub | Mul | Pow ;;
type expression =
| Num of float
| Var
| Binop of binop * expression * expression
| Neg of expression
;;
Num a represents the floating-point number a. Var represents the only variable, which we’ll call x.
Binop o e1 e2 represents a binary operation on subexpressions e1 and e2. The binary operation is given
by o of type binop which can be Add, Sub, Mul or Pow. Neg e is the negation of e.
For example, we could represent −3.0x
2 + x + 2.0 as
Binop (Mul, Neg (Num 3.0), Binop (Pow, Var, Num 2.0)),
Var),
Num 2.0)
(There are other ways we could represent it too; as we’ll learn soon when we start talking about parsing,
the grammar for expressions is ambiguous.)

## 3.1 Parsing expressions

We’ve provided functions (below the definition of the data types, in a large block of code you can ignore)
for parsing strings into expression datatypes. You may find this helpful when testing your code, unless you
particularly like manually typing things like the expression above. The parser is reasonably robust, but not
great, so you may need to fiddle around a bit to get your strings in the right format. We can get the above
expression by running
parse “~3.0*x^2 + x + 2.0”;;
Note that we use ~ instead of – to represent negation. This lets the parser distinguish between negation
and subtraction. We also need to explicitly include the * between ~3.0 and x^2 rather than just concatenating
the two like we do when writing math normally.
Continued on next page
3 CS 440 Homework 1

Implement a fold function over expressions. Fold for lists and trees only needed to take 2 arguments
(in addition to the list and tree being folded over) because there were two things a list or tree could be:
nil and cons for lists, or leaf or node for trees. Fold for lists needs to know two things: what to do on nil
(return the base case) and what to do on a cons (apply a function to the head and the recursive result of
the tail). Each of the “things” needs to return the same type.
Fold for expressions needs to take four things, all of which take a different kind of expression and return
a ’a.
1. What to do on a Num. This is a float -> ’a because it needs to know the number.
2. What to do on a Var. Like the base case for lists and trees, this is just a ’a because there’s no other
information at a Var.
3. What to do on a Binop. This is a binop -> ’a -> ’a -> ’a because it needs to know the operation
and the result of folding over both subexpressions.
4. What to do on a Neg. This is a ’a -> ’a because it just needs to know the result of folding over
the subexpression.

## Task 3.2 (Programming, 5 points).

Write a function contains_var : expression -> bool that returns true if and only if an expression
contains a variable. Your implementation may be recursive (it need not be tail-recursive) or may be
non-recursive and use fold_expr (even if you didn’t do that task; we’ll test your implementation with a
working version of fold_expr.)
Write a function evaluate : expression -> float -> float. The application evaluate e v should
substitute v for x in the given expression and evaluate it to a float. For example,
• evaluate (parse “~3.0*x^2 + x + 2.0”) 0.0 = 2.0
• evaluate (parse “~3.0*x^2 + x + 2.0”) 1.0 = 0.0
• evaluate (parse “~3.0*x^2 + x + 2.0”) 2.0 = -8.0
Your implementation may be recursive (it need not be tail-recursive) or may be non-recursive and use
fold_expr (even if you didn’t do that task; we’ll test your implementation with a working version of
fold_expr.)

## Task 3.4 (Written, 0 points).

How long (approximately) did you spend on this homework, total, not including bonus questions? (You
can tell us how long you spent on bonus questions, if applicable, separately if you want.) Your honest
feedback will help us with future homeworks.
Who, if anyone, did you collaborate with (and in what way), and what outside sources, if any, did you
consult in working on this homework?
Continued with one more bonus task on next page
4 CS 440 Homework 1

If you’d like, you can practice some Calc I and take the derivative of expressions.
Remember, if e, e1 and e2 are expressions potentially containing the variable x:
d
dx a = 0 (Constants have 0 derivative)
d
dx x = 1 (Variable)
d
dx (e)
a = a · (e)
(a−1)
·
d
dx e (Power rule, chain rule)
d
dx (e1 + e2) = d
dx e1 +
d
dx e2 (Sum Rule)
d
dx (e1 − e2) = d
dx e1 −
d
dx e2 (Sum Rule)
d
dx (e1 · e2) = e2
d
dx e1 + e1
d
dx e2 (Product Rule)

## Bonus Task 3.6 (Programming, 2 points).

Write a function derivative : expression -> expression that returns another expression that is
the derivative of the given expression using the rules above. Note that the rule for power above only applies
if a is a constant (i.e., does not depend on x). There’s another, more complicated rule for the derivative
of e
e2
1 where e1 and e2 both contain x, but you’re not required to use it. Instead, if the exponent depends
on x, raise the exception NotPolynomial.
For 1 bonus point, implement derivative recursively (it need not be tail-recursive).
For 2 bonus points, implement derivative using fold_expr (this is slightly trickier than you might think).
5 CS 440 Homework 1