```mylist.sortedBy{it.quantity}.map { NewElement(...

# getting-started

[Daniele B]

mylist.sortedBy{it.quantity}.map {
	NewElement(
		position = (how to retrieve the index of the element in the sorted list?)
		name = it.name,
		quantity = it.quantity
	)
}

is there any way to retrieve the index of the element?

[nanodeath]

.mapIndexed

[nanodeath]

also I'd lead with .asSequence() and end with .toList() to avoid allocating more lists than you need.

[Daniele B]

great! thanks

[Daniele B]

@nanodeath do you mean like this?

[Daniele B]

list.asSequence().sortedBy{it.quantity}.toList().mapIndexed { index, elem ->
	NewElement(
		position = index
		name = elem.name,
		quantity = elem.quantity
	)
}

[nanodeath]

toList() should usually be at the end, otherwise you got it

[Daniele B]

can you explain better what are the advantages of using asSequence() and then toList() ?

[Daniele B]

because actually what I am doing here, it is to create a new list of a NewElement type

[nanodeath]

so if you say foo.map {}.map {}.reduce{} or whatever, every step generates a new list. if you say asSequence().map {}.map{}.map{}.reduce{}.toList() (or toSet()), it minimizes the use of intermediate collections by doing fancy lazy iterator stuff

[nanodeath]

the general rule I follow is if you're doing more than one operation (e.g. map or reduce) in a chain, use a sequence

[nanodeath]

(granted it's a bit of a moot point in this particular case because sortedBy almost certainly creates a list internally, but...things like map don't)

[Daniele B]

Ok, I see, thanks for the explanation

[nanodeath]

Sequences are lazy the same way Streams are -- they don't "do" anything unless there's a terminal node (like toList or toSet) to pull the values through

[nanodeath]

👍