Kotlin readln and readlnOrNull Functions

The readln and readlnOrNull functions in Kotlin are used to read a line of input from the standard input (usually the console). The readln function throws an exception if the end of input is reached, whereas readlnOrNull returns null.

Table of Contents

1. Introduction
2. readln and readlnOrNull Function Syntax
3. Understanding readln and readlnOrNull
4. Examples
   • Basic Usage
   • Reading Multiple Lines
   • Reading Input Until a Specific Condition
   • Using readlnOrNull for Safe Reading
5. Real-World Use Case
6. Conclusion

Introduction

The readln and readlnOrNull functions allow you to read a single line of input from the standard input. These functions are useful for scenarios where you need to interact with the user, read configuration data, or process text input.

readln and readlnOrNull Function Syntax

The syntax for the readln and readlnOrNull functions is straightforward:

fun readln(): String
fun readlnOrNull(): String?

Returns:

• readln: A String representing the line of input read. Throws NoSuchElementException if the end of input is reached.
• readlnOrNull: A String? (nullable string) representing the line of input read, or null if the end of input has been reached.

Understanding readln and readlnOrNull

• readln: Reads a single line of text from the standard input and returns it as a string. Throws an exception if the end of input is reached.
• readlnOrNull: Reads a single line of text from the standard input and returns it as a nullable string. Returns null if the end of input is reached.

Examples

Basic Usage

To demonstrate the basic usage of readln and readlnOrNull, we will read a single line of input from the console.

Example

fun main() {
    println("Please enter your name:")
    val name = readln()
    println("Hello, $name!")
}

Output:

Please enter your name:
[User enters "Alice"]
Hello, Alice!

Reading Multiple Lines

This example shows how to read multiple lines of input from the console using readlnOrNull.

Example

fun main() {
    println("Enter your favorite quotes (type 'end' to finish):")

    val quotes = mutableListOf<String>()
    while (true) {
        val line = readlnOrNull() ?: break
        if (line.lowercase() == "end") break
        quotes.add(line)
    }

    println("Your favorite quotes:")
    quotes.forEach { println(it) }
}

Output:

Enter your favorite quotes (type 'end' to finish):
[User enters "To be or not to be"]
[User enters "The only thing we have to fear is fear itself"]
[User enters "end"]
Your favorite quotes:
To be or not to be
The only thing we have to fear is fear itself

Reading Input Until a Specific Condition

This example shows how to read input until a specific condition is met using readln.

Example

fun main() {
    println("Enter numbers (type '0' to finish):")

    val numbers = mutableListOf<Int>()
    while (true) {
        val line = readln()
        val number = line.toIntOrNull() ?: continue
        if (number == 0) break
        numbers.add(number)
    }

    println("You entered: $numbers")
}

Output:

Enter numbers (type '0' to finish):
[User enters "5"]
[User enters "10"]
[User enters "0"]
You entered: [5, 10]

Using readlnOrNull for Safe Reading

This example demonstrates how to use readlnOrNull to handle end-of-input scenarios safely.

Example

fun main() {
    println("Enter text (press Ctrl+D to end):")

    val lines = mutableListOf<String>()
    while (true) {
        val line = readlnOrNull() ?: break
        lines.add(line)
    }

    println("You entered:")
    lines.forEach { println(it) }
}

Output:

Enter text (press Ctrl+D to end):
[User enters "Line 1"]
[User enters "Line 2"]
[User presses Ctrl+D]
You entered:
Line 1
Line 2

Real-World Use Case

Interactive Command-Line Interface

In real-world applications, the readln and readlnOrNull functions can be used to create interactive command-line interfaces for various tasks, such as configuration or data entry.

Example

fun main() {
    while (true) {
        println("Enter command (type 'exit' to quit):")
        val command = readln()
        if (command == "exit") {
            println("Goodbye!")
            break
        }
        println("You entered: $command")
    }
}

Output:

Enter command (type 'exit' to quit):
[User enters "hello"]
You entered: hello
Enter command (type 'exit' to quit):
[User enters "exit"]
Goodbye!

Conclusion

The readln and readlnOrNull functions in Kotlin provide simple and effective ways to read lines of input from the standard input. By understanding and using these functions, you can efficiently manage and process text input in your Kotlin applications, ensuring that you can handle various input scenarios according to your requirements.