Type Conversion

Since readLine() returns a nullable string (null at end of input), you'll need to convert that input when you want to work with numbers. Kotlin provides conversion functions to transform strings into other types.

To convert a string to an integer, use toInt():

fun main() {
    val input = readLine()!!
    val number = input.toInt()
    println(number + 10)
}

If the user enters "5", the program converts it to the integer 5, then adds 10 to get 15. Without conversion, "5" + 10 concatenates text and produces "510". Converting first gives numeric addition.

For decimal numbers, use toDouble():

fun main() {
    val price = readLine()!!.toDouble()
    println("With tax: ${price * 1.1}")
}

You can chain the conversion directly after readLine()!! for cleaner code. Other useful conversions include toLong() for large integers and toBoolean() for boolean values.

These conversions assume the input is valid. If someone enters "hello" when you expect a number, the program will crash. You'll learn to handle such errors later in the course.