Is there a function to insert an element at i-th position in a list?

[AvailableUsername]

Hello,
I’ve been looking at
https://package.elm-lang.org/packages/elm/core/latest/List
and
https://package.elm-lang.org/packages/elm-community/list-extra/latest/List.Extra
and I couldn’t find a function to insert an element in a list at position i.
I’m a bit surprised because there seem to be a lot of functions in these two modules, that I guess would be used less frequently than inserting an element at the i-th position.
I could write this function myself, but I was thinking maybe it’s already in there and I didn’t see it.
Thanks everyone.

[MartinS]

That surprises me as well that there isn’t a List.Extra.insert. That said, it’s not too hard to implement

insert i value list = List.take i list ++ [ value ] ++ List.drop i list

[dta]

It’s probably not available because it looks efficient but actually isn’t. You would have the same runtime bound by converting to an array, doing the insert, and then converting back to a list. If you find you need to insert at arbitrary positions, you should consider using an array instead of a list the whole time.

[AvailableUsername]

Using an array is not practical at all, and this isn’t more efficient.
Suppose I have an array that looks like this

“a”, “b”, “c”, “d”, “e”

Suppose I want to insert “X” at the position 2 in the array to obtain

“a”, “b”, “X”, “c”, “d”, “e”

You can’t do this with better than linear time with an array. Not even considering the fact that you have to grow the array, these “c”, “d”, “e” values have to be copied to their new positions.

[rupert]

@AvailableUsername

You can’t do this with better than linear time with an array.

You can, because Array.slice and Array.append are log time not linear. Of course, you will only see a difference for large enough arrays.

Check out this example of a gap buffer capable of working smoothly with 1M lines of text, implemented using arrays and making use of Array.slice and Array.append to quickly insert or refocus the ‘gap’:

https://kind-colden-ebafc2.netlify.app/

Efficient Text Editing Buffer

If a simple Array is used to hold lines of text, whenever a line is edited, the array will need to be sliced, lines modified, then put back together again. It works fine up to a limit at which point the editor becomes noticeably unresponsive. I scratched my head over this one for a while, thinking about various tree structures, or skip lists, and so on. Then the answer came to me - make a data structure like a zip list, but use arrays instead of lists. There are lots of packages implementing zi…

[AvailableUsername]

Ok, I thought Array was a simple array, but apparently it’s more high tech than that. I just read about RRB trees.
Then there should be an insertAt function in the Array type!
Thanks everybody for your replies.

[dta]

There is! It’s in the elm-community/array-extra package.

[AvailableUsername]

Amazing. Thanks dta.