```mylist sortedBy it quantity map NewElement position = how kotlinlang #getting-started

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

08/08/2020, 9:57 PM

.mapIndexed

also I'd lead with

.asSequence()

and end with

.toList()

to avoid allocating more lists than you need.

👍 1

Daniele B

08/08/2020, 10:01 PM

great! thanks

@nanodeath do you mean like this?

Copy code

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

nanodeath

08/08/2020, 10:08 PM

toList()

should usually be at the end, otherwise you got it

Daniele B

08/08/2020, 10:08 PM

can you explain better what are the advantages of using

asSequence()

and then

toList()

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

nanodeath

08/08/2020, 10:09 PM

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

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

(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)

👍 1

Daniele B

08/08/2020, 10:11 PM

Ok, I see, thanks for the explanation

nanodeath

08/08/2020, 10:11 PM

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

👍

2 Views