I need help creating a program using Scala. object subtyping { // Instances of C
ID: 3905169 • Letter: I
Question
I need help creating a program using Scala.
object subtyping {
// Instances of Counter have a integer field that can be incremented, decremented, or read.
class Counter {
private var n = 0
def increment () = { n = n + 1 }
def decrement () = { n = n - 1 }
def get () : Int = n
}
// EXERCISE 1: complete the following function.
// The observeCounter function has one parameter f: a function that accepts (a reference to) a Counter instance but returns nothing.
// The observeCounter function should call f with (a reference to) an object (of a class extending Counter).
// Your class that extends Counter must keep track of the total number of times that increment/decrement have been called.
// I.e., if the increment method is called 3 times on an instance, and the decrement method is called 2 times on the same instance, then it should store 5 (somewhere other than the existing field n).
// observeCounter should call f, and then return the total number of times that increment/decrement were called on the instance by f.
def observeCounter (f : Counter => Unit) : Int = {
// TODO: Provide definition here.
-1
}
// EXERCISE 2: complete the following function.
// It is the same as observeCounter except that f has a parameter of type List[Counter] not Counter.
// f will insist that the List[Counter] has length 3.
// You must return a List[Int] not an Int.
// The first element of the result List[Int] must correspond to the number of times that increment/decrement were called on the first element of type List[Counter], similarly for the second and third elements.
def observeCounterList (f : List[Counter] => Unit) : List[Int] = {
// TODO: Provide definition here.
List (-1, -1, -1)
}
// EXERCISE 3: complete the following function.
// It is the same as observeCounterList except that f has a parameter of type Array[Counter] not List[Counter].
// f will insist that the Array[Counter] has length 3.
// You must return a Array[Int] not a List[Int].
// The first element of the result Array[Int] must correspond to the number of times that increment/decrement were called on the first element of type Array[Counter], similarly for the second and third elements.
def observeCounterArray (f : Array[Counter] => Unit) : Array[Int] = {
// TODO: Provide definition here.
List (-1, -1, -1).toArray
}
}
Explanation / Answer
object subtyping {
// Instances of Counter have a integer field that can be incremented, decremented, or read.
class Counter {
private var n = 0
def increment () = { n = n + 1 }
def decrement () = { n = n - 1 }
def get () : Int = n
}
// EXERCISE 1: complete the following function.
// The observeCounter function has one parameter f: a function that accepts (a reference to) a Counter instance but returns nothing.
// The observeCounter function should call f with (a reference to) an object (of a class extending Counter).
// Your class that extends Counter must keep track of the total number of times that increment/decrement have been called.
// I.e., if the increment method is called 3 times on an instance, and the decrement method is called 2 times on the same instance, then it should store 5 (somewhere other than the existing field n).
// observeCounter should call f, and then return the total number of times that increment/decrement were called on the instance by f.
def observeCounter (f : Counter => Unit) : Int = {
class Count extends Counter {
private var x = 0
private var n = 0
override def increment () = {
n = n + 1
x = x + 1
}
override def decrement () = {
n = n - 1
x = x + 1
}
override def get () : Int = {
x
}
}
val y = new Count
f(y)
val a : Int = y.get
a
}
// EXERCISE 2: complete the following function.
// It is the same as observeCounter except that f has a parameter of type List[Counter] not Counter.
// f will insist that the List[Counter] has length 3.
// You must return a List[Int] not an Int.
// The first element of the result List[Int] must correspond to the number of times that increment/decrement were called on the first element of type List[Counter], similarly for the second and third elements.
def observeCounterList (f : List[Counter] => Unit) : List[Int] = {
class Count extends Counter {
private var x = 0
private var n = 0
override def increment () = {
n = n + 1
x = x + 1
}
override def decrement () = {
n = n - 1
x = x + 1
}
override def get () : Int = {
x
}
}
val x = new Count
val y = new Count
val z = new Count
val countList = List (x, y, z)
f(countList)
val a = List(x.get, y.get, z.get)
a
}
// EXERCISE 3: complete the following function.
// It is the same as observeCounterList except that f has a parameter of type Array[Counter] not List[Counter].
// f will insist that the Array[Counter] has length 3.
// You must return a Array[Int] not a List[Int].
// The first element of the result Array[Int] must correspond to the number of times that increment/decrement were called on the first element of type Array[Counter], similarly for the second and third elements.
def observeCounterArray (f : Array[Counter] => Unit) : Array[Int] = {
class Count extends Counter {
private var x = 0
private var n = 0
override def increment () = {
n = n + 1
x = x + 1
}
override def decrement () = {
n = n - 1
x = x + 1
}
override def get () : Int = {
x
}
}
val x = new Count
val y = new Count
val z = new Count
val countArray:Array[Counter] = Array (x, y, z)
f(countArray)
val a = Array (x.get, y.get, z.get)
a
}
}
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