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--- fp:lab04 [2021/04/01 11:47]
pdmatei created
+++ fp:lab04 [2022/05/15 23:54] (current)
vbadoiu old revision restored (2022/03/25 10:15)
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-====== Higher order functions ======
+===== 4. Lists in Scala =====
-===== Map, foldl, foldr, filter, zipWith =====
+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.
+<code scala>
+def atLeastk(k: Int, l: List[Int]): Boolean =
+  if (k == 0) ???
+  else ???
+  }
+</code>
+**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.
+<code scala>
+def take(n: Int, l: List[Int]): List[Int] = ???
+//take(3,List(1,2,3,4,5)) = List(1,2,3)
+</code>
+**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.
+<code scala>
+def drop(n: Int, l: List[Int]): List[Int] = ???
+//drop(3,List(1,2,3,4,5)) = List(4,5)
+</code>
+**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**.
+<code scala>
+def takeP(p: Int => Boolean)(l: List[Int]): List[Int] = ???
+//takeP(_%2 == 0)(List(1,2,3,4,5,6)) = List(2,4,6)
+</code>
+**4.1.5.** Write a function which uses a predicate to partition (split) a list.
+<code scala>
+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))
+</code>
+==== 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:
+<code scala>
+val l = List(1,2,3,4,5,6,7,8,9)
+l.take(3)
+l.drop(3)
+l.partition(_%2 == 0)
+</code>
+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:
+<code scala>
+val gradebook = List(("G",3), ("F", 10), ("M",6), ("P",4))
+</code>
+To make the type signatures more legible, we can introduce type aliases in Scala:
+<code scala>
+type Gradebook = List[(String,Int)] //the type Gradebook now refers to a list of pairs of String and Int
+</code>
+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.
+<code scala>
+def increment(g: Gradebook): Gradebook =
+  g.map(???)
+</code>
+**4.2.2.** Find the average grade from a gradebook. You must use ''foldRight''.
+<code scala>
+def average(g: Gradebook): Double = ???
+</code>
+**4.2.3.** Write a function which takes a gradebook and returns the percentage of failed vs. passed students, as a pair (x,y).
+<code scala>
+def percentage(g: Gradebook): (Double,Double) = ???
+</code>
+**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.
+<code scala>
+def pass(g: Gradebook): List[String] = ???
+</code>
+**4.2.5.** Implement merge-sort (in ascending order) over gradebooks:
+<code scala>
+def mergeSort(l: Gradebook): Gradebook = {
+   def merge(u: Gradebook, v: Gradebook): Gradebook = ???
+   ???
+}
+</code>
+**4.2.6** Write a function which takes a gradebook and reports all passing students in **descending** order of their grade.
+<code scala>
+def honorsList(g: Gradebook): List[String] = ???
+</code>