`ListAppend` formula for Nested Lists

Suggestion Box
1

Not possible to create nested lists with ListCombine because it removes all nested lists.

I would like to be able to take an existing nested list, e.g.

List(
  List("A", 1), 
  List("B", 2)
)

And add

List("C", 3)

To the end to end up with:
image

Not currently possible with ListCombine as

List(
  List("A", 1), 
  List("B", 2)
).ListCombine(List("C", 3))

Returns:
image

ListCombine might better be called ListMerge or ListFlatten due to this behavior.

2

Ok, there is a way to do this. It’s super hacky but I guess the community could benefit from it since I didn’t see any posts on how to make nested lists.

Let’s say you have an existing formula called MyList that looks like this:

image
image1110×380 14.3 KB

You can append List("C", 3) to that list with the formula:

Sequence(1, MyList.Count() + 1).FormulaMap(
  If(CurrentValue <= MyList.Count(),
    List(
      MyList.Nth(CurrentValue).Nth(1),
      MyList.Nth(CurrentValue).Nth(2)
    ), 
    List("C", 3)
  )
)

Just to show it works:
Shared with CloudApp

Of course all of this could be solved much more elegantly with syntax like:

MyList.ListAppend(List("C", 3))
1 Like
3

@cnr there’s a Splice() formula specifically for list modification, i.e. inserting or deleting items:

List(
  List("A", "B"),
  List("C", "D")
).Splice(3, 0, List(List("E", "F")))

means at position 3 delete zero items and insert a List("E", "F") and return [A, B], [C, D], [E, F]:

image

Please note the List(List(...)) — if you just submitted a non-nested list to the Splice formula it would insert list items one by one and would get [A, B], [C, D], E, F. The Splice function unwraps one list but doesn’t unwrap the second.

5 Likes
5

I didn’t know this worked! Not exactly intuitive, but useful to know. Thank you.

I suppose the most general way to implement Append using Splice would be:

WithName(
  List(
    List("A", "B"),
    List("C", "D")
  ), TargetList,
  TargetList.Splice(TargetList.Count(), 0, List(List("E", "F"))
)

So that the TargetList can be of any length.

1 Like