CODEWITHSUNDEEP
scalalist
user11039395
user11039395

Reputation:

How would you pop an element from the end of a list in scala, and add it to the end of another list?

I am currently working with two lists, and I wanted to know what was the best way to pop the last element from one list and add it to the end of another list in Scala? I can't find the pop equivalent of Python in Scala so I'm a little confused.

For example:

list1: List1[Int] = List(4,5,6)

list2: List2[Int] = List(5,7,8)

From there I want to add the last element of List 2 to List1, and return 

list3: List1[Int] = List(4,5,6,8)

list4: List2[Int] = List(5,7)

Upvotes: 1

Views: 829

Answers (5)

Raman Mishra
Raman Mishra

Reputation: 2686

You don't need to use mutable list you can do that by using simple operations like last and drop.

val list1 = List(4,5,6)

val list2= List(5,7,8)

what you can do is:

//val lastElem = list1.last // can cause an error if list is empty

can use takeRight instead

val lastElem = list1.takeRight(1)

print(list1 :+ lastElem, list2.drop(1)) // output, List(4,5,6,8), List(5, 7)

You can also save these values into new val

val appenedList = list1:+lastElem
val popedList = list2.drop(1)

Upvotes: 1

Tim
Tim

Reputation: 27356

It is easy to compute list4 using init, but list3 needs care because list2 might be empty. This is a safe way to do it:

list3 = list1 ++ list2.takeRight(1)
list4 = list2.init

Upvotes: 2

Manoj Sehrawat
Manoj Sehrawat

Reputation: 1303

Since you want to manipulate the list, you can use mutable collections of Scala, which is highly not recommended. But just to give a flavour of it, this how you can do it: 

  val list1 = mutable.ListBuffer(4,5,6)
  val list2 = mutable.ListBuffer(5,7,8)

  val dropped = list2.remove(list2.size - 1)

  println(list2)
  list1.append(dropped) // OR you can use => 
  // list1 += dropped
  println(list1)

Update: Here are few options for without making List mutable. Now you can either make new lists out of it like newList1 and newList2 or make them variable var .

Disclaimer: having vars in your Scala code is a big NO, unless you are really sure about why you are doing it.

Here is the sample (similar to the above) code for both approaches:

  1. With newList1 and newList2
 val list1 = List(4,5,6)
 val list2 = List(5,7,8)

 val (newList2, dropped) = list2.splitAt(list2.size - 1)

 val newList1 = list1 ++ dropped

 println(newList2)
 println(newList1)
  1. With vars 
var list1 = List(4,5,6)
var list2 = List(5,7,8)

val result = list2.splitAt(list2.size - 1)
val dropped = result._2
list2 = result._1
list1 = list1 ++ dropped

println(list2)
println(list1)

Upvotes: 1

Ra Ka
Ra Ka

Reputation: 3055

You can simply use init(which is opposite of tail) and last (which is opposite of head) method of scala list.

val list1 = List(1,2,3)
val list2 = List(4,5,6)

val newList1 = list1 ++ List(list2.last)   // return 1,2,3,6
val newList2 = list2.init   // return 4,5

Upvotes: 0

Shanti Swarup Tunga
Shanti Swarup Tunga

Reputation: 641

If you can change you approach to use immutable list, you can achieve it by

val list1 = List(4,5,6)
val list2 = List(5,7,8)
val list3 = list1 :+ list2.last
val list4 = list2.dropRight(1)
println(list3, list4)

output

(List(4, 5, 6, 8),List(5, 7))

Upvotes: 1

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