Android के लिए Kotlin Basics

fun main() {
    val appName = "Student Diary"      // val = cannot change
    var notes = 3                      // var = can change
    notes += 2
    val price: Double = 49.5
    val isPro = false
    println("$appName has $notes notes")
    println("Price: ₹$price, pro = $isPro")
    println("Next year: ${2026 + 1}")
}

fun grade(marks: Int): String = when {
    marks >= 90 -> "A+"
    marks >= 75 -> "A"
    marks >= 60 -> "B"
    marks >= 33 -> "Pass"
    else -> "Fail"
}

fun main() {
    val marks = 78
    if (marks >= 33) println("Passed") else println("Failed")
    println("Grade: ${grade(marks)}")

    val day = 3
    val name = when (day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3 -> "Wednesday"
        else -> "Other day"
    }
    println(name)
}

fun main() {
    for (i in 1..5) print("$i ")
    println()
    for (i in 10 downTo 0 step 5) print("$i ")
    println()
    val subjects = listOf("Maths", "Physics", "English")
    for ((index, s) in subjects.withIndex()) println("${index + 1}. $s")
    var tries = 0
    while (tries < 3) { tries++ }
    println("tries = $tries")
}

fun main() {
    val subjects = listOf("Maths", "Physics", "English")          // read-only
    val notes = mutableListOf("Homework")                           // can change
    notes.add("Revision")
    notes.remove("Homework")
    println(subjects.size)
    println(notes)

    val marks = mapOf("Maths" to 92, "Physics" to 81, "English" to 74)
    println(marks["Maths"])
    for ((subject, m) in marks) println("$subject → $m")

    val high = marks.filter { it.value >= 80 }.keys
    println(high)
    println(marks.values.average())
}

fun greet(name: String, city: String = "Aligarh"): String {
    return "Namaste $name from $city"
}

fun square(n: Int) = n * n          // single-expression function

fun percent(obtained: Int, total: Int = 100): Double = obtained * 100.0 / total

fun main() {
    println(greet("Riya"))
    println(greet("Aman", city = "Delhi"))   // named argument
    println(square(7))
    println("%.1f%%".format(percent(425, 500)))
}

// The same Kotlin inside an Activity
val count = notes.size                                   // val
binding.tvTitle.text = "You have $count notes"           // string template
val color = when (note.priority) {                       // when
    2 -> R.color.priority_high
    0 -> R.color.priority_low
    else -> R.color.priority_medium
}
val pending = notes.filter { !it.isDone }                // collections + lambda