Collection library for Scala Programming Language
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Questions tagged [scala-collections]
1639 questions
7
votes
4 answers
Difference between Tuple and List[Any] in Scala?
Currently, I am learning Scala and reading this book Programming in Scala and which says, " Unlike an array or list, a tuple can hold objects with different types." For example, the following tuple contain Int, String and Float.
val tup = (1,…
Ra Ka
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7
votes
1 answer
scala 2.8 collections inconsistency?
why the methods transform (in-place mutation version of map) and retain (in-place mutation version of filter) are defined on only mutable.Map but not on mutable.Buffer and mutable.Set? shouldnt all mutable collections support these methods?
user7841255
- 71
- 1
7
votes
3 answers
What is the most succinct Scala way to reverse a Map?
What is the most succinct Scala way to reverse a Map? The Map may contain non-unique values.
EDIT:
The reversal of Map[A, B] should give Map[B, Set[A]] (or a MultiMap, that would be even better).
missingfaktor
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7
votes
1 answer
Spark DataFrame not respecting schema and considering everything as String
I am facing a problem which I have failed to get over for ages now.
I am on Spark 1.4 and Scala 2.10. I cannot upgrade at this moment (big distributed infrastructure)
I have a file with few hundred columns, only 2 of which are string and rest all…
Run2
- 1,839
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7
votes
6 answers
Scala: transform a collection, yielding 0..many elements on each iteration
Given a collection in Scala, I'd like to traverse this collection and for each object I'd like to emit (yield) from 0 to multiple elements that should be joined together into a new collection.
For example, I expect something like this:
val input =…
GreyCat
- 16,622
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7
votes
2 answers
Can I "pimp my library" with an analogue of TraversableLike.map that has nicely variant types?
Suppose I want to add functionality like map to a Scala List, something along the lines of list mapmap f, which applies the function f to each element of list twice. (A more serious example might be implementing a parallel or distributed map, but I…
Scott Morrison
- 3,100
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7
votes
1 answer
The difference and conversion between Seq[Int] and List[Int]
When I input Seq(1,2,3) in REPL, it returns me List(1,2,3)
scala> Seq(1,2,3)
res8: Seq[Int] = List(1, 2, 3)
Therefore, I thought the List(1,2,3) may be of type List[Int]. And I tried to specify the type for the variable who are assigned to…
Hanfei Sun
- 45,281
- 39
- 129
- 237
7
votes
3 answers
filter a List according to multiple contains
I want to filter a List, and I only want to keep a string if the string contains .jpg,.jpeg or .png:
scala> var list = List[String]("a1.png","a2.amr","a3.png","a4.jpg","a5.jpeg","a6.mp4","a7.amr","a9.mov","a10.wmv")
list: List[String] =…
Govind Singh
- 15,282
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7
votes
1 answer
Why is TraversableOnce.toSeq returning a Stream?
Here is an example:
scala> val xs = List(1,2,3).toIterator.toSeq
xs: Seq[Int] = Stream(1, ?)
A sequence is a materialized collection (it's a List by default), so I expected that toSeq would return a List, not a Stream
The implementation is in…
Adrian
- 3,762
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7
votes
2 answers
Scala equivalent of Python's "in" operator for sets?
In Scala, it's possible to check if an item is a member of a Set using "Contains":
object Main extends App {
val the_set = Set(1, 2, 3, 4)
if( the_set contains 3 ) println("The set contains 3!")
}
However, I'd like to do a similar…
bnsmith
- 1,667
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7
votes
4 answers
Efficient way to check if a traversable has more than 1 element in Scala
I need to check if a Traversable (which I already know to be nonEmpty) has a single element or more.
I could use size, but (tell me if I'm wrong) I suspect that this could be O(n), and traverse the collection to compute it.
I could check if…
Lorenzo Dematté
- 7,638
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- 77
7
votes
4 answers
Scala collections: array groupBy and return array indexes for each group
I have an array, something like that:
val a = Array("a", "c", "c", "z", "c", "b", "a")
and I want to get a map with keys of all different values of this array and values with a collection of relevant indexes for each such group, i.e. for a given…
GreyCat
- 16,622
- 18
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7
votes
1 answer
Scala Map.mapValues stackoverflowerror
The following code results in a StackOverflowError on the last line.
object StackTest extends App{
@tailrec def incrementValues(acc: Map[String, Int], inc: Int): Map[String, Int] = {
if(inc == 0) acc
else…
Pengin
- 4,692
- 6
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- 62
7
votes
1 answer
Merging Maps using `aggregate`
For any given collection of Map, for instance,
val in = Array( Map("a" -> 1, "b" -> 2),
Map("a" -> 11, "c" -> 4),
Map("b" -> 7, "c" -> 10))
how to use aggregate on in.par so as to merge the maps into
Map ( "a" ->…
elm
- 20,117
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7
votes
2 answers
Scala Map pattern matching
How to do pattern matching on a Map in Scala ?
A (non working) attempt includes,
Map("a"->1, "b"->2, "c"->3) match {
case Map(a,b,_*) => a
}
which errs with
value Map is not a case class, nor does it have an unapply/unapplySeq member
…
elm
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- 113