What is the easiest way to determine if `list` contains item kotlinlang #announcements

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Title

igor.wojda

10/31/2018, 11:09 AM

What is the easiest way to determine if

list

contains items with the same value (we don’t know what value it may be). Is there any operator or combination fo operators to achieve this ? eg `

listOf("A", "A", "A")  //true
listOf("A", "A", "B")  //false
`

👍 1

edwardwongtl

10/31/2018, 11:10 AM

.all {}

➕ 2

👍 1

arekolek

10/31/2018, 11:11 AM

I understand he wants something like

.toSet().size == 1

➕ 2

👍 3

👍🏻 1

igor.wojda

10/31/2018, 11:12 AM

Exactly @arekolek. Thanks

ribesg

10/31/2018, 11:16 AM

I always have problems building entirely new collections just to check something. My algorithmics teacher would cry seeing this

arekolek

10/31/2018, 11:16 AM

If you prefer to avoid creating the intermediary set, you could do for example

.asSequence().zipWithNext().all { (a, b) -> a == b }

ribesg

10/31/2018, 11:16 AM

Still creating a lot of stuff there, aren’t you?

I would create and extension that does a simple while loop

arekolek

10/31/2018, 11:18 AM

Only a

Sequence

object that uses the iterator

ribesg

10/31/2018, 11:18 AM

It’s probably irrelevant with nowadays available power anyway, so I guess you should use the solution which is the shortest to both write and understand

@arekolek

.all

does stuff too

arekolek

10/31/2018, 11:20 AM

it iterates the

Sequence

, doesn’t create any objects

ribesg

10/31/2018, 11:20 AM

The lambda itself is one

arekolek

10/31/2018, 11:20 AM

inlined though

ribesg

10/31/2018, 11:22 AM

I meant that the lambda compiled as a whole new class, and you’d use an instance of that class there

I would create an extension function with a clear name which does a simple loop. Shorter than the sequence solution to write and understand and faster to understand than the set solution.

Egor Trutenko

10/31/2018, 11:25 AM

But why would you do it, if

all

already does all the work 🤔

arekolek

10/31/2018, 11:28 AM

I guess you could do it with just

all

like this

all { first() == it }

, is that what you mean?

👍 2

igor.wojda

10/31/2018, 11:32 AM

distinct

may be also an option

listOf(1, 1, 1).distinct().size == 1 //true

Egor Trutenko

10/31/2018, 11:32 AM

Oh, I just noticed that you don't know what value is inside the collection. I'm sorry, yes,

all

doesn't help

arekolek

10/31/2018, 11:33 AM

I mean it kind of does,

.run { all { first() == it } }

makes more sense to me than the set or sequence ones

Egor Trutenko

10/31/2018, 11:39 AM

Well, actually, you're right. For some weird reason I though that comparing each element to the first one is undeterministic

dsavvinov

10/31/2018, 12:02 PM

Just bear in mind that

all

over an empty collection is always

true

, no matter what the passed predicate is (i.e. behaviour is different from the `toSet`/`distinct` approach) See this demo: https://pl.kotl.in/r12sFGv2X

igor.wojda

10/31/2018, 1:07 PM

Great tip @dsavvinov

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