How to combine two lists using scala?

Question

I have following lists -

val A = List(("A","B","C","D"),("A1","B1","C1","D1"),("A2","B2","C2","D2"))

and

val B = List(("P","Q","R","S","T"),("P1","Q1","R1","S1","T1"),("P2","Q2","R2","S2","T2"),("P3","Q3","R3","S3","T3"))

I want to merge 1st element of list A with first element of list B and so on. here list A have 3 elements and B have 4. I want to consider number of elements in list A while merging.

output as below

val combineList = List(("A","B","C","D","P","Q","R","S","T"),("A1","B1","C1","D1","P1","Q1","R1","S1","T1"),
        ("A2","B2","C2","D2","P2","Q2","R2","S2","T2"))

possible duplicate of stackoverflow.com/questions/7621705/… — Rickard B.
– Rickard B., Commented Jan 9, 2015 at 11:17

kiritsuku · Accepted Answer · 2015-01-09 19:33:42Z

7

If you can use shapeless, then you can simply do

scala> import shapeless.syntax.std.tuple._
scala> A.zip(B).map{case(a,b) => a ++ b}
res1: List[(String, String, String, String, String, String, String, String, String)] = List((A,B,C,D,P,Q,R,S,T), (A1,B1,C1,D1,P1,Q1,R1,S1,T1), (A2,B2,C2,D2,P2,Q2,R2,S2,T2))

It will work on arbitrary size of tuples.

edited Jan 9, 2015 at 19:33

kiritsuku

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answered Jan 9, 2015 at 12:16

mohit

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3 Comments

user5102379 Over a year ago

Sorry. As I understand you mean shapeless library (github.com/milessabin/shapeless/wiki/…).

Kevin Meredith Over a year ago

is there a non-shapeless way to answer the OP's question for arbritrary tuples?

mohit Over a year ago

@KevinMeredith - There is not an easy way. Scala does not provide a built in method to add or remove an element from a Tuple(A list is a more appropriate data structure for that while maintaining type safety). A non shapeless way will include first calculating size of the two tuples to determine what will be the -arity of the Tuple(the easiest part) After that create a tuple by calling the appropriate Tuple class(Tuple2, Tuple6 etc) and feed the values programatically.

Dima · Accepted Answer · 2015-01-09 11:24:03Z

1

A.zip(B).map { case ((a,b,c,d), (p,q,r,s,t)) => (a,b,c,d,p,q,r,s,t) }

answered Jan 9, 2015 at 11:24

Dima

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2 Comments

Soumya Simanta Over a year ago

Will this work for tuples with different sizes i.e, if tuples are of size 5 and 6 or 2 and 4 ?

Dima Over a year ago

Different tuples are different types, so, no. You'd have to write a different function for that ... or use shapeless, like @mohit suggested in his answer, although, that would not be my advice.

Sergii Lagutin · Accepted Answer · 2015-01-09 11:26:00Z

-1

First zip tuples in sequnces:

val lists = for {i <- 0 until A.length
     a = A(i)
     b = B(i)
} yield (a.productIterator ++ b.productIterator).toList

Next define converter from Seq back to Product:

def toTuple(seq: Seq[_]): Product = {
  val clz = Class.forName("scala.Tuple" + seq.size)
  clz.getConstructors()(0).newInstance(seq.map(_.asInstanceOf[AnyRef]): _*).asInstanceOf[Product]
}

And finally do map:

lists.map(toTuple)

This code does not consider different arity of input lists and tuple size's limitation.

answered Jan 9, 2015 at 11:26

Sergii Lagutin

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3 Comments

Dima Over a year ago

This results in List[Product], not List[(String,String,String,String,String,String,String,String,String)]

Sergii Lagutin Over a year ago

@Dima This code is about tuples of variadic length. And your comment is about tuples of fixed arity.

Dima Over a year ago

Huh? Where do you see anything about "variadic length" in the question? In general tuple means fixed arity. There is a different name for "tuples of variafic length": they are called lists :)

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