Is there a better way to build a map from 2 lists ...

# getting-started

[Youssef Shoaib [MOD]]

Is there a better way to build a map from 2 lists such that it's based on their order in the list? Currently, I'm doing list1.zip(list2).toMap() but it turns them into pairs in the process and I don't quite like that. Is there a neater 1-liner for it? I looked at associateWtih and associateBy but I can't seem to get them to work for this use case.
Edit: Neatest one-liner I've found: (0..list1.lastIndex).associateBy(list1::get, list2::get)

[Chris Lee]

A map is a collection of Pairs to hold the key, value mappings - no way around that. Some map types have no defined order (HashMap), others are insertion-order based (LinkedHashMap), others are Comparable/Comparator based (TreeMap).

[Youssef Shoaib [MOD]]

Yes but I mean that the intermediate step where zip turns it into a List<Pair<A, B>>. I want to avoid the creation of that list. In other words, I just want the map to be directly created out of my initial 2 lists. I can do this imperatively like so:

buildMap {
    for(i in 0..list1.lastIndex){
        put(list1[i], list2[i])
    }
}

But I would like something neater

[Chris Lee]

perhaps (list1 + list2).toMap (or associateBy etc)

[Youssef Shoaib [MOD]]

Got it! (0..list1.lastIndex).associateBy(list1::get, list2::get)

[ephemient]

list1.asSequence().zip(list2.asSequence()).toMap()

[Youssef Shoaib [MOD]]

Wouldn't that still create intermediate `Pair`s though? That's mainly what I'm trying to avoid

[ephemient]

depends on what you are after. it avoids the List but has other costs, including Pair.

[Youssef Shoaib [MOD]]

The intermediate list is fine enough for me, the Pairs feel inelegant though. Is the `(0..list1.lastIndex)`clear enough, in your opinion? Or is it hard to understand at first glance?

[ephemient]

list1.indices is undoubtedly clearer, but I'd use 0 until minOf(list1.size, list2.size) for consistency with zip

[ephemient]

I'd be more concerned about the list (which is built with arrays and mutation internally) than the pairs (short-lived immutable objects, which the GC can easily reclaim while in the nursery) but what matters more is somewhat situational

[Youssef Shoaib [MOD]]

Oh yeah right, I completely forgot about indices. Thanks for all the suggestions. Tbh currently performance isn't that big of a goal, but obviously I don't want to write inefficient code that could very easily be optimised. I think going with indices will potentially be the most efficient (because an IntRange seems quite cheap) and I think I might just define a fun maxOf(IntRange, IntRange) To top it all off!

[ephemient]

for the record, Kotlin will optimize away the IntRange in for (i in list1.indices) or for (i in 0 until maxOf(list1.size, list2.size)), but it won't (ending up going through the Iteratable and Iterator interfaces) in list1.indices.associateBy or a custom maxOf(IntRange, IntRange)