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--- fp:lab04 [2021/04/08 15:35]
pdmatei
+++ fp:lab04 [2022/05/15 23:54] (current)
vbadoiu old revision restored (2022/03/25 10:15)
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-====== 4. Higher order functions ======
+===== 4. Lists in Scala =====
-===== 4.1. 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)
-==== Map ====
+==== 4.1. Common list operations ====
-The function ''map'' has signature: ''(a->b) -> [a] -> [b]''. It takes as parameter:
-   - a **transformer** function ''f :: a->b''
-   - a list of objects of type ''a''
-It returns:
+**4.1.1.** Write a function which returns true if a list of integers has at least k elements. Use patterns.
-   - a list of transformed objects of type ''b''
+<code scala>
+def atLeastk(k: Int, l: List[Int]): Boolean =
+  if (k == 0) ???
+  else ???
+  }
+</code>
-.1.1. Write a function which takes a list of integers and adds 4 to **each element**.
+**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>
-.1.2. Write a function which takes a list of strings and adds a whitespace at the **beginning** of each string.
+**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>
-.1.3. Write a function which takes a value k, a list of **strings** and returns a list with the first k characters of each string.
+**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>
-==== Filter ====
+**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>
-The function ''filter :: (a -> Bool) -> [a] -> [a]'' takes as parameter:
+==== 4.2. Gradebooks ====
-  * a **predicate** (or condition) of type ''(a -> Bool)''
+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:
-  * a list of elements of type ''a''
+<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>
-It returns:
+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:
-  * a list of filtered elements using the predicate.
+<code scala>
+val gradebook = List(("G",3), ("F", 10), ("M",6), ("P",4))
+</code>
-.1.4. Write a function which removes all strings which are equal to the empty string.
+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.
-.1.5. Write a function which takes a list of strings and returns only those which start with a capital.
+**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>
-==== Foldr ====
+**4.2.2.** Find the average grade from a gradebook. You must use ''foldRight''.
+<code scala>
+def average(g: Gradebook): Double = ???
+</code>
-The function ''foldr'' has type ''(a -> b -> b) -> b -> [a] -> b''. It takes as parameter:
+**4.2.3.** Write a function which takes a gradebook and returns the percentage of failed vs. passed students, as a pair (x,y).
-  * a **binary** operation ''a -> b -> b''. It takes a "list" element of type ''a'', and an **accumulator** of type ''b'' and returns an element of type ''b''
+<code scala>
-  * an **accumulator** (or initial value) of type ''b''
+def percentage(g: Gradebook): (Double,Double) = ???
-  * a list with elements of type ''a''
+</code>
-It returns:
+**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.
-  * a **folded** (or reduced) value of type ''b''.
+<code scala>
+def pass(g: Gradebook): List[String] = ???
+</code>
-Example:
+**4.2.5.** Implement merge-sort (in ascending order) over gradebooks:
-<code haskell>
+<code scala>
-foldr (*) 0 [1,2,3] = 1 * (2 * (3 * 0))
+def mergeSort(l: Gradebook): Gradebook = {
+   def merge(u: Gradebook, v: Gradebook): Gradebook = ???
+   ???
+}
 </code>
-.1.6. Write a function using foldr which computes the product of a list of integers.
+**4.2.6** Write a function which takes a gradebook and reports all passing students in **descending** order of their grade.
+<code scala>
-.1.7. Write a function which computes the maximum of a list of **positive** integers using foldr.
+def honorsList(g: Gradebook): List[String] = ???
-.1.8. Write a function which concatenates a list of strings using foldr.
-==== Foldl ====
-The function ''foldl'' has type ''(b -> a -> b) -> b -> [a] -> b''. It takes as parameter:
-  * a **binary** operation ''b -> a -> b''. It takes an **accumulator** of type ''b'', a "list" element of type ''a'' and returns an element of type ''b''
-  * an **accumulator** (or initial value) of type ''b''
-  * a list with elements of type ''a''
-It returns:
-  * a **folded** (or reduced) value of type ''b''.
-Unlike foldr, foldl reduces element in a different order:
-Example:
-<code haskell>
-foldl (*) 0 [1,2,3] = ((0 * 1) * 2) * 3
 </code>
-.1.9. Write a function which uses foldl to check if an element is member of a list.
-.1.10. Write a function which uses foldl to implement the reversal of a list.
-==== zipWith ====
-.1.11. Use ''ghci> :t zipWith'' to inspect the type of ''zipWith''. Think about a possible example to run this function. What does it do?