How to handle objects that are value in map?

Question

Lets say there is a class

case class User(id: Long, firstName: String, surName: String, password: String)

and a map

var userMap = Map(
    1 -> User(1, "firstName1", "surName1", "1111"),
    2 -> User(2, "firstName2", "surName2", "2222"),
    3 -> User(3, "firstName3", "surName3", "3333")
)

by doing that

userMap find { case (a,b) => b == (1, "firstName1", "surName1", "1111") } isDefined

I can find if there is that value or not, but

how can I find if there is, for instance, surName1 in the userMap, if there is to change it and return me true or false if there is no such a surname?

Thank you.

Do you mean `b == User(1, "firstName1", "surName1", "1111")`? — om-nom-nom, Sep 26 '13 at 16:40
You may want a lens: look at the second answer [here](http://stackoverflow.com/questions/3900307/cleaner-way-to-update-nested-structures). — WeaponsGrade, Sep 26 '13 at 19:54
@WeaponsGrade Possibly more advanced than the OP is quite ready for, but definitely something anybody should be interested in. — itsbruce, Sep 27 '13 at 12:41

Brian · Answer 1 · 2013-09-26T20:08:49.467

2

Almost what you have now except for matching on all properties of the value just explicitly match on the surName property.

userMap.find{case (i, user) => user.surName == "surName1"}.isDefined
res0: Boolean = true

scala> userMap.find{case (i, user) => user.surName == "foo"}.isDefined
res1: Boolean = false

edited Sep 26 '13 at 20:08

answered Sep 26 '13 at 16:40

Brian

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om-nom-nom · Answer 2 · 2013-09-26T16:48:14.580

1

Instead of using find ... isDefined you would be better to opt to exists if you want to just check existence of said item:

userMap exists { 
   case (_, user) => user == User(1, "firstName1", "surName1", "1111") 
}

Next, if you want to

how can I find if there is, for instance, surName1 in the userMap, if there is to change it and return me true or false if there is no such a surname

There is not much to change to you code:

val found = userMap find { case (k,user) => user == User(_, _, "surName1", _) } 
// or with field access as @brian showed
found.foreach { case (key, value) => // do update with userMap by found key } 
found.isDefined

edited Sep 26 '13 at 16:48

answered Sep 26 '13 at 16:42

om-nom-nom

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Why not userMap.values.exists for the first? – itsbruce Sep 26 '13 at 18:38
@itsbruce yes, it could be `userMap.values.exists` as well – om-nom-nom Sep 26 '13 at 18:40

itsbruce · Answer 3 · 2013-09-26T19:44:15.240

scala>  userMap.values.exists{ case User(_, surName1, _, _ ) => true }
res1: Boolean = true

But if you're going to make a change, don't check for true then do it. That's imperative non-scala way of thinking. Here's a mildly evil way of doing that:

var changed = false
userMap map { x => x match {
    case (n, User(i, f, "surName1", p)) => {
      changed = true
      (n, User(i, f, "Appleyard", p))
    }
    case _ => x
  }
}

What this does is return a copy of userMap (because your userMap is probably immutable unless you specified a mutable map), with any "surName1" users changed to "Appleyards". If any matching users were found, "changed" will be set to true as a side-effect. I call this mildly evil because it uses side effects. Mind you, there's a very simple way of avoiding the side-effect...

scala> newMap = userMap  map { x => x match {
    case (n, User(i, f, "surName1", p)) => {
      (n, User(i, f, "Appleyard", p))
    }
    case _ => x
  }
}

scala> newMap == userMap
res2: Boolean = false

Now here's something more formal

def surnameA2B(oldName: String, newName: String, m: Map[Int,User]): (Int, Map[Int,User]) = {
  val changed = m collect {
    case (n, User(i, f, s, p)) if s == oldName =>
      (n, User(i, f, newName, p))
  }
  (changed.size, m ++ changed)
}

surnameA2B("surName1", "Appleyard", userMap)
res1: (Int, Map[Int,User]) = (1,Map(1 -> User(1,firstName1,Appleyard,1111), 2 -> User(2,firstName2,surName2,2222), 3 -> User(3,firstName3,surName3,3333)))

Given a name to match, a replacement name and a usermap, it returns a count of the changed users and a new usermap with the changes in place. 0 means no changes, so you can derive your boolean just by testing if the count is 0 or not.

Checking for a condition, then acting if it is true, then setting a condition based on the actions is a very imperative, non-Scala way of doing things. In fact, it is rarely necessary to check if a condition is true, when you can chain functions so that A will naturally happen if B completed (and for each relevant iteration of B, if necessary. Scala makes it easy to try this approach and it results in cleaner code.

How to handle objects that are value in map?

3 Answers3