Recursively iterate through a Scala list

See if this works for you.

def removeFours(lst: List[Int], acc: List[Int] = List.empty): List[Int] = {
  lst match {
     case Nil    => acc.reverse
     case 4 :: t => removeFours( t, acc )
     case h :: t => removeFours( t, h :: acc )
  }
}

Usage:

scala> removeFours( List(3,7,4,9,2,4,1) )
res84: List[Int] = List(3, 7, 9, 2, 1)

Using an inner function and pattern matching to de-structure the list. If the head in the list is 4, then do not add it to the accumulator. If it is, append it to the accumulator.

def removeFours(lst: List[Int]): List[Int] = {
  def loop(lst: List[Int], acc: List[Int]): List[Int] = lst match {
    case Nil => acc
    case h :: t =>
      if (h == 4) {
        loop(t, acc)
      }else{
        loop(t, acc :+ h)
      }
  }
  loop(lst, List())
}

The preferred way to do this is with guards in the pattern match but the if else statement may look more familiar if you're just getting started with scala.

def removeFours(lst: List[Int]): List[Int] = {
  def loop(lst: List[Int], acc: List[Int]): List[Int] = lst match {
    case Nil => acc
    case h :: t if (h == 4) => loop(t, acc)
    case h :: t  => loop(t, acc :+ h)
  }
  loop(lst, List())
}