✎ pp:2025:scala:l05 [books]

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====== Lab 5. Data types in Scala ======

Objectives:
  * get familiar with **algebraic data types**
  * get familiar with **pattern matching** and **recursion** with them

==== 5.1 Natural Numbers ====
Given the following implementation of the natural numbers, solve the next few exercises.
<code scala>
trait Nat
case object Zero extends Nat
case class Succ(x: Nat) extends Nat
</code>

**5.1.1** Write a function which takes two natural numbers, and returns their sum.
<code scala>
def add(x: Nat, y: Nat): Nat = ???
</code>

**5.1.2** Write a function which takes two natural numbers, and returns their product.
<code scala>
def multiply(x: Nat, y: Nat): Nat = ???
</code>

**5.1.3** Write a function which takes an int and converts it to a Nat.
<code scala>
def toNat(x: Int): Nat = ???
</code>

==== 5.2 Option ====
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.

** 5.2.1** 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.
<code scala>
def realrealtrycatch(t: => Option[Int], c: => Int): Int = {
  ???
}
</code>

**(!) 5.2.2** Refactor the function toNat(), so that it takes an integer (a positive or negative number) and returns a "container" of a Nat.
<code scala>
def toNatOpt(x: Int): Option[Nat] = ???
</code>

**(!) 5.2.3** Refactor the function add(), so that it takes two "containers" of Nats and returns a "container" of a Nat.
<code scala>
def addOpt(x: Option[Nat], y: Option[Nat]): Option[Nat] = ???
</code>

==== 5.3 Binary Trees ====
Given the implementation of binary trees from the previous lab:
<code scala>
trait BTree

case object TVoid extends BTree

case class Node(left: BTree, info: Int, right: BTree) extends BTree

def leaf_node(value : Int) : BTree = {
  Node(TVoid, value, TVoid)
}

object BTreePrinter {
  case class PrintInfo(len: Int, center: Int, text: List[String])

  def pp(tree: BTree): PrintInfo = tree match {
    case TVoid => PrintInfo(3, 2, List("Nil"))

    case Node(left, value, right) =>
      val strValue = value.toString
      val ppL = pp(left)
      val ppR = pp(right)
      val nlen = ppL.len + ppR.len + 1
      val ncenter = ppL.len + 1

      require(strValue.length <= nlen, "Nice try")

      val alignedX = " " * (ncenter - (strValue.length / 2) - 1) + strValue
      val centerLine = " " * (ncenter - 1) + "|"
      val dottedLine = " " * (ppL.center - 1) + "-" * (nlen - ppL.center - (ppR.len - ppR.center + 1) + 2)
      val downLines = " " * (ppL.center - 1) + "|" + " " * (nlen - ppL.center - (ppR.len - ppR.center +1)) + "|"

      val combinedLines = zipPad("",(l, r) => l ++ (" " * (ppL.len - l.size + 1 )) ++ r, ppL.text, ppR.text)
      
      PrintInfo(nlen, ncenter, alignedX :: centerLine :: dottedLine :: downLines :: combinedLines)
  }
  def zipPad[A](pad: A, f: (A, A) => A, left: List[A], right: List[A]): List[A] = (left, right) match {
    case (Nil, Nil) => Nil
    case (x :: xs, Nil) => x :: zipPad(pad, f, xs, List(pad))
    case (Nil, y :: ys) => f(pad, y) :: zipPad(pad, f, List(pad), ys)
    case (x :: xs, y :: ys) => f(x, y) :: zipPad(pad, f, xs, ys)
  }

  def printTree(tree: BTree): String = "\n" + pp(tree).text.mkString("\n")
}

extension(t: BTree) {
  def toStringTree: String = BTreePrinter.printTree(t)
}

</code>

And arborică:
<code scala>
val arborica = Node(Node(leaf_node(-1), 5, TVoid), 1, Node(leaf_node(4), 2, Node(leaf_node(3), 6, leaf_node(7))))
arborica.toStringTree
</code>

