Kotlin Basics I — Variables, Operators, Control Flow

Key concepts

• val vs var (immutable vs mutable)
• Type inference and explicit types
• Arithmetic and comparison operators
• if as an expression
• when expression and ranges
• for, while, do-while loops

Lectures in this day

1. 01

   Course Overview

   Here is what you will do in 12 days. First 2 days: learn the Kotlin language (the words we write). Next 2 days: learn how to build with classes (little machines). Next 2 days: make your first real Android screens. After that: connect screens, save data on the phone, then finish with a full app that uses Firebase (a free cloud database).

   Real life · Think of it like a 12-day sports camp. On Day 1 you just warm up. By Day 12 you can play a full match.

   Practice · 3 quick questions

   1. Q1.How many days is this roadmap?

      A.7B.12C.30

   2. Q2.What do the first two days focus on?

      A.FirebaseB.Kotlin basicsC.Google Maps

   3. Q3.What is Firebase used for in Day 11+?

      A.Drawing UIB.Cloud databaseC.Compiling code

   Check answersPick one answer per question.
2. 02

   Declaring Variables (Part 1)

   A variable is a labeled box that holds a value. Kotlin has two kinds. `val` is a box you fill once and never change (like your date of birth). `var` is a box you can refill any time (like today's score in a game).

   Real life · `val` is your birthday — written once, never changes. `var` is the money in your wallet — it goes up and down all day.
   kotlin

   Copy

   val id: String = "NID-1990-42"   0
   var balance = 1500                1
   balance += 500                    2
   println("Hi $id, balance = $balance")

   Practice · 3 quick questions

   1. Q1.Which keyword makes a value you cannot change again?

      A.varB.valC.let

   2. Q2.Which one CAN be reassigned later?

      A.valB.varC.const

   3. Q3.What does `balance += 500` do?

      A.Prints 500B.Adds 500 to balanceC.Deletes balance

   Check answersPick one answer per question.
3. 03

   Declaring Variables (Part 2)

   Sometimes a box is allowed to be empty. We show that with a `?` after the type (like `String?`). We can also glue values into text using `$name` inside quotes — that's called a string template. `const val` is a value that never changes AND is known before the app runs.

   Real life · A middle-name box on a form can be empty — some people don't have one. Kotlin makes you say 'this box might be empty' with `?`.
   kotlin

   Copy

   const val APP_NAME = "TourBook"     0
   val middleName: String? = null      1
   
   2
   val label = "${APP_NAME} user: ${middleName ?: "—"}"
   println(label)   3

   Practice · 3 quick questions

   1. Q1.What does `String?` mean?

      A.A numberB.A String that can be nullC.A list of strings

   2. Q2.Inside "Hi $name", what is `$name`?

      A.A fileB.A string templateC.An error

   3. Q3.What is `const val` best for?

      A.Values known before the app runsB.Random numbersC.User input

   Check answersPick one answer per question.
4. 04

   Arithmetic Operators

   Kotlin does math like a calculator: `+` add, `-` subtract, `*` multiply, `/` divide, `%` remainder (what's left over). One trick: if you divide two whole numbers, the answer is a whole number (the decimal is thrown away). To keep the decimal, make one side a decimal (`3.0` instead of `3`).

   Real life · You have 1000 taka and 3 friends. `1000 / 3` gives 333 (whole taka each). `1000.0 / 3` gives 333.33 (with paisa).
   kotlin

   Copy

   val total = 1000
   val each = total / 3          0
   val eachExact = total / 3.0   1
   val leftover = total % 3      2
   println("$each each, $leftover left over, exact=$eachExact")

   Practice · 3 quick questions

   1. Q1.What is `10 % 3`?

      A.3B.1C.0

   2. Q2.What is `7 / 2` in Kotlin (both Int)?

      A.3.5B.3C.4

   3. Q3.How do you keep the decimals?

      A.Use `7 / 2`B.Use `7.0 / 2`C.Add quotes

   Check answersPick one answer per question.
5. 05

   If Statement

   `if` lets your program pick between paths: if a rule is true do this, otherwise do that. In Kotlin, `if` also gives back a value, so you can save its result into a variable.

