Coming from C++, Kotlin's withDefault feature doesn't quite work as expected:
val m = mutableMapOf<Int, MutableList<Int>>().withDefault { mutableListOf() }
...
m.getValue(1).add(2) // does not work if m[1] == null!
Now if for performance reasons I want to do only a single lookup, how can I do that?
A way with two lookups would be
if (m[1] == null) m[1] = mutableListOf(2)
else m[1].add(2)
Is there any way with a single lookup?
// Create the map with some example entries
val m = mutableMapOf(
1 to mutableListOf(),
2 to mutableListOf(1, 2, 3)
)
m.putIfAbsent(2, mutableListOf(789))?.add(5)
// Will append 5 to [1, 2, 3] as the key 2 exists
m.putIfAbsent(3, mutableListOf(789))?.add(5)
// Will create a new map entry 3 to [789] as the key 3 does not exist
m.toList().forEach(::println)
Output:
(1, [])
(2, [1, 2, 3, 5])
(3, [789])