For / While fun forAndWhile(){ val students = arrayListOf("joyce", "james", "jenny", "jennifer") for (name in students) { println(name) } for ((index, name) in students.withIndex()) { println("${index+1}번재 학생: ${name}") } var name: String? = "something" name?.let { println(it) } var sum: Int = 0 for (i in 1..100) { sum += i } println("sum -> ${sum} ") } Nullable / NonNull fun nullcheck() { //NPE..