Hello, World! What would be the easiest way to ge...

# stdlib

[bod]

Hello, World! What would be the easiest way to get an "inverted" version of a Map (by that I mean the values become the keys and vice versa)?

[tddmonkey]

What would you do with duplicate values? Would a Map<K, V> -> Map<V, List<K>>?

[bod]

very good point. Didn't think of it because in my case it's a 1-1 mapping.

[tddmonkey]

tbh I thought there was a built in operator, but this will do the trick, albeit a bit untidily:

map.entries
        .map { it.value to it.key }
        .groupBy { it.first }
        .mapValues { it.value.map { it.second } }

[bod]

thanks! Isn't there a specific .map for Maps ? Something that takes a (k: K1, v:V1) -> Pair<K2, V2> and returns a Map<K2, V2> ?

[bod]

I recently tried to transform a java.util.Properties to a Map<String, String> and somehow this wasn't as trivial as I wanted, and I missed something like this

[bod]

I mean it must be frequent to need to transform a map of something something to a map of something else something else 😛

[nkiesel]

mapOf("A" to 1, "B" to 2, "C" to 3).map { it.value to it.key }.toMap() should work for 1:1 maps

[nils]

mapOf(...).entries.associateBy({ it.value }) { it.key } should also work for 1:1 maps

[bod]

Thanks a lot! Very nice

[Bornyls Deen]

You could add a little sugar to it by creating an extension function on the Map interface if this is functionality you find yourself requiring often 🙂

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