Scala Lists - Map and Filter

Two very common List operations in Scala are map and filter. Let's start by looking at map. This function is used when you need to map all of the elements in the list to some over value. For example, if we have a list of integer values and we need to produce a new list where each value in the original list is multiplied by 3, we can write the following map implementation in Scala.

    val l: List[Int] = List(1, 2, 2, 3, 4, 5, 6)
    
    val newList = l.map(x => x * 3)

Another useful function is filter. Filter is like map, in that it traverses every element in the list. However, filter will return a new list where all of the elements in the original list hold true for a particular predicate/test. For example, if we wanted to filter out all odd integer values in a list, we can write the following in Scala.

    val l: List[Int] = List(1, 2, 2, 3, 4, 5, 6)
    
    val newList = l.filter(x => (x %2 == 0))

Of course, we can combine both map and filter. Here's an example.

    val l: List[Int] = List(1, 2, 2, 3, 4, 5, 6)
    
    val newList = l.map(x => x * 3).filter(x => (x % 2) == 0)