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--- pp:2023:scala:l03 [2023/03/08 13:48]
andrei.cirpici created
+++ pp:2023:scala:l03 [2023/04/02 17:28] (current)
alexandra.udrescu01
@@ Line 1: / Line 1: @@
-====== Lab 3. High order functions ======
+====== Lab 3. Lists in Scala ======
 Objectives:
-  * implement and use **higher-order** functions. A **higher-order** function takes other functions as parameter or returns them
+  * get familiar with **pattern matching** lists, as well as common list operations from Scala and how they work
-  * implement **curry** and **uncurry** functions, and how they should be properly used (review lecture).
+  * get familiar with common **higher-order functions** over lists (partition, map, foldRight, foldLeft, filter)
-** Create a new Scala worksheet to write your solutions **
+==== 3.1. Common list operations ====
-===== 3.1 Intro. Functions as parameters =====
+**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.
-**1.1.1** Write a function ''apply'' that takes an integer and return the result of the applied function on the given integer. Start from the code stub below:
 <code scala>
-def apply(n: Int, f: Int => Int): Int = {
+def atLeastk(k: Int, l: List[Int]): Boolean =
-   ???
+  if (k == 0) ???
-}
+  else ???
+  }
+def atLeastkPred(pred: Int => Boolean)(k: Int, l: List[Int]): Boolean = ???
 </code>
-<hidden>
-Solution:
+**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.
 <code scala>
-def apply(n: Int, f: Int => Int): Int = {
+def take(n: Int, l: List[Int]): List[Int] = ???
-   f(n)
+//take(3,List(1,2,3,4,5)) = List(1,2,3)
-}
 </code>
-</hidden>
-**1.1.2** Write a function ''doubler'' that returns a function that doubles the input it receives (an integer). Start from the code stub below:
+**3.1.3.** Write a function which //drops// the first ''n'' elements from a given list.
 <code scala>
-def doubler(): Int => Int = {
+def drop(n: Int, l: List[Int]): List[Int] = ???
-   ???
+//drop(3,List(1,2,3,4,5)) = List(4,5)
-}
 </code>
-<hidden>
-Solution:
+**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**.
 <code scala>
-def doubler(): Int => Int = {
+def takeP(p: Int => Boolean)(l: List[Int]): List[Int] = ???
-   x => 2*x
+//takeP(_%2 == 0)(List(1,2,3,4,5,6)) = List(2,4,6)
-}
 </code>
-or
+**3.1.5.** Write a function which uses a predicate to partition (split) a list.
 <code scala>
-def doubler(): Int => Int = {
+def part(p: Int => Boolean)(l: List[Int]): (List[Int], List[Int]) = ???
-  def double(x: Int): Int = {
+// part(_%2 == 0)(List(1,2,3,4,5,6)) = (List(2,4,6),List(1,3,5))
-*x
-  }
-  double
-}
 </code>
-</hidden>
-===== 3.2 Custom high order functions =====
+==== 3.2. String processing ====
-**3.2.1** Define the function ''foldWith'' which uses an operation ''op'' to reduce a range of integers to a value. For instance, given that ''op'' is addition (+), the result of folding the range 1 to 3 will be 1+2+3=6. ''foldWith'' should be curried (it will take the operation and return another function which expects the bounds).
+In what follows, we shall encode a String as a list of characters, using the type defined below:
 <code scala>
-def foldWith (op: (Int,Int) => Int)(start: Int, stop: Int): Int = {
+type Str = List[Char]
-  def tail_fold(crt: Int, acc: Int): Int  = ???
-  ??
-}
 </code>
-<hidden>
+Add this type alias to your code before solving the following exercises.
-Solution:
+The following is an input test. You can add more examples to it:
 <code scala>
-def foldWith (op: (Int,Int) => Int)(start: Int, stop: Int): Int = {
+val l: List[Str] = List("matei@gmail.com", "mihai@gmail.com", "tEst@mail.com", "email@email.com", "short@ax.ro").map(x => x.toList)
-  def tail_fold(crt: Int, acc: Int): Int  = {
-      if crt == stop then
-        acc
-      else
-        tail_fold(crt + 1, op(acc, crt))
-  }
-  tail_fold(start + 1, start)
-}
 </code>
-</hidden>
-**3.2.2** Define the function ''foldConditional'' which extends ''foldWith'' by also adding a predicate ''p: Int => Int''. ''foldConditional'' will reduce only those elements of a range which satisfy the predicate.
+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.
 <code scala>
-def foldConditional(op: (Int,Int) => Int, p: Int => Boolean)(start: Int, stop: Int): Int = ???
+def remUpper(list: List[Str]): List[Str] = ???
 </code>
-<hidden>
-Solution:
+**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).
 <code scala>
-def foldConditional(op: (Int,Int) => Int, p: Int => Boolean)(start: Int, stop: Int): Int = {
+def longer(k: Int, list: List[Str]): List[Str] = ???
-  def tail_fold(crt: Int, acc: Int): Int = {
+</code>
-    if crt == stop then {
-      acc
-    }
-    else {
-      if p(crt) then {
-        tail_fold(crt + 1, op(acc, crt))
-      }
-      else {
-        tail_fold(crt + 1, acc)
-      }
-    }
-  }
-  if p(start) then {
+**3.2.3.** Count the number of emails longer than k characters. Use ''foldRight''.
