Scala best way of turning a Collection into a Map-by-key?

Question

You can use

c map (t => t.getP -> t) toMap

but be aware that this needs 2 traversals.

You can construct a Map with a variable number of tuples. So use the map method on the collection to convert it into a collection of tuples and then use the : _* trick to convert the result into a variable argument.

scala> val list = List("this", "maps", "string", "to", "length") map {s => (s, s.length)}
list: List[(java.lang.String, Int)] = List((this,4), (maps,4), (string,6), (to,2), (length,6))

scala> val list = List("this", "is", "a", "bunch", "of", "strings")
list: List[java.lang.String] = List(this, is, a, bunch, of, strings)

scala> val string2Length = Map(list map {s => (s, s.length)} : _*)
string2Length: scala.collection.immutable.Map[java.lang.String,Int] = Map(strings -> 7, of -> 2, bunch -> 5, a -> 1, is -> 2, this -> 4)

In addition to @James Iry's solution, it is also possible to accomplish this using a fold. I suspect that this solution is slightly faster than the tuple method (fewer garbage objects are created):

val list = List("this", "maps", "string", "to", "length")
val map = list.foldLeft(Map[String, Int]()) { (m, s) => m(s) = s.length }

This can be implemented immutably and with a single traversal by folding through the collection as follows.

val map = c.foldLeft(Map[P, T]()) { (m, t) => m + (t.getP -> t) }

The solution works because adding to an immutable Map returns a new immutable Map with the additional entry and this value serves as the accumulator through the fold operation.

The tradeoff here is the simplicity of the code versus its efficiency. So, for large collections, this approach may be more suitable than using 2 traversal implementations such as applying map and toMap.

Another solution (might not work for all types)

import scala.collection.breakOut
val m:Map[P, T] = c.map(t => (t.getP, t))(breakOut)

this avoids the creation of the intermediary list, more info here: Scala 2.8 breakOut