Hi! I have a List of objects, and depending on a f...

# getting-started

[Steve]

Hi! I have a List of objects, and depending on a flag, I need to sort this ascending or descending by a field. How can I best accomplish this?

[Joffrey]

val maybeSortedList = if (flag) list.sortedBy { it.prop } else list sortedDescendingBy { it.prop }

[Steve]

i was hoping there'd be a one liner. oh well

[Joffrey]

It can be on one line :)

[Joffrey]

You can make an extension as well

[ephemient]

not quite one-liners but

val ascList = list.sortedBy { it.prop }
val sortedList = if (asc) ascList else ascList.asReversed()

val comparator = compareBy(T::prop)
val sortedList = list.sortedWith(if (asc) comparator else comparator.reversed())

[Norbert Kiesel]

How about

fun <T, R: Comparable<R>> List<T>.sortedBy(descending: Boolean = false, selector: (T) -> R?): List<T> {
    return if (descending) sortedByDescending(selector) else sortedBy(selector)
}

listOf(3, 1, 4).sortedBy(true) { it } ⇒ 4, 3, 1

[loke]

You could also set an integer value to 1 or -1 depending on whether you want it ascending or descending, then multiply the result of the compareTo function by this value. This allows you to avoid an if statement in the comparator which is faster (if you care about that).

[Joffrey]

This is an option, but that only works if the type of the comparable field is numeric.

Also, there is no if statement in the comparator in the above approaches. The if only runs once, and then all elements are processed, just like the if that would set the 1 / -1 value. The -1 approach also add a multiplication for each element, if we're splitting hair. But anyway these considerations are most likely pointless.

[loke]

The compareTo method always returns an integer when comparing two objects. regardless of type.

[loke]

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-comparable/compare-to.html

[Joffrey]

Ah my bad, I read too quickly, I thought you meant to multiply in the sortedBy function, not compareTo

[ephemient]

multiply by -1 doesn't work if the comparator returns Int.MIN_VALUE

[Joffrey]

And if you're using compareTo by hand I fail to see how this would be an improvement in terms of conciseness/ style

[loke]

@ephemient that's actually a pretty astute observation. I was going to say "that'll never happen" (and it doesn't in the cases where I use it, at least in my current project, I just checked), but it is indeed a valid point.