   Real life · A traffic light. Red → stop. Yellow → slow. Green → go. Same idea: check, then act.
   kotlin

   Copy

   fun fare(distanceKm: Int): Int =
       if (distanceKm <= 2) 40                       0
       else if (distanceKm <= 10) 40 + (distanceKm - 2) * 15
       else 40 + 8 * 15 + (distanceKm - 10) * 12
   
   println(fare(6))   1

   Practice · 3 quick questions

   1. Q1.What does `if` do?

      A.Loops foreverB.Picks a path based on a conditionC.Declares a variable

   2. Q2.Can `if` return a value in Kotlin?

      A.YesB.No

   3. Q3.Which comparison is 'less than or equal'?

      A.=<B.<=C.==

   Check answersPick one answer per question.
6. 06

   Range and When Statement

   `when` is a cleaner way to check many cases at once — like a big list of 'if this, do that'. `1..10` is a range meaning 'every whole number from 1 to 10'. `in` asks 'is this value inside that range?'.

   Real life · Report card grades: 80–100 = A, 70–79 = B, and so on. You look at the score and pick the band.
   kotlin

   Copy

   0
   fun grade(score: Int): String = when (score) {
       in 80..100 -> "A"   1
       in 70..79  -> "B"
       in 60..69  -> "C"
       in 50..59  -> "D"
       else       -> "F"   2
   }
   
   println(grade(72))   3
   println(grade(45))   4

   Practice · 3 quick questions

   1. Q1.What does `1..10` mean?

      A.1 divided by 10B.Whole numbers 1 to 10C.A decimal

   2. Q2.`x in 1..5` is true when…

      A.x is 6B.x is 3C.x is 0

   3. Q3.Why use `when` instead of many `if/else`?

      A.It's faster to type and read for many casesB.It runs on GPUC.It saves memory

   Check answersPick one answer per question.
7. 07

   Loops in Kotlin

   A loop repeats the same steps many times. `for` walks through a range or a list. `while` keeps going as long as a rule is true. `downTo` counts backwards. `step 2` skips every other number.

   Real life · An elevator that goes floor by floor: `1..10` goes up. `10 downTo 1` comes down. `step 2` visits only even floors.
   kotlin

   Copy

   for (i in 1..5) print("$i ")             0
   println()
   for (i in 10 downTo 2 step 2) print("$i ") 1
   println()
   
   var n = 3
   while (n > 0) {                          2
       println("tick $n")
       n--                                  3
   }

   Practice · 3 quick questions

   1. Q1.How many times does `repeat(3) { ... }` run the block?

      A.1B.3C.0

   2. Q2.`for (i in 1..3)` prints i as…

      A.1,2,3B.0,1,2C.3,2,1

   3. Q3.What does `while (x < 5)` do?

      A.Runs onceB.Runs while x is less than 5C.Never runs

   Check answersPick one answer per question.

Real-life analogies

Code examples

val name: String = "Rakib"   0
var age = 24                   1
age += 1
println("Hello $name, you are $age")

fun grade(score: Int): String = when (score) {
    in 80..100 -> "A"
    in 70..79  -> "B"
    in 60..69  -> "C"
    in 50..59  -> "D"
    else        -> "F"
}

println(grade(84)) 0

for (i in 1..5) print("$i ")           0
for (i in 10 downTo 1 step 2) print("$i ") 1

var n = 3
while (n > 0) { println(n); n-- }

Hands-on checklist

• Install IntelliJ IDEA Community or use kotlinlang.org/play
• Write a calculator with + − × ÷ using when
• Write a grade evaluator using ranges
• Write the same loop with for, while, and downTo

Milestone

Common pitfalls

• Forgetting else in a when used as an expression — compile error.
• Using == vs === (structural vs referential equality) — always use == for value comparison.