How to get the current index in for each Kotlin

Question

How to get the index in a for each loop? I want to print numbers for every second iteration

For example

for (value in collection) {
    if (iteration_no % 2) {
        //do something
    }
}

In java, we have the traditional for loop

for (int i = 0; i < collection.length; i++)

How to get the i?

also see https://stackoverflow.com/questions/46825787/does-kotlin-have-an-enumerate-function-like-python/46826172#46826172 — s1m0nw1, Feb 21 '18 at 08:04

score 659 · Accepted Answer · answered Feb 21 '18 at 05:06

659

In addition to the solutions provided by @Audi, there's also forEachIndexed:

collection.forEachIndexed { index, element ->
    // ...
}

answered Feb 21 '18 at 05:06

zsmb13

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It works on Arrays and Iterables, what else do you need it to work on? – zsmb13 Feb 21 '18 at 09:40
1

Sorry confused with primitive java arrays. – Adolf Dsilva Feb 21 '18 at 09:59
1

Any way to use `break` inside ? – Levon Petrosyan Feb 28 '19 at 00:15
1

You can't break from the entire loop, the only similar thing you can do is `return@forEachIndexed` which will essentially serve as a `continue` to skip to the next element. If you need to break, you'd have to wrap it in a function, and use `return` in the loop to return from that enclosing function. – zsmb13 Feb 28 '19 at 06:05
Using this will prevent you to work with `continue` use @Audi answer if you need such function – Mihae Kheel Apr 11 '21 at 15:45
any performance difference for each vs for loop? – Bolt UIX May 03 '21 at 15:07

score 228 · Answer 2 · answered Feb 21 '18 at 04:21

228

Use indices

for (i in array.indices) {
    print(array[i])
}

If you want value as well as index Use withIndex()

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

Reference: Control-flow in kotlin

answered Feb 21 '18 at 04:21

Adolf Dsilva

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I think this answers is better because don't needs to learn something else, just the simple for loop +1 – underfilho Nov 15 '18 at 19:19

Anoop M Maddasseri · Answer 3 · 2020-07-15T04:45:24.550

69

Alternatively, you can use the withIndex library function:

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

Control Flow: if, when, for, while: https://kotlinlang.org/docs/reference/control-flow.html

edited Jul 15 '20 at 04:45

answered Mar 22 '19 at 10:12

Anoop M Maddasseri

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score 41 · Answer 4 · answered Oct 15 '18 at 16:08

41

try this; for loop

for ((i, item) in arrayList.withIndex()) { }

answered Oct 15 '18 at 16:08

ali ozkara

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While this code may answer the question, providing additional context regarding how and/or why it solves the problem would improve the answer's long-term value. – Reinstate Monica Oct 15 '18 at 18:04
How can i put a limit for this loop? Like it goes until half or some numbers before end – E.Akio Jul 17 '20 at 14:59
@E.Akio One option would be to work with a sub list: `arrayList.subList(0, arrayList.size/2)`. – Dario Seidl Nov 18 '21 at 14:48

score 37 · Answer 5 · answered Aug 06 '20 at 13:08

37

Working Example of `forEachIndexed` in Android

Iterate with Index

itemList.forEachIndexed{index, item -> 
println("index = $index, item = $item ")
}

Update List using Index

itemList.forEachIndexed{ index, item -> item.isSelected= position==index}

answered Aug 06 '20 at 13:08

Hitesh Sahu

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score 13 · Answer 6 · edited Feb 26 '18 at 09:00

13

It seems that what you are really looking for is filterIndexed

For example:

listOf("a", "b", "c", "d")
    .filterIndexed { index, _ ->  index % 2 != 0 }
    .forEach { println(it) }

Result:

b
d

edited Feb 26 '18 at 09:00

Kirill Rakhman

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answered Feb 21 '18 at 07:02

Akavall

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also consider using a function reference `.forEach(::println)` – Kirill Rakhman Feb 26 '18 at 09:00
@KirillRakhman, is using function references preferred style in situations like that? I am new to Kotlin, so I am still figuring this stuff out. – Akavall Feb 26 '18 at 17:53
I tend to use function references whenever possible. When you have more than one parameter, you save a bunch of characters compared to using a lambda. But it's a matter of taste for sure. – Kirill Rakhman Feb 26 '18 at 21:13

score 10 · Answer 7 · answered May 03 '22 at 10:17

10

Please try this once.

yourList?.forEachIndexed { index, data ->
     Log.d("TAG", "getIndex = " + index + "    " + data);
 }

answered May 03 '22 at 10:17

Surendar D

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score 7 · Answer 8 · answered Feb 21 '18 at 08:21

7

Ranges also lead to readable code in such situations:

(0 until collection.size step 2)
    .map(collection::get)
    .forEach(::println)

answered Feb 21 '18 at 08:21

s1m0nw1

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alternatively `(0..collection.lastIndex step 2)` – Kirill Rakhman Feb 26 '18 at 21:14

score 0 · Answer 9 · edited Jul 01 '22 at 14:27

0

You can initialize a variable counter=0 and increment inside the loop:

for (value in collection){//do something then count++ }`

edited Jul 01 '22 at 14:27

Bill Tür stands with Ukraine

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answered Jul 01 '22 at 11:13

user19459910

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How to get the current index in for each Kotlin

9 Answers9

Working Example of `forEachIndexed` in Android

Linked

How to get the current index in for each Kotlin

9 Answers9

Working Example of forEachIndexed in Android

Linked

Working Example of `forEachIndexed` in Android