Lab 3. Lists in Scala

Objectives:

• get familiar with pattern matching lists, as well as common list operations from Scala and how they work
• get familiar with common higher-order functions over lists (partition, map, foldRight, foldLeft, filter)

3.1.1. Write a function which returns true if a list of integers has at least k elements. Use patterns. Write a second function which returns true if the list has at least k elements that satisfy a predicate.

def atLeastk(k: Int, l: List[Int]): Boolean =
  if (k == 0) ???
  else ???
  }
 
def atLeastkPred(pred: Int => Boolean)(k: Int, l: List[Int]): Boolean = ???

3.1.2. Write a function which returns the first n elements from a given list. The function should not be implemented as tail-recursive.

def take(n: Int, l: List[Int]): List[Int] = ???
//take(3,List(1,2,3,4,5)) = List(1,2,3)

3.1.3. Write a function which drops the first n elements from a given list.

def drop(n: Int, l: List[Int]): List[Int] = ???
//drop(3,List(1,2,3,4,5)) = List(4,5)

3.1.4. Write a function which takes a predicate p: Int ⇒ Boolean, a list l and returns a sublist of l containing those elements for which p is true. The function should be curried.

def takeP(p: Int => Boolean)(l: List[Int]): List[Int] = ???
//takeP(_%2 == 0)(List(1,2,3,4,5,6)) = List(2,4,6)

3.1.5. Write a function which uses a predicate to partition (split) a list.

def part(p: Int => Boolean)(l: List[Int]): (List[Int], List[Int]) = ???
// part(_%2 == 0)(List(1,2,3,4,5,6)) = (List(2,4,6),List(1,3,5))

In what follows, we shall encode a String as a list of characters, using the type defined below:

type Str = List[Char]

Add this type alias to your code before solving the following exercises.

The following is an input test. You can add more examples to it:

val l: List[Str] = List("matei@gmail.com", "mihai@gmail.com", "tEst@mail.com", "email@email.com", "short@ax.ro").map(x => x.toList)

Use map, foldr/foldl, instead of recursive functions.

3.2.1. Remove uppercases from emails. (Do not use recursion). Use the Internet to find the appropriate character function.

def remUpper(list: List[Str]): List[Str] = ???

3.2.2. Write a function which removes emails longer than a given size. Try to think of two ways to implement this using already defined functions (do not define your own auxiliary functions).

def longer(k: Int, list: List[Str]): List[Str] = ???

3.2.3. Count the number of emails longer than k characters. Use foldRight.

def howMany(k: Int)(list: List[Str]): Int = ???

3.2.4. Split the list between first names and email domains. What ingredients (auxiliary functions) are necessary? Use either a fold or a tail-recursive function in your implementation.

def namesEmails(list: List[Str]): List[(Str, Str)] = ???

3.2.5. Identify the list of the employed domain names (e.g. gmail.com). Remove duplicates. Use no recursion.

def domains(l: List[Str]): List[Str] = ???

(!) 3.2.6. In some previous exercise you have, most likely, used already defined function to split the emails. Try implementing a split function using foldRight. Try to figure out what the accumulator should do.

def mySplit(l: Str): List[Str] = ???

3.2.7. Generalize the former function for any given character. Use it to implement a function that return the domains without the dot (ex. gmail).

def domain(list: List[Str]): List[Str] = ???

More general implementations of taken, dropn and part are already implemented in Scala and can be used as member functions of lists. Examples are shown below:

val l = List(1,2,3,4,5,6,7,8,9)
l.take(3)
l.drop(3)
l.partition(_%2 == 0)

In what follows, we shall encode a gradebook as a list of pairs (<name>,<grade>), where <name> is a String and <grade> is an Int. Example:

val gradebook: List[(Str, Int)] = List((List('G'),3), (List('F'), 10), (List('M'),6), (List('P'),4))

To make the type signatures more legible, we can introduce type aliases in Scala:

type Gradebook = List[(Str,Int)] //the type Gradebook now refers to a list of pairs of String and Int

Add this type alias to your code before solving the following exercises.

3.3.1. Write a function which adds one point to all students which have a passing grade (>= 5), and leaves all other grades unchanged.

def increment(g: Gradebook): Gradebook =
  g.map(???) 

3.3.2. Find the average grade from a gradebook. You must use foldRight.

def average(g: Gradebook): Double = ???

3.3.3. Write a function which takes a gradebook and returns the percentage of failed vs. passed students, as a pair (x,y).

def percentage(g: Gradebook): (Double,Double) = ???

3.3.4. Write a function which takes a gradebook and returns the list of names which have passed. Use filter and map from Scala.

def pass(g: Gradebook): List[Str] = ???

3.3.5. Implement merge-sort (in ascending order) over gradebooks:

def mergeSort(l: Gradebook): Gradebook = {
   def merge(u: Gradebook, v: Gradebook): Gradebook = ???
   ???
}

3.3.6 Write a function which takes a gradebook and reports all passing students in descending order of their grade.

def honorsList(g: Gradebook): List[Str] = ???