Kotlin readLine Function

The readLine function in Kotlin is used to read a line of input from the standard input (usually the console). It is part of the Kotlin standard library and provides a simple way to read user input or input from a file.

Table of Contents

1. Introduction
2. readLine Function Syntax
3. Understanding readLine
4. Examples
   • Basic Usage
   • Reading Multiple Lines
   • Reading Input Until a Specific Condition
   • Using readLine with Files
5. Real-World Use Case
6. Conclusion

Introduction

The readLine function allows you to read a single line of input from the standard input. This is useful for scenarios where you need to interact with the user, read configuration data, or process text input.

readLine Function Syntax

The syntax for the readLine function is straightforward:

fun readLine(): String?

Returns:

• A String? (nullable string) representing the line of input read, or null if the end of input has been reached.

Understanding readLine

• readLine: Reads a single line of text from the standard input and returns it as a string. If the end of input is reached (e.g., end of file), it returns null.

Examples

Basic Usage

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

Example

fun main() {
    println("Please enter your name:")
    val name = readLine()
    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.

Example

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

    val quotes = mutableListOf<String>()
    while (true) {
        val line = readLine() ?: 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.

Example

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

    val numbers = mutableListOf<Int>()
    while (true) {
        val line = readLine() ?: break
        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 readLine with Files

This example shows how to use readLine to read lines from a file using BufferedReader.

Example

import java.io.File

fun main() {
    val file = File("example.txt")
    file.writeText("Line 1\nLine 2\nLine 3")

    val bufferedReader = file.bufferedReader()
    bufferedReader.useLines { lines ->
        lines.forEach { println(it) }
    }
}

Output:

Line 1
Line 2
Line 3

Real-World Use Case

Processing User Input

In real-world applications, the readLine function can be used to process user input for various tasks, such as command-line interfaces, interactive prompts, or configuration settings.

Example

fun main() {
    println("Enter your username:")
    val username = readLine()

    println("Enter your password:")
    val password = readLine()

    println("Username: $username")
    println("Password: $password")
}

Output:

Enter your username:
[User enters "user123"]
Enter your password:
[User enters "secret"]
Username: user123
Password: secret

Conclusion

The readLine function in Kotlin provides a simple and effective way to read a line of input from the standard input or from files. By understanding and using the readLine function, you can efficiently manage and process text input in your Kotlin applications, ensuring that you can handle various input scenarios according to your requirements.