Solve the next few exercises:

**5.3.1** Write a function which takes a BinaryTree and returns its depth.
<code scala>
def depth(tree: BTree): Int = ???
</code>

**5.3.2** Write a function which takes a BinaryTree and returns the number of nodes in its subtree.
<code scala>
def subtree(tree: BTree): Int = ???
</code>

**5.3.3** Write a function which takes a BinaryTree and returns the number of nodes with an even number of children.
<code scala>
def evenChildCount(tree: BTree): Int = ???
</code>

**5.3.4** Write a function which takes a BinaryTree and returns the number of nodes whose values follow a certain rule.
<code scala>
def countNodes(tree: BTree, cond: Int => Boolean): Int = ???
</code>

**(!) 5.3.5** 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 .
<code scala>
def append(tree1: BTree, tree2: BTree): Option[BTree] = ???
</code>

==== 5.4 Expression evaluation ====
Given the following implementation of expressions, solve the next few exercises.
<code scala>
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
</code>

**5.4.1** Write a function which takes an Expression and evaluates it.
<code scala>
def evaluate(e: Expr): Int = ???
</code>

**5.4.2** Write a function which takes an Expression and simplifies it. (Ex. a * (b + c) -> remove parentheses -> ab + ac) 
<code scala>
def simplify(e: Expr): Expr = ???
</code>

**5.4.3** Write a function which takes an Expression and removes 'useless' operations. (Ex. a * 1 -> a, a + 0 -> a)
<code scala>
def optimize(e: Expr): Expr = ???
</code>

<hidden>
If you work outside of worksheets, you can define the trait as:
<code scala>
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
</code>
With the operators defined, you can create expressions writting:
<code scala>
(Atom(1) + Atom(2) * Atom(3)) + Atom(4)
</code>
instead of:
<code scala>
Add(Add(Atom(1), Mult(Atom(2), Atom(3))), Atom(4))
</code>
</hidden>

==== 5.5 Matrix manipulation ====
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

$math[ \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:
<code scala> type Matrix = List[List[Int]] </code>
which makes the type-name ''Matrix'' stand for ''[ [Integer] ]''.

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

$math[ \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)] 

<code scala>
def scalarProd(m: Matrix)(v: Int): Matrix = ??? 
</code>

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

$math[ \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) ]

<code scala>
def hJoin(m1: Matrix, m2: Matrix): Matrix = ???
</code>

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

$math[ \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) ]

<code scala>
def vJoin(m1: Matrix, m2: Matrix): Matrix = ???
</code>

**5.5.4** Write a function which adds two matrices, element by element:

<code scala>
def matSum(m1: Matrix, m2: Matrix): Matrix = ???
</code>
**5.5.5** Write a function that transposed a matrix:

$math[ \left(\begin{array}{ccc} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \\ \end{array}\right)^{T} = \left(\begin{array}{ccc} 1 & 4 & 7 \\ 2 & 5 & 8 \\ 3 & 6 & 9 \\ \end{array}\right)] 

<code scala>
def transposed(m: Matrix): Matrix = ??? 
</code>

**5.5.6** Write a function that computes the matrix product:

$math[ \left(\begin{array}{ccc} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \\ \end{array}\right)  * \left(\begin{array}{ccc} 1 & 2 \\ 3 & 4 \\ 5 & 6 \\ \end{array}\right) = \left(\begin{array}{ccc} 22 & 28 \\ 49 & 64 \\ 76 & 100 \\ \end{array}\right)] 

(For an element of the result $math[a_{ij}] the value is obtained by selecting the i-th row and the j-th column, computing the products of the elements at the same index and adding all those results up:

E. g. 22 = 1 * 1 + 2 * 3 + 3 * 5)

<code scala>
def matrixProd(m1: Matrix, m2: Matrix): Matrix = ??? 
</code>