Adding Scala examples for Chapter 4 and Chapter 5 (#30)

* Add files via upload

* Update 01_countdown.scala

* Update 02_greet.scala

* Update 02_greet.scala

* Create scala

* Delete scala

* Add files via upload

* Add files via upload

04_quicksort/scala/01_loop_sum.scala

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+def sum(arr: List[Int]): Int = {
+  var total: Int = 0
+  for(x <- arr) total += x
+  total
+}
+println(sum(List(1,2,3,4)))

04_quicksort/scala/02_recursive_sum.scala

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+def sum(list: List[Int]): Int = {
+  if(list.isEmpty) return 0
+  else return list(0) + sum(list.drop(1))
+}

04_quicksort/scala/03_recursive_count.scala

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+def count(list: List[Int]): Int = {
+  if(list.isEmpty) return 0 
+  return 1 + count(list.drop(1))
+}

04_quicksort/scala/04_recursive_max.scala

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+def max(list: List[Int]): Int = {
+  if(list.length == 2){
+    if(list(0) > list(1)) return list(0) 
+    else return list(1)
+  }
+  val sub_max = max(list.drop(1))
+  if(list(0) > sub_max) list(0) 
+  else sub_max
+}

04_quicksort/scala/05_quicksort.scala

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+def quicksort(array: Array[Int]): Array[Int] = {
+  if(array.length < 2){
+    //base case, arrays with 0 or 1 element are already "sorted"
+    return array
+  }
+  else {
+    //recursive case
+    val pivot: Int = array(0)
+    //sub-array of all the elements less than the pivot
+    val less: Array[Int] = array.drop(1).filter(x => x < pivot)
+    //sub-array of all the elements greater than the pivot
+    val greater: Array[Int] = array.drop(1).filter(x => x > pivot)
+    return (quicksort(less) :+ pivot) ++ quicksort(greater)
+  }
+}
+quicksort(Array(10,5,2,3))

05_hash_tables/scala/01_price_of_groceries.scala

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+import scala.collection.mutable
+val book = mutable.Map[String, Double]()
+book += ("apple" -> 0.67)
+book += ("milk" -> 1.49)
+book += ("avocado" -> 1.49)
+println(book)

05_hash_tables/scala/02_check_voter.scala

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+import scala.collection.mutable
+val voted = mutable.Map[String, Boolean]()
+def check_voter(name: String): Unit = {
+  if(voted.exists(_ == (name,true))){
+    println("kick them out!")
+  } else{
+    voted += (name -> true)
+  }
+}
+check_voter("tom")
+check_voter("mike")
+check_voter("mike")