Map versus FlatMap on String

Question

Listening to the Collections lecture from Functional Programming Principles in Scala, I saw this example:

scala> val s = \"Hello World\"

scala> s.flat         


        

Answer 1

Use flatMap in situations where you run map followed by flattern. The specific situation is this:

• You’re using map (or a for/yield expression) to create a new collection from an existing collection.

• The resulting collection is a List of Lists.

• You call flatten immediately after map (or a for/yield expression).

When you’re in this situation, you can use flatMap instead.

Example: Add all the Integers from the bag

val bag = List("1", "2", "three", "4", "one hundred seventy five")

def toInt(in: String): Option[Int] = {
try {
Some(Integer.parseInt(in.trim))
} catch {
case e: Exception => None
}
}

Using a flatMap method

> bag.flatMap(toInt).sum

Using map method (3 steps needed)

bag.map(toInt) // List[Option[Int]] = List(Some(1), Some(2), None, Some(4), None)

bag.map(toInt).flatten //List[Int] = List(1, 2, 4)

bag.map(toInt).flatten.sum //Int = 7