Scala Collections - flatMap

March 28, 2017

flatMap is a frequently used combinator that combines mapping and flattening. flatMap takes a function that works on the nested lists and then concatenates the results back together. It can be very confusing at first. Let’s break it down with more examples.

With A Single List

flapMap can be used with a single list. Take note of List(x*2) signature. every item is multiplied by 2. After that, all lists are flattened as one list.

    val result: List[Int] = List(1, 2, 3, 4, 5, 6).flatMap(x => List(x * 2))
    val resultAsString: String = result.mkString(",")
    println("the result is: " + resultAsString)

the result is: 2,4,6,8,10,12

With Multiple Lists

flatMap can also be used for multiple lists. Then we use flatMap then map pattern. Similar to the first flatMap with the single list, every item is multiplied by 2 and all lists are flattened as one list.

    val result: List[Int] = List(List(1, 2, 3), List(3, 4, 5)).flatMap(x => x.map(i => i * 2))
    val resultAsString: String = result.mkString(",")
    println("the result is: " + resultAsString)

the result is: 2,4,6,6,8,10

flatMap as Filter

flatMap can also be used as a filter. Here, pattern matching and Option are used

    val result: List[Int] = List(1, 2, 3, 4, 5, 6).flatMap {
      case x if x % 2 == 0 => Some(x)
      case _ => None
    }
    val resultAsString: String = result.mkString(",")
    println("the result is: " + resultAsString)

the result is: 2,4,6

flatMap to Retrieve Known Data

To retrieve known data or value, flapMap can be used too. Similar to flatMap as filter, pattern matching and Option are used. Here, we extract 1, 2 and 4 which can be parsed by using Interger.parseInt. Those that cannot be parsed will return as None.

    val result: List[Int] = List("1", "2", "three", "4", "five", "scala").flatMap { x =>
      try {
        Some(Integer.parseInt(x.trim))
      } catch {
        case e: Exception => None
      }
    }
    val resultAsString: String = result.mkString(",")
    println("the result is: " + resultAsString)

the result is: 1,2,4

flatMap can also be used as a filter on the list with multiple types.

    val result: List[String] = List("no", 1, true, "life").flatMap {
      case s: String => Some(s)
      case _ => None
    }
    val resultAsString: String = result.mkString(",")
    println("the result is: " + resultAsString)

the result is: no,life