Question about use of lateinit var: I’ve only seen it used in class variables, but I recently came a...

# getting-started

[Rachel Carandang]

Question about use of lateinit var: I’ve only seen it used in class variables, but I recently came across it in a PR from someone new to Kotlin that used it inside a method call. What are the patterns and advice for using lateinit var inside methods? Or should it be avoided? Is the below use of lateinit valid?

fun degreesToDirection(degrees: Float?): Direction {
            var smallestDistance: Float = Float.POSITIVE_INFINITY
            lateinit var direction: Direction

            degreesMapping.entries.forEach {
                if (abs(it.value - (degrees ?: 0f)) < smallestDistance) {
                    direction = it.key
                    smallestDistance = (degrees ?: 0f) -  it.value

                }
            }

            return direction
        }

[Joffrey]

There is nothing really wrong about this lateinit var per se. But doing this sort of loop to initialize a variable in Kotlin is usually a smell. For instance, is it expected to iterate over all the mappings here? Usually the first match should stop the loop

[Javier]

I can't think a lot about it now, but you probably can change that to reduce + first (or fold + first and so on), so you can avoid mutability.

But not sure if the alternative is so readable compared to the current one

[Vampire]

fun degreesToDirection(degrees: Float?) =
    degreesMapping.minByOrNull { (_, mappedDegrees) ->
        abs(mappedDegrees - (degrees ?: 0f))
    }!!.key

[Vampire]

For instance, is it expected to iterate over all the mappings here? Usually the first match should stop the loop

First match, when searching for a minimum? 😉

[Vampire]

As far as I understood, lateinit is practially always a smell, be it as property or not and is mainly for integrate into existing frameworks like Spring that sets the value after creating the object and similar.

[Joffrey]

Lol indeed for a minimum you have to, I actually didn't really read the code in details (I'm on mobile) but the general pattern of declaring a var and init via a loop seemed wrong

[Starr]

isn't there `minBy`/`maxBy` for this kind of stuff?

[Vampire]

Yes, I just gave the imho proper code, just see my first comment

[Joffrey]

In Kotlin 1.7 you shouldn't need minByOrNull + !!

[Vampire]

Which else would I use?
I only found minBy which is deprecated and should be replaced by minByOrNull.

[Javier]

they removed minBy in 1.6 to bring back it in 1.7

[Javier]

the previous implementation of minBy can be nullable, so to follow the standard naming, they removed for one version so they can have both, minBy and OrNull in 1.7

[Vampire]

I see o_O

[Vampire]

Or

fun degreesToDirection(degrees: Float?) =
    degreesMapping.minWith(compareBy { (_, mappedDegrees) ->
        abs(mappedDegrees - (degrees ?: 0f))
    }).key

[Vampire]

Hm, https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-map/ does not display minBy

[Vampire]

And https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/min-by.html shows minBy only not deprecated for common, js, and native, but deprecated for jvm

[Joffrey]

Strange, they're supposed to be back:
https://kotlinlang.org/docs/whatsnew17.html#min-and-max-collection-functions-return-as-non-nullable

[Vampire]

Maybe a documentation problem due to the back and forth with deprecation?

[Rachel Carandang]

Wow thanks for the good discussion! Good to know