Activity for Scala Essentials¶

Instructions¶

For each of the following questions, enter (or copy-and-paste or recall-and-edit) and evaluate the corresponding expression within a Scala REPL.

Pay attention to both the result value and its complete type, e.g.,

res12: Seq[Int] = List(1, 2, 3, 4, 5)

List(1, 2, 3, 4, 5): scala.collection.immutable.List[Int]

To enhance your understanding of these techniques, you are also encouraged to evaluate different subexpressions of these expressions in the Scala REPL.
Make sure to leave some time for the last five or so examples!
You’re encouraged to work in pairs or small groups, and to discuss your results with each other.

Specific examples for this activity¶

Please evaluate each of these expressions in the Scala REPL, one at a time.

// basic sequence and list operations

 Seq("hello", "world",  "what", "up")(2)

 Seq("hello", "world",  "what", "up").take(2)

 Seq("hello", "world",  "what", "up").drop(2)

 // maps and filters

 (1 to 5).map(n => (n, n + 1))

 Seq(4, 3, 1, 5, 2).filter(_ >= 3)

 (1 to 5).filter(_ >= 3).map(n => (n, n + 1))

 (1 to 5).map(n => (n, n + 1)).filter{ case (x, y) => y >= 3 }

 (1 to 5).map(n => Seq(n, n + 1))

 (1 to 5).flatMap(n => Seq(n, n + 1))

 (1 to 5).flatMap(n => Seq(n, n + 1)).filter(_ >= 3)

 // fold left and right (repeated application of a binary operator)

 (1 to 5).foldLeft(1) { (result, n) =>
   result * n
 }

 (1 to 5).foldLeft(Seq.empty[Int]) { (result, n) =>
   n +: result
 }

 (1 to 5).foldRight(Seq.empty[Int]) { (n, result) =>
   n +: result
 }

 // for-yield expressions

 for {
   x <- 1 to 5
   y <- Seq(x, x + 1)
   if y >= 3 // like filter!
 } yield y

 for {
   x <- 1 to 5
   y <- Some(4)
   if y >= 3 // like filter!
 } yield x + y

 // grouping and aggregation

 Seq("hello", "world", "what", "up").map(s => s.length).max

 Seq(("s", 2), ("s", 3), ("s", 5), ("s", 3)).map(_._2).toSet

 Seq("hello", "world", "what", "up").zipWithIndex

 Seq("hello", "world", "what", "up", "hello", "hello", "up", "hello").zipWithIndex.groupBy(_._1)

 Seq("hello", "world", "what", "up", "hello", "hello", "up", "hello").zipWithIndex.groupBy(_._1).view.mapValues(_.length).toMap