-    tail_fold(start + 1, start)
-  }
+<code scala>
-  else {
+def howMany(k: Int)(list: List[Str]): Int = ???
-    if start < stop then {
-      foldConditional(op, p)(start + 1, stop)
-    }
-    else {
-    }
-  }
-}
 </code>
-</hidden>
-**3.2.3** Write a function ''foldMap'' which takes values $math[a_1, a_2, \ldots, a_k] from a range and computes $math[f(a_1)\;op\;f(a_2)\;op\;\ldots f(a_k)].
+**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.
-Use the ''apply'' and ''foldWith'' methods
 <code scala>
-def foldMap(op: (Int,Int) => Int, f: Int => Int)(start: Int, stop: Int): Int = ???
+def namesEmails(list: List[Str]): List[(Str, Str)] = ???
 </code>
-<hidden>
-Solution:
+**3.2.5.** Identify the list of the employed domain names (e.g. ''gmail.com''). Remove duplicates. Use no recursion.
 <code scala>
-def foldMap(op: (Int,Int) => Int, f: Int => Int)(start: Int, stop: Int): Int = {
+def domains(l: List[Str]): List[Str] = ???
-  def tail_aux(crt: Int, acc: Int): Int = {
-    if crt == stop then {
-      acc
-    }
-    else {
-      tail_aux(crt + 1, op(acc, f(crt)))
-    }
-  }
-  tail_aux(start + 1, f(start))
-}
 </code>
-</hidden>
+**(!) 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.
+<code scala>
+def mySplit(l: Str): List[Str] = ???
+</code>
-**3.2.4** Write a function which computes $math[1 + 2^2 + 3^2 + \ldots + (n-1)^2 + n^2] using ''foldMap''.
+**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'').
 <code scala>
-def sumSquares(n: Int): Int = ???
+def domain(list: List[Str]): List[Str] = ???
 </code>
-<hidden>
-Solution:
+==== 3.3. Gradebooks ====
+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:
 <code scala>
-def sumSquares(n: Int): Int = foldMap(_+_, x => x*x)(1, n)
+val l = List(1,2,3,4,5,6,7,8,9)
+l.take(3)
+l.drop(3)
+l.partition(_%2 == 0)
 </code>
-or
+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[(Str, Int)] = List((List('G'),3), (List('F'), 10), (List('M'),6), (List('P'),4))
+</code>
+To make the type signatures more legible, we can introduce type aliases in Scala:
 <code scala>
-def sumSquares(n: Int): Int = foldMap((x, y) => x + y, x => x*x)(1, n)
+type Gradebook = List[(Str,Int)] //the type Gradebook now refers to a list of pairs of String and Int
 </code>
-</hidden>
+Add this type alias to your code before solving the following exercises.
-**3.2.5** Write a function ''hasDivisor'' which checks if a range contains a multiple of k. Use ''foldMap'' and choose ''f'' carefully.
+**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.
 <code scala>
-def hasDivisor(k: Int, start: Int, stop: Int): Boolean = ???
+def increment(g: Gradebook): Gradebook =
+  g.map(???)
 </code>
-<hidden>
-Solution:
+**3.3.2.** Find the average grade from a gradebook. You must use ''foldRight''.
 <code scala>
-def hasDivisor(k: Int, start: Int, stop: Int): Boolean = foldMap(_ & _, _ % k)(start, stop) == 0
+def average(g: Gradebook): Double = ???
 </code>
-or
+**3.3.3.** Write a function which takes a gradebook and returns the percentage of failed vs. passed students, as a pair (x,y).
 <code scala>
-def hasDivisor(k: Int, start: Int, stop: Int): Boolean = foldMap((x, y) => x & y, x => x % k)(start, stop) == 0
+def percentage(g: Gradebook): (Double,Double) = ???
 </code>
-</hidden>
-**3.7.** We can compute the sum of an area defined by a function within a range a,b (the integral of that function given the range), using the following recursive scheme:
+**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.
-  * if the range is small enough, we treat f as a line (and the area as a trapeze). It's area is $math[(f(a) + f(b))(b-a)/2].
-  * otherwise, we compute the mid of the range, we recursively compute the integral from a to mid and from mid to b, and add-up the result.
-Implement the function ''integrate'' which computes the integral of a function f given a range:
 <code scala>
-def integrate(f: Double => Double)(start: Double, stop: Double): Double = ???
+def pass(g: Gradebook): List[Str] = ???
 </code>
-<hidden>
-Solution:
+**3.3.5.** Implement merge-sort (in ascending order) over gradebooks:
 <code scala>
-def integrate(f: Double => Double)(start: Double, stop: Double): Double = {
+def mergeSort(l: Gradebook): Gradebook = {
-  def aux(crt: Double, acc: Double): Double = {
+   def merge(u: Gradebook, v: Gradebook): Gradebook = ???
-    if crt >= stop then
+   ???
-      acc
-    else
-      aux(crt + 0.01, acc + (f(crt) + f(crt + 0.01))*0.01/2)
-  }
-  aux(start, 0)
 }
 </code>
-</hidden>
+**3.3.6** 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[Str] = ???
+</code>