From Java to Kotlin, first impression on for loops

#java #kotlin #webdev #beginners

What? This was supposed to be like Java? I can't even write a for loop?!

Or, at least, that was my first impression of Kotlin.
I tried to write for (int i = 0...) but then I quickly adapted to for (var i = 0...) and then for (var i = 0 : Int....) but realised that I was going nowhere.

After scratching my noggin and saying a few ~~curses~~ self-encouraging words. I opted in for the regular enhanced for loop instead. But alas for (n : ints) also resulted in a compilation error.

So lets dive in to what I should have done.

Quick disclaimer is that the IntRange object, as well as the other ranges in the kotlin.ranges package are easy, and often used in, for instance, loops. They can be easily created like the following examples:

val intRange = (1 .. 5)
val theEntireEnglishAlphabet = ('a' .. 'z')

// The ".." operator makes it an ascending range. 
// Another option, is to use the until or downTo keywords

val theSameAsIntRange = (1 until 5)
val descendingIntRange = (5 downTo 1)

So, back to what this article is all about, how do I use for loops?

Here are some examples!

// the regular, wonderful, enhanced for-loop

val someArr = intArrayOf(5, 3, 6, 32, 20, 1)

fun enhancedLoop() {
  for (num in someArr) {
    println(num) // -> console: 5, 3, 6, 32, 20, 1
  } 
}

Yes! A loop! 🙌 But how do I get the indexes? 🤔
There are 2 ways of easily achieving this. One using the .indicies property(?) that you'll find on all of the Arrays and Collections that returns a... you guessed it! IntRange.


// using the .indicies 

val someArr = intArrayOf(5, 3, 6, 32, 20, 1)

fun loopWithIndex() {
  for (i in someArr.indicies) {
    println(i) // -> 0, 1, 2, 3, 4, 5
  }
}

// and if you're looking to loop until a certain value.
// you can just create a new range

// top level: private const val MAX_LOOPS = 5 

fun loopWithInlineRange() {
  for (i in 0 .. MAX_LOOPS) { // you can also use the "until" keyword for increased readability 
    println(i) // -> 0, 1, 2, 3, 4, 5
  }
}

// and if you'd like to do it descending, most likely just 
// a "backwards loop" 

val someOtherArr = intArrayOf(1, 2, 10, 4, 5)

fun loopDescendingWithRange(){
  for (i in someOtherArr.size -1 downTo 0){
    println(someOtherArr[i]) // -> 5, 4, 10 2, 1
  }
}

"Nice! But that -1 feels very caveCode for being such a new language. Can't I just deconstruct the value and the index and keep it enhanced and safe from IndexOutOfBoundsExceptions?" you ask yourself.

Yes! Is the answer. By using the method withIndex() that you'll find on your arrays!


val someArr = intArrayOf(5, 3, 6, 32, 20, 1)

for ((i, num) in someArr.withIndex()) {
  println("$num has index $i") // -> "5 has index 0", "3 has index 1" .... 
}

// Important to note is that you're getting the index first, then the value

// If you want it reversed, you can just reverse the arr

for ((i, num) in someArr.reversed().withIndex()) {
  println("$num has index $i") // -> "1 has index 5", "20 has index 4" .... 
}

I hope you found this little read helpful. I missed my good 'ol for (int i = 0; i < whatHaveYou.length(); i++) in the beginning, but rather quickly grew fond of the usage of ranges instead.