Iterating through Stream - scala

Question

I'm very new to Scala and I have to find primes for numbers in range [i;j].
This is how I fill stream:
val stream = (i to j).toStream
and then I call this function which supposed to find primes:
def findPrimes(s: Stream[Int]): Stream[Int] = {
println("HEAD: " + s.head)
return findPrimes(s.tail)
// s.head #:: findPrimes(s.tail.filter( _ % s.head != 0 ))
}
Outputting stream to console, I am surprised why am I getting exception:
Exception in thread "main" java.util.NoSuchElementException: head of empty stream
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1104)
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1102)
at com.example.anna.app.HelloWorld$.findPrimes(HelloWorld.scala:43)
STREAM SIZE IS 100
at com.example.anna.app.HelloWorld$.prime(HelloWorld.scala:32)
HEAD: 1
at com.example.anna.app.HelloWorld$.delayedEndpoint$com$example$anna$app$HelloWorld$1(HelloWorld.scala:11)
HEAD: 2
HEAD: 3
at com.example.anna.app.HelloWorld$delayedInit$body.apply(HelloWorld.scala:3)
HEAD: 4
at scala.Function0.apply$mcV$sp(Function0.scala:34)
HEAD: 5
HEAD: 6
at scala.Function0.apply$mcV$sp$(Function0.scala:34)
HEAD: 7
at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
HEAD: 8
HEAD: 9
at scala.App.$anonfun$main$1$adapted(App.scala:76)
HEAD: 10
HEAD: 11
at scala.collection.immutable.List.foreach(List.scala:389)
HEAD: 12
at scala.App.main(App.scala:76)
HEAD: 13
at scala.App.main$(App.scala:74)
HEAD: 14
HEAD: 15
at com.example.anna.app.HelloWorld$.main(HelloWorld.scala:3)
HEAD: 16
at com.example.anna.app.HelloWorld.main(HelloWorld.scala)
HEAD: 17
starting from 17 up to end there's no single exception.
And also this commented line doesn't work - it throws same exception.
Any help & advice would be appreciated.

Answer 1

Stream that you pass to findPrimes is of limited size and is getting exhausted when you call findPrimes.
Create infinite stream using Stream.from method.
Scala REPL
scala> :paste
// Entering paste mode (ctrl-D to finish)
def primeStream(s: Stream[Int]): Stream[Int] =
Stream.cons(s.head, primeStream(s.tail filter { _ % s.head != 0 }))
val primes = primeStream(Stream.from(2))
// Exiting paste mode, now interpreting.
primeStream: (s: Stream[Int])Stream[Int]
primes: Stream[Int] = Stream(2, ?)
scala> primes.take(100).toList
res0: List[Int] = List(2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541)
Problem with your code.
scala> val primes = primeStream((1 to 100).toStream)
primes: Stream[Int] = Stream(1, ?)
scala> primes.take(100).toList
java.util.NoSuchElementException: head of empty stream
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1104)
at scala.collection.immutable.Stream$Empty$.head(Stream.scala:1102)
at .primeStream(<console>:12)
at .$anonfun$primeStream$1(<console>:12)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1169)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1159)
at scala.collection.immutable.Stream.$anonfun$take$2(Stream.scala:789)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1169)
at scala.collection.immutable.Stream$Cons.tail(Stream.scala:1159)
at scala.collection.generic.Growable.loop$1(Growable.scala:54)
at scala.collection.generic.Growable.$plus$plus$eq(Growable.scala:58)
at scala.collection.generic.Growable.$plus$plus$eq$(Growable.scala:50)
at scala.collection.mutable.ListBuffer.$plus$plus$eq(ListBuffer.scala:186)
at scala.collection.mutable.ListBuffer.$plus$plus$eq(ListBuffer.scala:44)
at scala.collection.TraversableLike.to(TraversableLike.scala:590)
at scala.collection.TraversableLike.to$(TraversableLike.scala:587)
at scala.collection.AbstractTraversable.to(Traversable.scala:104)
at scala.collection.TraversableOnce.toList(TraversableOnce.scala:294)
at scala.collection.TraversableOnce.toList$(TraversableOnce.scala:294)
at scala.collection.AbstractTraversable.toList(Traversable.scala:104)
... 28 elided

Answer 2

Why this throws an exception? The reason for this is the head method called on empty stream in the println statement.
So we can pattern-match this stream to return empty stream when the tail is empty as follows (there are better ways to traverse stream, but I tried not to change your code very much):
def findPrimes(s: Stream[Int]): Stream[Int] = s match {
case Stream.Empty =>
println("END") //consider removing println statements
Stream.Empty
case h #:: tl =>
println(s"HEAD: $h")
h #:: findPrimes(tl)
}