List operations

Learn list management with add, remove and contains.

List operations: add, remove, contains

On a mutable list the three most common methods are: add(x) appends an element, remove(x) takes out the first match, contains(x) returns a Boolean telling whether x is in the list.

val tasks = mutableListOf("Shopping", "Homework")
tasks.add("Sport")
tasks.remove("Homework")
tasks.contains("Sport")  // true

add always returns true; remove returns true if the element was found and removed, false if it was absent. So you can tell whether the operation succeeded.

Return values of contains and remove

contains(x) returns a Boolean: true if the element is in the list, false otherwise. That is why it fits easily in conditions like if (tasks.contains("Sport")).

val found = tasks.contains("Sport")
val removed = tasks.remove("Missing")
// found: true or false
// removed: false (element was absent)

The Boolean return of remove is handy when you want to remove conditionally: do one thing if it succeeded, try another path if not.

Exercise Add, remove and print size

Start with mutableListOf("Ava", "Liam", "Nora"). Add "Maya" to the list, remove "Liam" and print the size with println(names.size).

Show example solution

fun main() {
    val names = mutableListOf("Ava", "Liam", "Nora")
    names.add("Maya")
    names.remove("Liam")
    println(names.size)
}

Test yourself

Fill in the blank: append to the list
```
tasks.____("Shopping")
```
Show answer
```
tasks.add("Shopping")
```
What does nums.contains(5) return?
- Int (index)
- Unit
- Boolean (true or false)
- List<Int>
Show answer

Boolean (true or false) — contains returns a Boolean; true if present, false if not.
Match method to return value

add(x)

true (always)
remove(x)

true if removed
contains(x)

true if present
size

Int

Next lesson: List access

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