fun main() {

     // array
     val my1:Array<Int> = arrayOf<Int>(1, 2, 3) // bad practise
     val my2:IntArray = intArrayOf (10, 20, 30, 40, 50) // good practise

     // list
     val books1:ArrayList<String> = arrayListOf("1", "2");
     val books2:List<String> = listOf("1", "2");
     val books3:MutableList<String> = mutableListOf("1", "2");

     // set
     val numbers:Set<String> = setOf("1", "2", "3", "4")

     // map
     val booksToPrice:Map<String,Int> = mapOf("Green planet" to 10, "Angels and demons" to 25, "Sky flight" to 7)
}