Tutorial :Scala - how to build an immutable map from a collection of Tuple2s?



Question:

In Python, a dictionary can be constructed from an iterable collection of tuples:

>>> listOfTuples = zip(range(10), [-x for x in range(10)])  >>> listOfTuples  [(0, 0), (1, -1), (2, -2), (3, -3), (4, -4), (5, -5), (6, -6), (7, -7), (8, -8), (9, -9)]  >>> theDict = dict(listOfTuples)  >>> theDict  {0: 0, 1: -1, 2: -2, 3: -3, 4: -4, 5: -5, 6: -6, 7: -7, 8: -8, 9: -9}  >>>   

Is there an equivalent Scala syntax? I see that you can use a varargs type amount of Tuple2s to construct a map, e.g.

scala> val theMap = Map((0,0),(1,-1))  theMap: scala.collection.immutable.Map[Int,Int] = Map((0,0), (1,-1))    scala> theMap(0)  res4: Int = 0    scala> theMap(1)  res5: Int = -1    scala> val tuplePairs = List((0,0),(1,-1))  tuplePairs: List[(Int, Int)] = List((0,0), (1,-1))    scala> val mapFromIterable = Map(tuplePairs)  <console>:6: error: type mismatch;   found   : List[(Int, Int)]   required: (?, ?)         val mapFromIterable = Map(tuplePairs)                                   ^  

I could loop through and assign each value manually, but it seems like there must be a better way.

scala> var theMap:scala.collection.mutable.Map[Int,Int] = scala.collection.mutable.Map()     theMap: scala.collection.mutable.Map[Int,Int] = Map()    scala> tuplePairs.foreach(x => theMap(x._1) = x._2)                                         scala> theMap  res13: scala.collection.mutable.Map[Int,Int] = Map((1,-1), (0,0))  


Solution:1

Using Scala 2.8.0 final, you can do it like this:

  scala> val tuplePairs = List((0,0),(1,-1))  tuplePairs: List[(Int, Int)] = List((0,0), (1,-1))    scala> tuplePairs.toMap  res0: scala.collection.immutable.Map[Int,Int] = Map((0,0), (1,-1))  

If you're using Scala 2.7.7 you could do something like this, as an alternative to the method you used:

  scala> val tuplePairs = List((0,0),(1,-1))  tuplePairs: List[(Int, Int)] = List((0,0), (1,-1))    scala> Map(tuplePairs: _*)  res2: scala.collection.immutable.Map[Int,Int] = Map(0 -> 0, 1 -> -1)  

but as you can see, in 2.8.0 things have been much improved upon.


Solution:2

While there are some answers that give good advice, here is what I think is closest to the original python code.

// Scala 2.8  val listOfTuples = (0 until 10) zip (for (x <- 0 until 10) yield -x)  val theMap = Map(listOfTuples:_*)  

Scala 2.7 does not have zip on Ranges, yet, therefore you have to convert the Ranges to Lists in the first assignment:

// Scala 2.7  val listOfTuples = (0 until 10).toList zip (for (x <- 0 until 10) yield -x).toList  val theMap = Map(listOfTuples:_*)  


Solution:3

There are several options. First (but not recommmended, IMO), you can convert a List into a varargs using list:_* Alternatively, you can use something like the ++ function to add a list of values into a map (which is what Map.apply does anyways)

scala> (Map[Int,Int]()) ++ List((1,2),(3,4))  res4: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4)    scala> Map(List((1,2),(3,4)):_*)  res5: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4)  


Solution:4

I cannot leave a comment to the accepted answer (not enough reputation yet), but the proposed solution for Scala 2.7 is overly complex:

scala> Map() ++ (tuplePairs map (t => (t._1,t._2)))  res2: scala.collection.immutable.Map[Int,Int] = Map(0 -> 0, 1 -> -1)  

The "map" isn't doing anything, it's converting a Tuple2 into a Tuple2. This is enough:

scala> Map() ++ tuplePairs  res3: scala.collection.immutable.Map[Int,Int] = Map(0 -> 0, 1 -> -1)  

In my eyes it would be even better to use Map.empty:

scala> Map.empty ++ tuplePairs  res4: scala.collection.immutable.Map[Int,Int] = Map(0 -> 0, 1 -> -1)  

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