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Set operations

Learn set management with add, remove and contains.

Set operations: add, remove, contains

On a mutable set add(x) inserts a new element. If the element already exists it returns false, otherwise true. remove(x) takes the element out and returns true if it was present, false otherwise. contains(x) answers the presence question with a Boolean.

val tags = mutableSetOf("new", "important")
tags.add("urgent")      // true
tags.add("new")         // false (already there)
tags.remove("important")  // true

These Boolean returns are handy when you want to check whether an element was actually added.

Set operations: union, intersect, subtract

Kotlin sets support union, intersect and subtract. These return a new set; the originals are unchanged.

val a = setOf(1, 2, 3)
val b = setOf(2, 3, 4)
a union b       // {1, 2, 3, 4}
a intersect b    // {2, 3}
a subtract b     // {1}

These operations are handy when comparing tag, category or permission sets: which tags are shared, which are only on one side?

Practice Add, remove and print size on a set

Start with mutableSetOf("a", "b"). Add "c" with add, remove "a" with remove and print the size with println(colors.size).

Starter code

Main.kt
fun main() {
    val colors = mutableSetOf("a", "b")
    // Add "c", remove "a" and print size
}

Sample solution

Main.kt
fun main() {
    val colors = mutableSetOf("a", "b")
    colors.add("c")
    colors.remove("a")
    println(colors.size)
}

Expected output

2

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match method to return value

Term Meaning
add(x) true: added / false: already there
remove(x) true: was there / false: absent
contains(x) present? Boolean
size Unique element count

2 What does set.add("already there") return on a set?

  • true (always)
  • false (already there) Correct answer
  • Unit
  • Throws an exception

Explanation: add returns false when the element is already present; true when newly added.

3 Fill in the blank: add to the set

Question 
colors.____("yellow")
Answer 
colors.add("yellow")

Explanation: colors.add("yellow") appends the new color to the set.

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together