hi all, I've the following nested maps: `Map<St...

# getting-started

[Alexandru Gheorghe]

hi all, I've the following nested maps: Map<String, Map<String, Map<String, Int>>> and I'm trying to access it by nesting if statements. I am trying to increment the value (Int) to one provided as argument to a function.

In doing so, I'm encountering an issue: No set method providing array access when trying to do something like:

val keyMap = cls.parameter!!["key"] as Map<*, *>
// logic here to increment the value of the last key in the nested map
keyMap[nestedKey1][key] = new_value

the last statement is the one failing with the error shown above

my questions are:
1. how can I access nested maps in a safe manner and when I detect a key not existing, create it and nest in that if statement creating the rest of the map?
2. how can I set the map on an object (cls.parameter) such that I preserve other existing mappings and do not override with the new (and what would be the only if written) mapping?

[Ruckus]

The innermost map needs to be mutable in order to set the value. That is, you need Map<String, Map<String, MutableMap<String, Int>>>

[Alexandru Gheorghe]

Thanks. I've modified all Map< to `MutableMap<`and now I'm encountering the issue where I'm trying to access the key in order to set its value. E.g. map[key1][key2][key3] = value. In fact what I'm trying to do is Python equivalent of map[key1][key2][key3] += value. How can I achieve this value update for the innermost map and set it on a object while preserving the rest of the keys in the nested map?

[Ruckus]

In Python, map[key] will throw if the key isn't in the map, whereas in Kotlin it will just return null. You can work around this using other functions, but it won't be as pretty. Here's one example (not sure if there's a cleaner one):

map.getValue(key1).getValue(key2).let { it[key] = it.getOrDefault(key3, 0) + 1 }

[Alexandru Gheorghe]

Right! What I'm trying is map!![key1]?.get(key2) and I get an error on key2. Even though it's defined as a nested MutableMap.

[Ruckus]

What kind of error are you getting? It works for me: https://pl.kotl.in/EqlOoXvHx

[Alexandru Gheorghe]

Nevermind. Just implemented the way you coded it and it works. I have a question: what if getValue(key2) returns null? will the lambda create it? or do I have to deal with this case before calling the setting of the value?

[Ruckus]

No, it will throw like Python

[Alexandru Gheorghe]

I see. How to create non-existing keys in the nested mapping for each level?

[Ruckus]

You'll want to look into `getOrPut`: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/get-or-put.html

[Ruckus]

Something like

map.getOrPut(key1) { mutableMapOf() }
    .getOrPut(key2) { mutableMapOf() }
    .let { it[key3] = it.getOrDefault(key3, 0) + 1 }

[Ruckus]

You can also look into the withDefault function, but that doesn't actually clean up the API at all, just eliminates the chance of throwing.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/with-default.html

[Alexandru Gheorghe]

Thanks! Will report back should I encounter any issues.

[Konstantin]

Looks like you are fighting against type system. What domain are you trying to model?
Is it some config in json/xml?
You might want to create your generic class which can hold values of different types, provides type safe access/creation/defaults and supports nesting