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AssociateBy - List to Map in Kotlin

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AssociateBy - List to Map in Kotlin

associateBy is a Kotlin collection function that converts a list into a map, where we decide which property of each item becomes the key.

In this blog, we will learn about the Kotlin Collection Functions - associateBy that converts a list into a map.

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There are many useful collection functions in Kotlin. It is good to know about those and use those based on the requirement. One of those collection functions is associateBy.

associateBy let us convert a list into a map.

Let's learn by example.

Consider a data class Contact like below:

data class Contact(val name: String, val phoneNumber: String)

And, a list of Contact:

val contacts = listOf(
    Contact("Amit", "+9199XXX11111"),
    Contact("Messi", "+9199XXX22222"),
    Contact("Ronaldo", "+9199XXX33333"))

Now, let's use the associateBy function on this list of Contact to get a Map with the

  • key as name
  • value as phoneNumber
val nameToNumberMap = contacts.associateBy( {it.name}, {it.phoneNumber})
println(nameToNumberMap)

This will print the following:

{Amit=+9199XXX11111,
Messi=+9199XXX22222,
Ronaldo=+9199XXX33333}

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Now, let's get back to the topic.

If we go through the source code, we will find the following definition:

inline fun <T, K, V> Iterable<T>.associateBy(keySelector: (T) -> K, valueTransform: (T) -> V): Map<K, V>

Note:

  • Returns a Map containing the values provided by valueTransform and indexed by keySelector functions applied to elements of the given collection.
  • If any two elements is having the same key returned by keySelector then, the last one will get added to the map.
  • Maintain the original order of items.

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Frequently Asked Questions

What happens if two elements have the same key in associateBy?

The last one gets added to the map. If any two elements return the same key from the keySelector, the later element overwrites the earlier one. So, the map keeps only one value for each key.

Does associateBy keep the original order of the list?

Yes. The associateBy function maintains the original order of the items. The resulting map has its entries in the same order as the elements of the list.

What do the two lambdas in associateBy do?

The first lambda is the keySelector, which picks the key for each element. The second lambda is the valueTransform, which picks the value. For a list of contacts, we can use the name as the key and the phone number as the value.

That's it for now.

Thanks

Amit Shekhar
Founder @ Outcome School

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