Scala updating/creating value in a hashmap

Question

I don't understand this with Scala hasmaps: How do I create a value or update one if it does not exist?

I am tryng to count the number of characters in a list of Strings.

I've tried this code but it doesn't work :

 def times(chars: List[Char]): List[(Char, Int)] = {
     val map = new HashMap[Char, Int]()
     chars.foreach(
        (c : Char) => {
           map.update(c, map.get(c) + 1)
        })
 }

I understand the returning type isn't correct. But is my foreach loop wrong? Is there a prettier way to write it?

I would suggest anonymizing code snippets so that they are not directly taken from the Coursera course assignments, especially in light of the recent "academic fraud" alerts. — ach
– ach, Commented Oct 15, 2012 at 17:32

oluies · Accepted Answer · 2012-10-12 22:46:54Z

3

I think this will answer your question:

scala> "abaccdba".groupBy(identity).mapValues(_.length)
res3: scala.collection.immutable.Map[Char,Int] = Map(b -> 2, d -> 1, a -> 3, c -> 2)

Oh, and btw HashMap has a method getOrElseUpdate as to your original question

edited Oct 12, 2012 at 22:46

oluies

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answered Oct 12, 2012 at 21:31

Nikita Volkov

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3 Comments

bruce Over a year ago

Thank you very much. I think that updating or creating a map value using the previous value (if there is) can be interesting. How Is it possible?

Nikita Volkov Over a year ago

See the API

bruce Over a year ago

Well beleving the API GetOrElseUpdate update only if there is no value. Whereas I'd like to update always. There is an update method but it doesn't use the previous value.

bruce · Accepted Answer · 2012-10-13 00:21:53Z

1

If someone wonder how to use GetOrElseUpdate and find this post here is the exemple I found :

val map = Map('a' -> 1, 'b' -> 2)         //> map  :  
scala.collection.immutable.Map[Char,Int] = Map(a -> 1, b -> 2)
val newval = map.getOrElse('b', 0) + 1      //> newval  : Int = 3
val updated = map + ('b' -> (newval))        //> updated  : 
scala.collection.immutable.Map[Char,Int] = Map(a -> 1, b -> 3)

answered Oct 13, 2012 at 0:21

bruce

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1 Comment

ach Over a year ago

You can also say "Map(...) withDefaultValue 0" instead of using getOrElse.

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