if I have a

List<List<T>>

which can be conceptualized as a "list of columns", (where each inner

List<T>

is a column), whats the most elegant collection method to use to get a list of rows?

I would use sth. like

list.map { it[index] }

. EDIT: Sorry, understood that question wrong.

There are some edge cases that make this inconvenient, eg. lists with different sizes, an empty list and a list of empty lists. This works in the easy cases though:

list.first().indiches.map { i -> list.map{ it[i] } }

list.flatMap { row -> row.mapIndexed { index, t -> index to t }}. groupBy({ it.first}) { it.second }.values

This sould return a list of rows, even when they aren't the same length or sth. is empty. But if you know, that the columns are all the same length, you should use Karels solution. It's way more efficient.

I might consider a one dimensional array to store elements, then some iterators for both ways