Maps and Sets

Created By: Geoffrey Challen
/ Updated: 2022-09-12

This lesson introduces two new extremely useful data structures: maps and sets. Together, maps and lists can be used to solve almost any problem. And sets have their uses as well. So let's get started!


Maps represent a fundamentally new data structure for us to consider. So far we've looked at data structures that put items in order—like arrays and lists. We've also discussed using higher-dimensional arrays when needed to represent higher-dimensional data.

Maps are quite different. They don't put items in order. Rather, they allow us to store and look up mappings between one thing and other.

More specifically, maps store mappings between keys and values. Each key in a map maps to just one value. However, the same value can be mapped to by multiple keys.

Map Operations

Let's make this more concrete by looking at a few examples. Just like Kotlin Lists, Maps are built right in and don't require an imports:

Note that, like Lists, Kotlin Maps also utilize type parameters within the angle brackets: <String, String> in the example above. However, Maps require two type parameters: one for the key, and a second for the value. Similar to Lists, Kotlin has both mutable and immutable Maps. We'll be working primarily with mutable ones.

We can also initialize a Map with some initial values, which allows Kotlin to infer the type of the Map:

Note the 1 to "one" syntax above, which adds a mapping to the map between 1 (the key) and "one" (the value).

The first map we created above can be used to map Strings to other Strings:

We can add mappings to our Map using bracket notation, similar to what we used for modifying arrays and lists! But instead of just integers inside the brackets, now we put the key inside the brackets and the value on the right side of the assignment. So map["challen"] = "Geoff Challen" adds a mapping to the map between "challen" (the key) and "Geoff Challen" (the value). We also show how a second assignment to "challen" overwrites the first, since each key in the map maps to a single value.

In the example above, we show how we can establish two mappings to the same value—both "student1" and "student2" initially map to "A Student".

To retrieve values from a Map we also use bracket notation and place the key to retrieve inside the brackets. The result is the value for the key, or null if the map does not contain that key. Sometimes we also refer to this as looking up the key in the map: so looking up the mapping for "challen" or "student1" in the example above.

The Elvis Operator

As a brief but useful digression, one of the useful operators that Kotlin provides for working with null safely is called the "null-coalescing" operator. Or, more colorfully, the "Elvis operator", for reasons that will be obvious shortly.

The Elvis operator evaluates to the left side if it is not null, otherwise to its right side:

The Elvis operator is particularly useful when using Kotlin Maps, as we'll show in the following example, since it allows us to set a default value to use when a map does not contain a particular key.

Map Example

Maps are great for solving problems where we need to save and look up information based on a key. Let's look at an example that may hit close to home: Recording scores on a homework problem!

Walk through how to complete the example above by loading the highest scores for each student into the Map. Show how to use Elvis notation to establish a default value when retrieving a value from the map, and how to use assert for testing.

Map Iteration

If you want to iterate over all of the mappings in a Kotlin Map, there are a few different ways to do that:

Show how to complete the homework problem above. Feel free to cover multiple approaches!


Before we wrap up, let's briefly examine one other potentially-useful data structure: sets. A set represents an unordered collection of distinct elements. We can add and remove items from a set, but the set either contains the item or does not, which we can test using .contains. Items in a set don't have an index and are not ordered or counted.

Sets are generally less useful that lists or maps. But they do come in hand sometimes, particularly when you need to record membership but don't care about counts or ordering. Let's look at an example where a set might come in handy:

Walk through an example using a Set and lowercase to clean the attendance String so that we can record who was at the Muppet meeting. Provide some practice with CSV-like String parsing, setting up the method properly, and using assert for testing.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Show how to complete the homework problem above. Feel free to cover multiple approaches!

More Practice

Need more practice? Head over to the practice page.