How to merge lists of objects, based on time key

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Say I have multiple (MANY) Lists, each containing multiple Record objects that contain one long constructor parameter "time" (and a string parameter for debugging purposes).

Here is an example:

val list1 = listOf(Record(0, "A1"), Record(1, "A2"), Record(2, "A3"))
val list2 = listOf(Record(0, "B1"), Record(2, "B2"))
val list3 = listOf(Record(1, "C1"), Record(2, "C2"))

I want to combine the lists into one list, so that at any given time present in the original lists, the latest value from each list is present in the list.

Here is an example:

val output = listOf(Record(0, "A1+B1"), Record(1, "A2+B1+C1"), Record(2, "A3+B2+C2"))

Assume that Records can be added to create new Records containing the data of both.

(Technically, Records are a typealias for a key-value map containing data, but I thought that was out of scope for this question.)

kotlin
2 Answers

you can use 'associateByTo' inline method. Assuming that：

data class Record(val time: Int, var text: String)

then：

    val list1 = listOf(Record(0, "A1"), Record(1, "A2"), Record(2, "A3"))
    val list2 = listOf(Record(0, "B1"), Record(2, "B2"))
    val list3 = listOf(Record(1, "C1"), Record(2, "C2"))

    val map = hashMapOf<Int, Record>()
    val output = listOf(list1, list2, list3).flatten().associateByTo(map,
        { it.time },
        {
            //if the record id exist in map, append time
            map[it.time]?.apply { text += "+${it.text}" } ?: it
        })

    println(output.values)

output.values are what you want.

user_46890240

Assuming that

data class Record(val time:Long , val param2:String)

Then you can do like this

val list1 = listOf(Record(0, "A1"), Record(1, "A2"), Record(2, "A3"))
val list2 = listOf(Record(0, "B1"), Record(2, "B2"))
val list3 = listOf(Record(1, "C1"), Record(2, "C2"))

val output = listOf(list1,list2,list3)
             .flatten()
             .groupBy { it.time }
             .map { (key,value) -> 
                    // you could define your own separator if needed
                   val newParam2 = value.joinToString(separator = "+") { it.param2 }
                   Record(key,newParam2)

              }

println(output)
//[Record(0, "A1+B1"), Record(1, "A2+B1+C1"), Record(2, "A3+B2+C2")]
user_156927000
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