Let's Code Kotlin: For & While

Last week I went over If, Else, And When. This week I explore two more pieces of Kotlin syntax. Lucky for us, they are almost equivalent to their Java counterparts: For and While. The latter may seem new, but you'll come to find that it's an old concept with a new name.

For

You can loop through a collection in Kotlin similar to how you can in Java.

for (spell in spellbook) {
    print("${spell.name}")
}

If you don't need anything more advanced, this will work for you just fine in many cases without any added complexity. Actually, you can perform this version of looping in another way by making use of a convenient extension function on Collection: forEach.

spellbook.forEach {
    print("${it.name}")
}

spellbook.forEach { spell ->
    print("${spell.name}")
}

In the first example, it is the default name for each item in the collection. The second example shows you how to rename it to something a bit more specific.

Looping through a range of numbers is a bit different. You may be used to writing for (int i = 0; i < 10, i++). That's not a complex thing to write, but it's much simpler in Kotlin.

for (i in 0..10) {
    // this will loop, from 0 to 12, inclusive.
}

Despite being simpler, however, I find that I almost never loop through things like this. In Java, it was customary to start at 0, go until the size of the list, incrementing by one each time. In the code block of that for loop, I'd make a variable for the item at the given index. I've abandoned this method in Kotlin. I think I've used it maybe once; I find myself using the .forEach extension method more.

It's worth noting that ranges can get complex. They're not too difficult and open up some possibilities that could be useful, like looping from ten to zero but only going through the even indices. Again, I don't loop through ranges anymore, but it's interesting to see how this differs from its Java implementation.

for (i in 10 downTo 0 step 2) {
    // this will loop from 10 to 0, but only the event numbers, inclusive
}

You can find more documentation on For loops on the official Kotlin page (and also the page about Ranges).

While

For the sake of completion, while and do while are pretty much exactly like they are in Java.

while (x > 10) {
    x += 2
}

do {
    update()
) while (x != 3)

Short and simple! Check out my other posts and stay tuned for more Kotlin tips!

Comments

[Andika Kurniawan]

Nice article. Sometimes, I forgot how to use looping although I have experience more than 1 year in Kotlin. Great.....

[Alexander Girke]

Same same

[Tavon]

I forget too! When I looked at the docs while writing this, I definitely shocked myself because I didn't remember how to do the ranges. They almost never come up though, so I'm not toooo harsh on myself lol I like .forEach because it does pretty much everything I need.