Read standard input

Use the readln() function to read data from the standard input. It reads the whole line as a string:

// Reads and stores the user input in a variable. For example: Hi there!
val myInput = readln()

println(myInput)
// Hi there!

// Reads and prints the user input without storing it in a variable. For example: Hi, Kotlin!
println(readln())
// Hi, Kotlin!

To work with data types other than strings, you can convert the input using conversion functions like .toInt(), .toLong(), .toDouble(), .toFloat(), or .toBoolean(). It is possible to read multiple inputs of different data types and store each input in a variable:

// Converts the input from a string to an integer value. For example: 12
val myNumber = readln().toInt()
println(myNumber)
// 12

// Converts the input from a string to a double value. For example: 345 
val myDouble = readln().toDouble()
println(myDouble)
// 345.0

// Converts the input from a string to a boolean value. For example: true
val myBoolean = readln().toBoolean()
println(myBoolean)
// true

These conversion functions assume the user enters a valid representation of the target data type. For example, converting "hello" to an integer using .toInt() would result in an exception as the function expects a number in the string input.

To read several input elements separated by a delimiter, use the .split() function specifying the delimiter. The following code sample reads from the standard input, splits the input into a list of elements based on the delimiter, and converts each element of the list into a specific type:

// Reads the input, assuming the elements are separated by spaces, and converts them into integers. For example: 1 2 3 
val numbers = readln().split(' ').map { it.toInt() }
println(numbers)
//[1, 2, 3] 

// Reads the input, assuming the elements are separated by commas, and converts them into doubles. For example: 4,5,6
val doubles = readln().split(',').map { it.toDouble() }
println(doubles)
//[4.0, 5.0, 6.0]

Handle standard input safely

You can use the .toIntOrNull() function to safely convert user input from a string to an integer. This function returns an integer if the conversion is successful. However, if the input is not a valid representation of an integer, it returns null:

// Returns null if the input is invalid. For example: Hello!
val wrongInt = readln().toIntOrNull()
println(wrongInt)
// null

// Converts a valid input from a string to an integer. For example: 13
val correctInt = readln().toIntOrNull()
println(correctInt)
// 13

The readlnOrNull() function is also helpful in safely handling the user input. The readlnOrNull() function reads from the standard input and returns null if the end of the input is reached, whereas readln() throws an exception in such a case.

Last modified: 03 July 2024