Extract list from a list in Kotlin based on object

I am trying to parse a list of data object define by:

data class BookInfo(
        val id: Int?,
        val description: String)

I receive a list of this BookInfo which could be up to 50.

I am trying to regroup the list of book by id and then run a forEach on each list of regrouped BookInfo by id

Something like:

val tmpList: List = [BookInfo(1, "test"), BookInfo(2, "tit"),BookInfo(1, "tkllt"),BookInfo(3, "test"),BookInfo(1, "test"),BookInfo(2, "test"),BookInfo(3, "test"),BookInfo(2, "test")]

then using a tmpList.<function_in_kotlin_condition_on_id>.forEach() will run a forEach and the result is a subList of BookInfo with the same id.

It's like extracting a list using .filter to get a sublist of all BookInfo having the same id and then runing a forEach on all various id

tmpList.<extract_by_id>.forEach { sublist .... and the sublist will be

BookInfo(1, "test"), BookInfo(1, "tkllt"), BookInfo(1, "test"), next loop on the forEach will be BookInfo(2, "tit"),BookInfo(2, "test"),BookInfo(2, "test") and finally BookInfo(3, "test"),BookInfo(3, "test"),

I know it's seems strangely explained

Any idea, how to do it

You can do groupBy and then take the values of the resulting map. Example:

fun main() {

    val books = arrayOf(
        BookInfo(1,"a"),
        BookInfo(2,"b"),
        BookInfo(1,"c"),
        BookInfo(2,"d"),
        BookInfo(1,"e"),
        BookInfo(3,"f")
    )

    books.groupBy { it.id }.values.forEach {
        println(it)
    }

}

data class BookInfo(
    val id: Int?,
    val description: String)

Output:

[BookInfo(id=1, description=a), BookInfo(id=1, description=c), BookInfo(id=1, description=e)]
[BookInfo(id=2, description=b), BookInfo(id=2, description=d)]
[BookInfo(id=3, description=f)]