How to merge two Maps properly? - kotlin

Question

I've been trying to solve a problem, where I should merge two Maps to get the new one with no equal values for equal keys.
fun main() {
mergePhoneBooks(mapOf("Emergency" to "112"),
mapOf("Emergency" to "911", "Police" to "02"))
}
fun mergePhoneBooks(mapA: Map<String, String>, mapB: Map<String, String>): Map<String, String> {
val result = mutableMapOf<String, String>()
for ((keyA, valueA) in mapA) {
for ((keyB, valueB) in mapB) {
result[keyA] = valueA
result[keyB] = valueB
if (keyA == keyB) {
if (valueA != valueB) {
result[keyA] = "$valueA, $valueB"
}
}
}
}
println(result)
return result
}
What I need is:
{Emergency=112, 911, Police=02},
but all I get is:
{Emergency=112, Police=02}

Answer 1

fun main() {
val m1 = mapOf("Emergency" to "112")
val m2 = mapOf("Emergency" to "911", "Police" to "02")
val r = (m1.entries + m2.entries).groupBy({ it.key }, { it.value })
// or use Sequence
// val r = (m1.asSequence() + m2.asSequence()).distinct().groupBy({ it.key }, { it.value })
println(r) // {Emergency=[112, 911], Police=[02]}
println(r.mapValues { it.value.joinToString(", ") }) // {Emergency=112, 911, Police=02}
}

Answer 2

result[keyA] = valueA
result[keyB] = valueB
You've assigned the valueB to the key (if keyA was same as keyB) again so the condition if (valueA != valueB) { is never going to be true.
You should remove the above lines and do it like this:
if (keyA == keyB && valueA != valueB) {
result[keyA] = "$valueA, $valueB"
} else {
result[keyA] = valueA
}

Answer 3

A map can only have one value per key. If you need to preserve multiple values, you need to change the value type to something like a List or Pair.
fun <K, V> mergeMapsValues(mapA: Map<K, V>, mapB: Map<K, V>): Map<K, List<V>> {
val outMap = mapA.mapValues { (_, value) -> mutableListOf(value) }.toMutableMap()
mapB.forEach { (key, value) ->
outMap.getOrPut(key, ::mutableListOf).add(value)
}
return outMap
}

Answer 4

I think below code will solve your issue:
fun main() {
val first = mapOf("A" to "0", "B" to "1", "C" to "2")
val second = mapOf("A" to "4", "C" to "2")
val result = (first.asSequence() + second.asSequence()).distinct()
.groupBy({ it.key }, { it.value })
.mapValues { it.value.joinToString(",") }
print(result) // {A=0,4, B=1, C=2}
}

Answer 5

You can implement mergeWith extension function:
infix fun Map<String, String>.mergeWith(anotherMap: Map<String, String>): Map<String, String> {
return (keys + anotherMap.keys).associateWith {
setOf(this[it], anotherMap[it]).filterNotNull().joinToString()
}
}
And use it:
val mapC = mapA mergeWith mapB