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Lesson 7 - Arrays in Kotlin

In the previous lesson, Type system: Null safety in Kotlin, we learned how to use nullable types. In today's lesson, we're going to introduce you all to the array data structure and show you what it's capable of accomplishing.


Imagine that you want to store some information of multiple items, e.g. you want to keep 10 numbers in the memory, each of the fields of a checkerboard or names of 50 users. Perhaps you realize that there must be an easier way than to start typing variables like user1, user2 ... up until user50. Despite the fact that there may be 1000 of them. How would go about searching for something in there? Definitely not like that! :)

If we need to store a larger amount of variables of the same type, we can solve this problem using an array. We can imagine it as a row of boxes, each of them containing one item. The boxes are numbered by indexes, the first one has index 0.

Array structure in Kotlin

(We see an array of 8 numbers in this picture)

Programming languages are very different in the way they work with arrays. In some languages (especially the older ones, compiled), it wasn't possible to create an array with a size specified at runtime (e.g. to specify its size using some variable). Arrays had to be declared with a constant size in the source code. A workaround was made by inventing pointers and custom data structures, which often lead to errors in manual memory management and to program instability (e.g. in C++). On the other hand, some interpreted languages allow us to not only declare arrays of any size but to also change the size of an existing array (it's possible e.g. in PHP). We know that Kotlin belongs to the modern ones, you don't have to think about the size of arrays (you don't even have to specify it) and you can add new items to an already existing array.

We declare an array using the arrayOf() function:

var numbers = arrayOf<Int>()

numbers is obviously a name of our variable.

First, let's add new items using the += operator. In our case, after the operator, we'll specify a new number as Int, which will be stored at the end of the array:

var array = arrayOf<Int>()
array += 34

We can also add another array of the same type to an array (meaning the Int type, in our case) whose items will be added to the array.

We access array items through brackets with the item index specified. Of course, we can do so only when there's really an item at the given index. Let's try it:

var numbers = arrayOf<Int>()
numbers += 34

We printed the item with the index 0, i.e. the first one, since the item indexes are zero-based. In the output, we can see the number 34 which is stored at that index.


Filling the array manually like this wouldn't be too effective. Let's use a loop and fill the array with numbers from 1 to 10. We'll use the for loop to fill the array.

var numbers = arrayOf<Int>()
for (i in 1..10) {
    numbers += i

If we want to print this array, we need to add this piece of code after the one above:

for (i in numbers) {
    print("$i ")

Now we can see the true power of the for loop in Kotlin. All we have to do is to pass the array after the in keyword instead of a numeric range and the loop will go through all the items. In the loop's body, we can access the items and for example print them:

1 2 3 4 5 6 7 8 9 10

Arrays have the size property, in which the number of items is stored.

Of course, we can fill an array manually even without going through each index to assign the values. We can use the arrayOf() function and list the items in the brackets, separated by commas. Now, let's try to create an array of strings:

val simpsons = arrayOf("Homer", "Marge", "Bart", "Lisa", "Meggie")

We don't want anyone to change our array now, therefore we declare it using val which makes the array constant. We omit the type specification since Kotlin easily recognizes from the items that it's a String array. Of course, however, nothing stops us from specifying it:

val simpsons: Array<String> = arrayOf("Homer", "Marge", "Bart", "Lisa", "Meggie")

Arrays are often used to store intermediate results, which are used later in the program. When we need some result 10 times, we won't calculate the same thing 10 times, instead, we'll calculate it once and put it into the array, then we just read the result.

Array methods

Kotlin provides us with several utility methods for working with arrays. Let's look at them.

sort() and sorted()

As the name suggests, the methods sort our array. The sort() method sorts an already existing arrays (it must be declared using var) and sorted() returns a new sorted array, so we musn't forget to assign it to a variable. The methods are even so clever that they work accordingly to what is stored in the array. It sorts Strings by name and numbers by their value. Let's try to sort and print our Simpsons family:

val simpsons: Array<String> = arrayOf("Homer", "Marge", "Bart", "Lisa", "Meggie")
for (simpson in simpsons) {

The output:


And the same thing using sorted():

val simpsons: Array<String> = arrayOf("Homer", "Marge", "Bart", "Lisa", "Meggie")

val simpsons_sorted = simpsons.sortedArray()
for (simpson in simpsons_sorted) {

The output:

\---code Bart Homer Lisa Maggie Marge \---

Try to create an array of numbers and verify that it actually works for them as well.

reverse() and reversedArray()

These methods reverse the array (the first item will be the last, and so on). The principle is the same as with the sorting. The reversed() method sorts an existing array, reversedArray() returns a new reversed array. We can use reversing e.g. for descending sorting:

val simpsons: Array<String> = arrayOf("Homer", "Marge", "Bart", "Lisa", "Meggie")
for (simpson in simpsons) {

The output:



The method will find the index of the first occurrence of an element in the array and return its index as Int. The element may not be in the array, the method returns -1 in this case. We'll create a simple program that will assume that the Simpsons are sorted by popularity. When the user enters one, we'll print on which position the Simpson is, or that the input doesn't exist in the array.

val simpsons = arrayOf("Homer", "Marge", "Bart", "Lisa", "Meggie")
println("Hi, enter your favorite Simpson (the Simpsons family): ")
val simpson = readLine()!!
val position = simpsonovi.indexOf(simpson)
if (position != -1) {
    println("Yeah, that's my number ${position + 1} Simpson!")
} else {
    println("Hey that's not Simpson!")

The output:

Hi, enter your favorite Simpson (the Simpsons family):
Yeah, that's my number 1 Simpson!


We've already mentioned size, it contains the number of elements in the array. It's not a method, but a property, so we don't use brackets () here.


As you may guess, this method will return true if the array is empty. Its notation is more readable than array.size == 0. The code clearly says we're interested in the possibility the array is empty.

min() and max()

Math methods returning the lowest element (min()) and the greatest element (max()) of the array. The result is returned as a nullable type, in case of the array being empty.

first() and last()

Just by looking at the names of the methods, we can guess they return the first and the last element. The returned values are nullable again.


The method returns true/false depending on whether an element given through the method parameter exists in the array.

That's enough for today, you can play with arrays for a while if you'd like. In the next lesson, Solved tasks for Kotlin lesson 7, I've got a surprise for you I think you might like ;-)

In the following exercise, Solved tasks for Kotlin lesson 7, we're gonna practice our knowledge from previous lessons.


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