associateBy

The associateBy function builds a Map from a DataFrame by selecting a key for each row using a row expression.
The rows themselves (or values derived from them) become the map values.

If multiple rows produce the same key, only the last row (or value) for that key is kept. This matches the behavior of Kotlin’s standard kotlin.collections.associateBy function.

df.associateBy { keySelector }

keySelector: (DataRow) -> Key

toMap — converts a DataFrame into a Map by using column names as keys and their values as map values.
associate — builds a map from key–value pairs produced by transforming each row.

Example

Create a map with names as keys:

df.associateBy { "${name.firstName} ${name.lastName}" }

Output:

{
  Alice Cooper: { name:{ firstName:Alice, lastName:Cooper }, age:15, city:London, weight:54, isHappy:true },
  Bob Dylan: { name:{ firstName:Bob, lastName:Dylan }, age:45, city:Dubai, weight:87, isHappy:true },
  Charlie Daniels: { name:{ firstName:Charlie, lastName:Daniels }, age:20, city:Moscow, isHappy:false },
  Charlie Chaplin: { name:{ firstName:Charlie, lastName:Chaplin }, age:40, city:Milan, isHappy:true },
  Bob Marley: { name:{ firstName:Bob, lastName:Marley }, age:30, city:Tokyo, weight:68, isHappy:true },
  Alice Wolf: { name:{ firstName:Alice, lastName:Wolf }, age:20, weight:55, isHappy:false },
  Charlie Byrd: { name:{ firstName:Charlie, lastName:Byrd }, age:30, city:Moscow, weight:90, isHappy:true }
}

18 September 2025

associateBy

Related functions

Example