Flow Control

fun main() {
    val temperature = 31

    // statement form
    if (temperature > 30) {
        println("Hot day")
    } else if (temperature > 20) {
        println("Warm day")
    } else {
        println("Cool day")
    }

    // expression form: the chosen branch is the value
    val advice = if (temperature > 30) "Drink water" else "Enjoy"
    println(advice)

    val max = if (temperature > 25) {
        println("checking a block branch")
        temperature      // last expression = the value
    } else {
        25
    }
    println("max = $max")
}

fun describe(x: Any): String = when (x) {
    0 -> "zero"
    1, 2, 3 -> "small number"
    in 4..9 -> "single digit"
    is Int -> "a bigger Int"
    is String -> "text of length ${x.length}"   // smart cast: x is a String here
    else -> "something else"
}

fun main() {
    println(describe(0))
    println(describe(2))
    println(describe(7))
    println(describe(250))
    println(describe("hello"))
    println(describe(4.5))
}

fun grade(score: Int): String = when {
    score !in 0..100 -> "invalid"
    score >= 90 -> "A"
    score >= 75 -> "B"
    score >= 50 -> "C"
    else -> "F"
}

fun main() {
    for (s in listOf(95, 75, 60, 12, 140)) {
        println("$s -> ${grade(s)}")
    }
}

fun main() {
    for (i in 1..3) print("$i ")
    println()

    for (i in 0 until 3) print("$i ")
    println()

    for (i in 10 downTo 0 step 5) print("$i ")
    println()

    for (ch in "abc") print("$ch-")
    println()

    val fruits = listOf("apple", "banana", "cherry")
    for (i in fruits.indices) println("$i: ${fruits[i]}")
    for ((index, fruit) in fruits.withIndex()) println("#${index + 1} $fruit")

    val stock = mapOf("pens" to 12, "pads" to 3)
    for ((item, count) in stock) println("$item=$count")
}

fun main() {
    // How many times can 1000 be halved before it drops below 1?
    var n = 1000.0
    var halvings = 0
    while (n >= 1) {
        n /= 2
        halvings++
    }
    println("halvings: $halvings")

    // Collatz: keep going until we reach 1
    var x = 6
    val path = mutableListOf(x)
    while (x != 1) {
        x = if (x % 2 == 0) x / 2 else 3 * x + 1
        path.add(x)
    }
    println(path)

    // do-while runs the body once even though the condition is already false
    var tries = 10
    do {
        println("attempt $tries")
        tries++
    } while (tries < 3)
}

fun main() {
    // Re-ask until the input is a valid number between 1 and 5
    var rating: Int?
    do {
        print("Rate 1-5: ")
        val line = readln()
        rating = line.toIntOrNull()?.takeIf { it in 1..5 }
        if (rating == null) println("'$line' is not valid")
    } while (rating == null)
    println("Thanks for rating $rating")
}

fun main() {
    for (n in 1..10) {
        if (n % 3 == 0) continue   // skip multiples of 3
        if (n > 7) break           // stop completely after 7
        print("$n ")
    }
    println()

    val grid = listOf(
        listOf(4, 8, 15),
        listOf(16, 23, 42),
        listOf(7, 1, 9),
    )
    val target = 23
    outer@ for ((r, row) in grid.withIndex()) {
        for ((c, value) in row.withIndex()) {
            if (value == target) {
                println("found $target at row $r, col $c")
                break@outer          // leave BOTH loops
            }
        }
    }
    println("search done")
}

fun main() {
    val evens = 2..20 step 4
    println("first=${evens.first} last=${evens.last} step=${evens.step}")
    println(evens.toList())

    val working = 9..17
    println(13 in working)
    println(working.count())
    println((1..10).filter { it % 2 == 1 })
    println((1..5).reversed().toList())
    println((1..5).map { it * it })
    println(('a'..'z' step 5).joinToString(""))

    repeat(3) { println("line ${it + 1}") }
    println("-".repeat(10))
}