How to iterate on a list of object but in specific field Kotlin

Question

Lets say that i have a list of AccountDto (getContats() -> List)that each has a column accountId

List<Int> list = accountsDao.getContactIds()

and i have a list of MessageDto, where messageDto has a field 'fromAccountId'. I want to loop through the messageList and find what new fromAccountId i have that dont exist in my DB.

getAccounts().value?.let {
            for ((every accountId from it.accountDto) in --(every fromAccountId in newMessages.list)--) {
                if (it.contains(newFromAccountId))
                    println("fount $newFromAccountId")
            }
        }

Is there an elegant way to do that in Kotlin...?

Answer 1

You are using nested loops which is not a very good idea, since time complexity of this operation is O(n^2), performace will degrade very fast as the size of your lists increase.

Better approach on the expense of some extra memory would be to first create a set of all the accountId's from your Database, and then iterate over the messageList and for every accoutId check if the set contains this accountId.

// Store all the accountId's in a Set so that lookup is fast
var accountIds = getAccounts().map{it.accountId}.toSet()
// Iterate over the messageList and find the Id's that are not in Set
messageList.asSequence()
           .filter { !accountIds.contains(it.fromAccountId) }
           .forEach { println("fount $it") }