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Logical operators

Learn boolean logic with &&, || and !.

Logical operators

Logical operators combine Boolean values: && (and), || (or), ! (not). && yields true when both sides are true. || yields true when at least one side is true. ! flips true to false.

val a = true
val b = false
println(a && b)   // false
println(a || b)   // true
println(!a)        // false

Practice Try logical

Write a fun main(). Declare val age = 20. Check the age range with println(age >= 18 && age <= 65).

Sample solution

Main.kt
fun main() {
    val age = 20
    println(age >= 18 && age <= 65)
}

Expected output

true

Short-circuit evaluation

&& and || are short-circuit operators. In a && b, if a is false, b is not evaluated; the result is already false. In a || b, if a is true, b is not evaluated.

This behavior is especially useful for null checks: x != null && x.length > 0 is safe; if x is null, the second part is not run.

Questions and answers

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

1 What does `true && false` return?

  • true
  • null
  • false Correct answer
  • It throws an error

Explanation: && yields true only when both sides are true.

2 Logical operators: match the terms

Term Meaning
&& And
|| Or
! Not
== Equal

3 Logical operators: fill in the missing part

Question 
val result = (3 > 2) ____ (5 > 10)
println(result)  // true
Answer 
val result = (3 > 2) || (5 > 10)
println(result)  // true

Explanation: With || the result is true if at least one condition holds.

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