Question

I wish to create a Map keyed by name containing the count of things with that name. I have a list of the things with name, which may contain more than one item with the same name. Coded like this I get an error “type mismatch; found : String required: (String, Int)”:

//variation 0, produces error 
(Map[String, Int]() /: entries)((r, c) => { r + (c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })

This confuses me as I though (a, b) was a Tuple2 and therefore suitable for use with Map add. Either of the following variations works as expected:

//variation 1, works
(Map[String, Int]() /: entries)((r, c) => { r + Tuple2(c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
//variation 2, works
(Map[String, Int]() /: entries)((r, c) => { 
    val e = (c.name, if (r.contains(c.name)) (c.name) + 1 else 1) })
    r + e

I’m unclear on why there is a problem with my first version; can anyone advise. I am using Scala-IDE 2.0.0 beta 2 to edit the source; the error is from the Eclipse Problems window.

Answer 1

When passing a single tuple argument to a method used with operator notation, like your + method, you should use double parentheses:

(Map[String, Int]() /: entries)((r, c) => { r + ((c.name, r.get(c.name).map(_ + 1).getOrElse(1) )) })

I’ve also changed the computation of the Int, which looks funny in your example…