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Let's say that I have a List of objects of the following class.
class Contact( val name: String // ... ) I would like to retrieve a Map<String, Int> which maps a name to its number of occurences.
On a SQL-based database I would query:
SELECT name, count(*) FROM Contact GROUP BY name; What is the best way to do this in Kotlin with higher order functions?
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If you want to group elements and then apply an operation to all groups at one time, use the function groupingBy() . It returns an instance of the Grouping type. The Grouping instance lets you apply operations to all groups in a lazy manner: the groups are actually built right before the operation execution.
Inside main() , create a variable called numbers of type List<Int> because this will contain a read-only list of integers. Create a new List using the Kotlin standard library function listOf() , and pass in the elements of the list as arguments separated by commas.
If contacts is of type List<Contact> you can do the following:
val numOccurencesMap = contacts.groupingBy { it.name }.eachCount() numOccurencesMap will be of type Map<String, Int>.
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