Idiomatic way to transform map in kotlin?

Question

In Scala, it's just the map function. For example, if hashMap is a hashMap of strings, then you can do the following:

val result : HashMap[String,String] = hashMap.map(case(k,v) => (k -> v.toUpperCase))

In Kotlin, however, map turns the map into a list. Is there an idiomatic way of doing the same thing in Kotlin?

How about `mapKeys`, `mapValues` or applying `groupBy` to the result of simple `map`? — yeputons, Feb 07 '17 at 04:14
@yeputons. I don't know. You tell me if that's idiomatic. Feel free to add an answer — U Avalos, Feb 07 '17 at 04:18

James Bassett · Answer 1 · 2021-07-02T21:58:08.413

176

I don't think one person's opinion counts as idiomatic, but I'd probably use

// transform keys only (use same values)
hashMap.mapKeys { it.key.uppercase() }

// transform values only (use same key) - what you're after!
hashMap.mapValues { it.value.uppercase() }

// transform keys + values
hashMap.entries.associate { it.key.uppercase() to it.value.uppercase() }

Note: or toUpperCase() prior to Kotlin 1.5.0

edited Jul 02 '21 at 21:58

answered Feb 07 '17 at 05:35

James Bassett

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This is what I would do too although if you are only transforming values then you can use `mapValues` instead (e.g. `hashMap.mapValues { it.value.toUpperCase() }`) but I personally don't like the signature of `mapValues` because it uses `transform: (Map.Entry) -> R` instead of `transform: (V) -> R` which seems to me to be inconsistent with `filterValues(predicate: (V) -> Boolean)` meaning you can't write `hashMap.mapValues(String::toUpperCase)` so I just use `hashMap.entries.associate { ... }`. – mfulton26 Feb 07 '17 at 14:39
Updated to include `mapValues()` as well, thanks @mfulton26 – James Bassett Feb 07 '17 at 19:37
The last line is beautiful. Thank you for this example, Kotlin is indeed awesome. – Lucas P. Dec 09 '20 at 18:49

score 31 · Answer 2 · answered Jun 03 '19 at 15:24

31

The toMap function seems to be designed for this:

hashMap.map { (key, value) ->
      key.toLowerCase() to value.toUpperCase()
    }.toMap()

It converts Iterable<Pair<K, V>> to Map<K, V>

answered Jun 03 '19 at 15:24

emu

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Here is an unnecessary transformation `toMap` For better performance and less code use methods from @James Bassett answer above – Roman Nazarevych Jul 30 '20 at 10:23
Not what I needed but led me to the solution to my problem. Thanks! – nrod Jun 11 '21 at 12:27
@RomanNazarevych sounds like premature optimization, this option is cleaner to my personal taste – Albert Bikeev Jan 30 '22 at 13:54

Ruckus T-Boom · Answer 3 · 2021-07-01T14:52:11.390

7

You could use the stdlib mapValues function that others have suggested:

hashMap.mapValues { it.value.uppercase() }

or with destructuring

hashMap.mapValues { (_, value) -> value.uppercase() }

I believe this is the most idiomatic way.

edited Jul 01 '21 at 14:52

answered Feb 07 '17 at 18:51

Ruckus T-Boom

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score -3 · Answer 4 · edited Jan 19 '18 at 15:43

-3

I found another variant. It seems to be more clear

val result = mapOf( *hashMap.map { it.key.toUpperCase() to it.value.toUpperCase() }.toTypedArray() )

It'll automatically infer the type of resulted map.

.toTypedArray() is required to use splat(*) operator.

edited Jan 19 '18 at 15:43

braX

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answered Jan 19 '18 at 15:02

Alexander Usikov

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Idiomatic way to transform map in kotlin?

4 Answers4