4. 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)
4.1. Common list operations
4.1.1. Write a function which returns true if a list of integers has at least k elements. Use patterns.
def atLeastk(k: Int, l: List[Int]): Boolean = if (k == 0) ??? else ??? }
4.1.2. 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)
4.1.3. Write a function which drops the first n
elements from a given list. The function should not be implemented as tail-recursive.
def drop(n: Int, l: List[Int]): List[Int] = ??? //drop(3,List(1,2,3,4,5)) = List(4,5)
4.1.4. 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)
4.1.5. 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))
4.2. Gradebooks
More general implementation 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(("G",3), ("F", 10), ("M",6), ("P",4))
To make the type signatures more legible, we can introduce type aliases in Scala:
type Gradebook = List[(String,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.
4.2.1. 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(???)
4.2.2. Find the average grade from a gradebook. You must use foldRight
.
def average(g: Gradebook): Double = ???
4.2.3. 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) = ???
4.2.4. 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[String] = ???
4.2.5. Implement merge-sort (in ascending order) over gradebooks:
def mergeSort(l: Gradebook): Gradebook = { def merge(u: Gradebook, v: Gradebook): Gradebook = ??? ??? }
4.2.6 Write a function which takes a gradebook and reports all passing students in descending order of their grade.
def honorsList(g: Gradebook): List[String] = ???