Merge sort in Scala using Actors - scala

Question

So this is the code :
object Main {
def main(args: Array[String]){
var myArray = Array(5,2,7,6,8,1,15,5/*,9,10,56*/)
var startSorting = new Starter(myArray)
startSorting.start
startSorting ! Begin
var i = 0
for( i <- 0 to myArray.length - 1){
println(myArray(i))
}
}
}
import scala.actors.Actor
import scala.actors.Actor._
abstract class SortArray
case object Sort extends SortArray
case object FinishedSubArraySorting extends SortArray
case object Begin extends SortArray
class Starter(toBeSorted :Array[Int]) extends Actor{
def act(){
var first:Array[Int] = Array()
var second:Array[Int] = Array()
loop{
react{
case Begin =>
var SortActor = new MergeSort(toBeSorted)
SortActor.start
SortActor ! Sort
case sortedArray :Array[Int] =>
var i = 0
println("Sortat:")
for( i <- 0 to sortedArray.length - 1){
println(sortedArray(i))
}
}
}
}
}
class MergeSort(toBeSorted :Array[Int]) extends Actor{
def act(){
var finishedSorting = 0
var thisArray = toBeSorted
var first:Array[Int] = Array();
var second:Array[Int] = Array();
var sortedSubArrays = 0;
loop{
react{
case Sort =>
if(thisArray.length == 1){
finishedSorting = 1
sender ! thisArray
exit('stop)
}else{
first = thisArray.slice(0,thisArray.length/2)
second = thisArray.slice(thisArray.length/2,thisArray.length)
var firstSort = new MergeSort(first)
var secondSort = new MergeSort(second)
firstSort.start
secondSort.start
firstSort ! Sort
secondSort ! Sort
}
case subSortedArray:Array[Int] =>
sortedSubArrays = sortedSubArrays + 1
if(sortedSubArrays == 1){
first = subSortedArray
}else{
second = subSortedArray
thisArray = merge(first,second)
finishedSorting = 1
sender ! thisArray
exit('stop)
}
}
}
}
def merge(firstArray :Array[Int],secondArray :Array[Int]):Array[Int] = {
var result:Array[Int] = new Array[Int](firstArray.length + secondArray.length)
var i = 0
var j = 0
var k = 0
while(i < firstArray.length && j < secondArray.length){
if(firstArray(i) <= secondArray(j)){
result(k) = firstArray(i)
i = i + 1
}else{
result(k) = secondArray(j)
j = j + 1
}
k = k + 1
}
while (i < firstArray.length)
{
result(k) = firstArray(i)
i = i + 1
k = k + 1
}
while (j < secondArray.length)
{
result(k) = secondArray(j)
j = j + 1
k = k + 1
}
return result
}
}
It uses 2 actors:One is for the beginning and end output and another one that is going to have multiple instances of it to merge sort the array and sub-arrays. An instance is first called with !Sort in order to divide the array in two and make two actors for each subArray and is also called afterwards with the resulting sorted sub-array from each of the two actors i mentioned earlier.
The problem with this code is that it manages to sort and merge only at the lowest levels(subarrays with max length 2) and then for some reasons the actors call different senders than the ones that called them so nothing happens anymore.

Answer 1

Your problem is that when the MergeSort actors receive the sublists from their children they then send the merged lists to sender - which will be their children instead of parents.
I added a parent field to MergeSort and changed sends to use this parent - here's the modified code
object Main extends App {
var myArray = Array(5,2,7,6,8,1,15,5/*,9,10,56*/)
var startSorting = new Starter(myArray)
startSorting.start
startSorting ! Begin
var i = 0
for( i <- 0 to myArray.length - 1){
println(myArray(i))
}
}
import scala.actors.Actor
import scala.actors.Actor._
abstract class SortArray
case object Sort extends SortArray
case object FinishedSubArraySorting extends SortArray
case object Begin extends SortArray
class Starter(toBeSorted :Array[Int]) extends Actor{
def act(){
var first:Array[Int] = Array()
var second:Array[Int] = Array()
loop{
react{
case Begin =>
var SortActor = new MergeSort(toBeSorted,self)
SortActor.start
SortActor ! Sort
case sortedArray :Array[Int] =>
var i = 0
println("Sortat:")
for( i <- 0 to sortedArray.length - 1){
println(">"+sortedArray(i))
}
}
}
}
}
class MergeSort(toBeSorted :Array[Int],parent:Actor) extends Actor{
def act(){
var finishedSorting = 0
var thisArray = toBeSorted
var first:Array[Int] = Array();
var second:Array[Int] = Array();
var sortedSubArrays = 0;
loop{
react{
case Sort =>
if(thisArray.length == 1){
finishedSorting = 1
println(this + " sending up "+thisArray.mkString("[",",","]"))
parent ! thisArray
exit('stop)
}else{
first = thisArray.slice(0,thisArray.length/2)
second = thisArray.slice(thisArray.length/2,thisArray.length)
var firstSort = new MergeSort(first,self)
var secondSort = new MergeSort(second,self)
firstSort.start
secondSort.start
firstSort ! Sort
secondSort ! Sort
}
case subSortedArray:Array[Int] =>
println(this + " received " + subSortedArray.mkString("[",",","]"))
sortedSubArrays = sortedSubArrays + 1
if(sortedSubArrays == 1){
first = subSortedArray
}else{
second = subSortedArray
thisArray = merge(first,second)
finishedSorting = 1
parent ! thisArray
exit('stop)
}
}
}
}
def merge(firstArray :Array[Int],secondArray :Array[Int]):Array[Int] = {
var result:Array[Int] = new Array[Int](firstArray.length + secondArray.length)
var i = 0
var j = 0
var k = 0
while(i < firstArray.length && j < secondArray.length){
if(firstArray(i) <= secondArray(j)){
result(k) = firstArray(i)
i = i + 1
}else{
result(k) = secondArray(j)
j = j + 1
}
k = k + 1
}
while (i < firstArray.length)
{
result(k) = firstArray(i)
i = i + 1
k = k + 1
}
while (j < secondArray.length)
{
result(k) = secondArray(j)
j = j + 1
k = k + 1
}
return result
}
}