Reading User Input

In a command-line program, readln() waits for the user to type a line and press Enter, then returns it as a String.

Example

fun main() {
    print("What's your name? ")
    val name = readln()
    println("Nice to meet you, $name!")
}

Expected output

What's your name? Ada
Nice to meet you, Ada!

(Ada is what the user typed.) Using print for the question keeps the cursor on the same line.

readlnOrNull() returns null instead of throwing when there's no more input — for example, when input is piped from a file that has ended.

Reading numbers

Input is always text, so convert it and handle invalid input:

fun main() {
    print("How old are you? ")
    val age = readln().trim().toIntOrNull()

    if (age == null) {
        println("Please enter a whole number.")
        return
    }

    println("In ten years you'll be ${age + 10}.")
}

return inside main ends the program.

Note: the Kotlin Playground and Android apps don't read keyboard input this way. Run these examples from IntelliJ IDEA or a terminal.

Practice

Ask the user for their favorite color and print it back in capitals: BLUE is a great color!.

fun main() {
    print("Favorite color? ")
    val color = readln()
    println("${color.uppercase()} is a great color!")
}