Lab 4. Data types in Scala

Objectives:

• get familiar with algebraic data types
• get familiar with pattern matching and recursion with them

Given the following implementation of the natural numbers, solve the next few exercises.

trait Nat
case object Zero extends Nat
case class Succ(x: Nat) extends Nat

4.1.1 Write a function which takes two natural numbers, and returns their sum.

def add(x: Nat, y: Nat): Nat = ???

4.1.2 Write a function which takes two natural numbers, and returns their product.

def multiply(x: Nat, y: Nat): Nat = ???

4.1.3 Write a function which takes an int and converts it to a Nat.

def toNat(x: Int): Nat = ???

Option = carrier (like a box or a container) for a single or no element, of a given type. (Ex. Some(_) or None)

We use Option to write robust functions, in case they return null or fail to return an accepted value.

4.2.1 Let's revisit the function realtrycatch now that we have a type that represents the possibility of error. If an error occurs (try function returns None), the catch function will be called instead.

def realrealtrycatch(t: => Option[Int], c: => Int): Int = {
  ???
}

(!) 4.2.2 Refactor the function toNat(), so that it takes an integer (a positive or negative number) and returns a “container” of a Nat.

def toNatOpt(x: Int): Option[Nat] = ???

(!) 4.2.3 Refactor the function add(), so that it takes two “containers” of Nats and returns a “container” of a Nat.

def addOpt(x: Option[Nat], y: Option[Nat]): Option[Nat] = ???

Given the following implementation of binary trees, solve the next few exercises.

trait BTree
case object EmptyTree extends BTree
case class Node(value: Int, left: BTree, right: BTree) extends BTree

4.3.1 Write a function which takes a BinaryTree and returns its depth.

def depth(tree: BTree): Int = ???

4.3.2 Write a function which takes a BinaryTree and returns the number of nodes in its subtree.

def subtree(tree: BTree): Int = ???

4.3.3 Write a function which takes a BinaryTree and returns the number of nodes with even number of children.

def evenChildCount(tree: BTree): Int = ???

4.3.4 Write a function which takes a BinaryTree and flattens it (turns it into a list containing the values of the nodes).

def flatten(tree: BTree): List[Int] = ???

4.3.5 Write a function which takes a BinaryTree and return the number of nodes whose values follow a certain rule.

def countNodes(tree: BTree, cond: Int => Boolean): Int = ???

4.3.6 Write a function which takes a BinaryTree and return mirrored BinaryTree.

def mirror(tree: BTree): BTree = ???

(!) 4.3.7 Write a function which takes two BinaryTree and tries to assign the second tree as a child of the first. It should return a “container” of a BinaryTree .

def append(tree1: BTree, tree2: BTree): Option[BTree] = ???

Given the following implementation of expressions, solve the next few exercises.

trait Expr
case class Atom(a: Int) extends Expr
case class Add(e1: Expr, e2: Expr) extends Expr
case class Mult(e1: Expr, e2: Expr) extends Expr

4.4.1 Write a function which takes an Expression and evaluates it.

def evaluate(e: Expr): Int = ???

4.4.2 Write a function which takes an Expression and simplifies it. (Ex. a * (b + c) → remove parentheses → ab + ac)

def simplify(e: Expr): Expr = ???

4.4.3 Write a function which takes an Expression and removes 'useless' operations. (Ex. a * 1 → a, a + 0 → a)

def optimize(e: Expr): Expr = ???

trait Expr {
    def + (that: Expr): Expr = Add(this, that)
    def * (that: Expr): Expr = Mul(this, that)
}
case class Atom(a: Int) extends Expr
case class Add(e1: Expr, e2: Expr) extends Expr
case class Mult(e1: Expr, e2: Expr) extends Expr

(Atom(1) + Atom(2) * Atom(3)) + Atom(4)

Add(Add(Atom(1), Mult(Atom(2), Atom(3))), Atom(4))

We shall represent matrices as lists of lists, i.e. values of type [ [Integer ] ]. Each element in the outer list represents a line of the matrix. Hence, the matrix

$ \displaystyle \left(\begin{array}{ccc} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \\ \end{array}\right)$

will be represented by the list [ [1,2,3],[4,5,6],[7,8,9] ].

To make signatures more legible, add the type alias to your code:

 type Matrix = List[List[Int]] 

which makes the type-name Matrix stand for [ [Integer] ].

4.5.1 Write a function that computes the scalar product with an integer:

$ \displaystyle 2 * \left(\begin{array}{ccc} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \\ \end{array}\right) = \left(\begin{array}{ccc} 2 & 4 & 6 \\ 8 & 10 & 12 \\ 14 & 16 & 18 \\ \end{array}\right)$

def scalarProd(m: Matrix)(v: Int): Matrix = ??? 

4.5.2 Write a function which adjoins two matrices by extending rows (horizontally):

$ \displaystyle \left(\begin{array}{cc} 1 & 2 \\ 3 & 4\\\end{array}\right) hjoin \left(\begin{array}{cc} 5 & 6 \\ 7 & 8\\\end{array}\right) = \left(\begin{array}{cc} 1 & 2 & 5 & 6 \\ 3 & 4 & 7 & 8\\\end{array}\right) $

def hJoin(m1: Matrix, m2: Matrix): Matrix = ???

4.5.3 Write a function which adjoins two matrices by adding new rows (vertically):

$ \displaystyle \left(\begin{array}{cc} 1 & 2 \\ 3 & 4\\\end{array}\right) vjoin \left(\begin{array}{cc} 5 & 6 \\ 7 & 8\\\end{array}\right) = \left(\begin{array}{cc} 1 & 2 \\ 3 & 4 \\ 5 & 6\\ 7 & 8\\ \end{array}\right) $

def vJoin(m1: Matrix, m2: Matrix): Matrix = ???

4.5.4 Write a function which adds two matrices, element by element:

def matSum(m1: Matrix, m2: Matrix): Matrix = ???