Essential Scala Collection Functions
Let me ask you a question. How many times you wrote similar code?
case class User(id : Int, userName: String)
val users: List[User] = // ....
val resultUsers: List[User] = // ....
for (i <- 0 until users.size) {
if (users(i).userName != "test") {
resultUsers += users(i)
}
}
May imperative languages like Java, C, C++ have very similar approach for iteration. Some of them have some syntactic sugar (like Java for each loop). There are many problems with this approach. I think the most important annoyance is that it’s hard to tell, what this code actually does. Of course, if you have 10+ years Java experience, you can tell it within a second, but what if the body of the for is more involving and has some state manipulatin? Let me show you another example:
def formatUsers(users: List[User]): String = {
val result = new StringBuilder
for (i <- 0 until users.size) {
val userName = users(i).userName
if (userName != "test") {
result append userName
if (i < users.size - 1) {
result append ", "
}
}
}
return result.toString
}
Was it easy to spot the purpose of this code? I think more important question: how do you read and try to understand this code? In order to understand it you need to iterate in your head - you create simple trace table in order to find out how values of i and result are changing with each iteration. And now the most important question: is this code correct? If you think, that it is correct, then you are wrong! It has an error, and I leave it to you to find it. It’s pretty large amount of code for such a simple task, isn’t it? I even managed to make an error it.
I’m going to show you how this code can be improved, so that intent is more clear and a room for the errors is much smaller. I’m not going to show you complete Scala collection API. Instead I will show you 6 most essential collection functions. These 6 function can bring you pretty far in your day-to-day job and can handle most of your iteration needs. I going to cover following functions: filter, map, flatMap, foldLeft, foreach and mkString.
Also note, that these functions will work on any collection type like List, Set or even Map (in this case they will work on tuple (key, value))