I have a list of people and each person has a TYPE Either DI kotlinlang #getting-started

I have a list of people and each person has a TYPE. Either DIRECTOR, PROFESSOR or STUDENT. How do I sort the list so that all students are listed after the DIRECTORS and PROFESSORS? As a first approach to this, I think it would just be easier to just have two identical lists. filter out the students from list 1. and then filter out prof and directors from list2 and add them together.

ephemient

11/30/2021, 3:57 AM

list.sortedBy { it.type == STUDENT }

since

false < true

although that's O(n log n) and is probably not the right solution unless you're also performing other sorting anyway

if this is the only reorder that you want to do, then

val (students, others) = list.partition { it.type == STUDENT }
others + students

is more or less your approach but built into stdlib

Colton Idle

11/30/2021, 4:03 AM

partition hm. Haven't heard of that.

is partition faster than n log n

ephemient

11/30/2021, 4:08 AM

O(n)

in theory, if you didn't need to preserve order, you could simply swap elements in place - that's what C++'s partition function does - but Kotlin's does the obvious thing of building two new lists and filling them in order

Colton Idle

11/30/2021, 4:12 AM

Interesting. Do the docs have these O(n) runtimes called out or is this just something you knonw?

ephemient

11/30/2021, 4:12 AM

unlike C++, the docs to not commit to any particular complexity, no

Colton Idle

11/30/2021, 4:13 AM

Cool. I'll take your word for it then. 😄

ephemient

11/30/2021, 4:14 AM

or just look at the implementation, https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/common/src/generated/_Collections.kt#L3046-L3057

ursus

11/30/2021, 8:40 PM

implement a normal comparator

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