Input, Output & Parsing

fun main() {
    val line = readln()                 // String, throws on EOF (Kotlin 1.6+)
    val maybe = readlnOrNull()          // String? — null at EOF
    val nums = readln().trim().split(" ").map { it.toInt() }
    val (a, b) = readln().split(" ").map { it.toInt() }
    println(nums.sum())
}

val all = generateSequence(::readlnOrNull).toList()      // all lines
val text = System.`in`.bufferedReader().readText()       // one string

import java.io.BufferedReader
import java.io.InputStreamReader
import java.util.StringTokenizer

fun main() {
    val br = BufferedReader(InputStreamReader(System.`in`))
    val (n, m) = br.readLine().split(" ").map { it.toInt() }
    val st = StringTokenizer(br.readLine())
    val xs = IntArray(n) { st.nextToken().toInt() }

    val out = StringBuilder()                 // batch output — never println in a loop
    for (x in xs) out.append(x * 2).append('\n')
    print(out)
}

println("plain")                       // stdout + newline
print("no newline")
System.err.println("diagnostics")      // stderr

"%.2f".format(3.14159)                 // "3.14"
String.format("%05d", 42)              // "00042"
"%,d".format(1234567)                  // "1,234,567" (locale-dependent)
val s = buildString { append("a"); appendLine("b") }

import java.io.File

val text = File("data.txt").readText()
val lines = File("data.txt").readLines()             // List<String>
File("data.txt").useLines { seq ->                    // lazy, auto-closes
    seq.filter { it.isNotBlank() }.count()
}
File("out.txt").writeText("hello\n")
File("out.txt").appendText("more\n")
File("big.txt").bufferedWriter().use { w ->           // `use` = try-with-resources
    w.write("line"); w.newLine()
}

// java.nio.file for paths/dirs
import java.nio.file.*
Files.createDirectories(Path.of("build/reports"))
Files.exists(Path.of("data.txt"))
File("dir").walk().filter { it.extension == "kt" }.toList()

// key=value pairs
val cfg = File("app.cfg").readLines()
    .filter { "=" in it }
    .associate { it.substringBefore("=").trim() to it.substringAfter("=").trim() }

// CSV-ish line
val cols = line.split(",").map { it.trim() }

// Regex extraction
val m = Regex("""(\d{4})-(\d{2})-(\d{2})""").find("due 2026-07-05")
val (y, mo, d) = m!!.destructured

Regex("""\d+""").findAll(text).map { it.value.toInt() }.toList()