val s: Array<LongArray> = Array(5) { LongArray(10) }
println("size" + s[0].size)

is this the only way to get col size ^^

This is an array of arrays. Each row can have a different-sized column.

s[0].size

is the number of columns in the first row. Also, note that your initializer,

Array(5) { LongArray(10) }

initializes an array of 5 rows, with the first row being a LongArray of 10 columns, and the remaining rows being null.

@Klitos Kyriacou so to declare some thing like java Long [] [] matrix , how can we do this in kotlin

Sorry, you're right, it's the latter. Ignore my original reply as it's wrong.

Klitos' post isn't completely wrong though. not every column needs to be the same length. You could define a triangular matrix like this:

Array(5) { LongArray(it+1) }

generates [0] [0] [1] [0][1] [2] [0] ...[2] ... [4] [0] ........ [4]

yes: an

Array<Array>

is not the same as a matrix because every row can have a different size. You can count the number of "cells" using `s.sumOf { it.size }`which also works (though slightly less efficient than

s.size * s[0].size

) for an array where all rows have the same length.

Thank you @Klitos Kyriacou @Stephan Schroeder @nkiesel for your valuable time at replying my question 🙂