As we’ve seen with lists and sets, a map can be declared mutable or immutable. So far, we’ve only worked with maps whose entries are used for read-only operations. Let’s explore how to declare mutable maps and have more control of the data they hold.

To declare a mutable map, use the term, mutableMapOf in a map declaration:

var/val mapName = mutableMapOf(key1 to val1, key2 to val2, key3 to val2)

The above syntax can be used to declare the following mutable map of students and their age:

var students = mutableMapOf("Willow" to 15, "Elijah" to 17, "Florence" to 16, "Muhammed" to 15) 

Using the [key] syntax, we are able to retrieve and reassign key values. Assume Willow just celebrated her birthday and turned 16. We can update the age value as so:

students["Willow"] = 16

Notice how we’ve used the assignment operator to assign a new value to the key, "Willow".

Note: In a mutable map, values can be updated, however, keys can never change. In order to update a key, the pair must be removed entirely and re-added. In the next exercise, we’ll see how to add and remove values from a map.