How to remove an item from a list in Scala having only its index?

Question

I have a list as follows:

val internalIdList: List[Int] = List()

internalIdList = List(11, 12, 13, 14, 15)

From this list would remove the third element in order to obtain:

internalIdList = List(11, 12, 14, 15)

I can not use a ListBuffer, are obliged to maintain the existing structure. How can I do?

Thanks to all

Solution 1

Simply use

val trunced = internalIdList.take(index) ++ internalIdList.drop(index + 1)

This will also work if index is larger than the size of the list (It will return the same list).

Solution 2

There is a .patch method on Seq, so in order to remove the third element you could simply do this:

List(11, 12, 13, 14, 15).patch(2, Nil, 1)

Which says: Starting at index 2, please remove 1 element, and replace it with Nil.

Knowing this method in depth enables you to do so much more than that. You can swap out any sublist of a list with arbitrary other.

Solution 3

An idiomatic way to do it is to zip the value with their index, filter, and then project the value again:

scala> List(11,12,13,14,15).zipWithIndex.filter(_._2 != 2).map(_._1)
res0: List[Int] = List(11, 12, 14, 15)

But you can also use splitAt:

scala> val (x,y) = List(11,12,13,14,15).splitAt(2)
x: List[Int] = List(11, 12)
y: List[Int] = List(13, 14, 15)

scala> x ++ y.tail
res5: List[Int] = List(11, 12, 14, 15)

Solution 4

If you insist on using the oldschool method, use collect:

List(1,2,3,4).zipWithIndex.collect { case (a, i) if i != 2 => a }

However, I still prefer the method in my other answer.

Solution 5

A generic function that implements Nicolas' first solution:

def dropIndex[T](list: List[T], idx: Int): List[T] =
  list.zipWithIndex.filter(_._2 != idx).map(_._1)

Usage:

scala> val letters = List('a', 'b', 'c')
scala> for (i <- 0 until letters.length) println(dropIndex(letters, i))
List(b, c)
List(a, c)
List(a, b)

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Comments

[Nicolas]

The if/else is useless: take and drop will behave as expected if the given index is greater than the size.

[Kevin eyeson]

This is amazing!

[Rok Kralj]

Glad you like it, still, I suggest my .patch() answer. ;)