User Input and Output

# CHAPTER 6

User Input and Output #

1. Chapter Introduction #

2. Learning Objectives #

• Output data clearly using println() and print().

• Inject variables into Strings using String Templates ($).

• Read user input using readln() (or readLine()).

• Convert input strings to numbers.

• Build a Mini Project: User Registration System.

3. Output and String Templates #

In Java, you have to use String Concatenation (+), which is messy: System.out.println("Hello " + name + ", you are " + age + " years old.");

Kotlin String Templates ($): Kotlin allows you to embed variables directly inside the double quotes using the $ symbol!

fun main() {
    val name = "Alice"
    val age = 28
    
    // Using String Templates (Clean and readable!)
    println("Hello $name, you are $age years old.")
}

If you need to evaluate an *expression* (like math) inside a string, wrap it in curly braces ${ }:

val items = 5
println("Total items plus one is: ${items + 1}")

4. Reading User Input (readln()) #

fun main() {
    print("Please enter your name: ")
    
    // Pauses and waits for the user to type and press Enter
    val name = readln() 
    
    println("Welcome to Kotlin, $name!")
}

5. Parsing Input to Numbers #

fun main() {
    print("Enter your age: ")
    val ageString = readln()
    
    // Convert the String to an Int using .toInt()
    val ageInt = ageString.toInt() 
    
    println("Next year, you will be ${ageInt + 1} years old.")
}

6. Safe Parsing (Preventing Crashes) #

fun main() {
    print("Enter your age: ")
    
    // Returns null if the input is not a valid number
    val age = readln().toIntOrNull() 
    
    // Using the Elvis operator to default to 0 if they typed garbage
    val finalAge = age ?: 0 
    
    println("Recorded age: $finalAge")
}

7. Mini Project: User Registration System #

fun main() {
    println("--- USER REGISTRATION ---")
    
    print("Enter Username: ")
    val username = readln()
    
    print("Enter Birth Year: ")
    // Chain readln() directly into toIntOrNull()
    val birthYear = readln().toIntOrNull() ?: 2000
    
    val currentYear = 2024
    val age = currentYear - birthYear
    
    println("\n--- REGISTRATION COMPLETE ---")
    println("Profile for $username created successfully.")
    println("Calculated Age: $age")
}

8. Common Mistakes #

• Using + for strings: While println("Hello " + name) works, it is considered unidiomatic Kotlin. Always use String Templates ($name).

• Forgetting to Parse: Trying to do math directly on the result of readln(): val x = readln() + 5. The compiler will throw an error because you are trying to add a number to text.

9. Best Practices #

• Always use toIntOrNull(): When dealing with human input, assume the user will make a mistake. Using OrNull parsing functions combined with the Elvis Operator ?: guarantees your program will not crash.

10. Exercises #

1. 1. Write a program that asks the user for the price of an item.

1. 2. Read the input and convert it safely to a Double. (Use toDoubleOrNull()).

1. 3. Calculate a 10% tax.

1. 4. Print the final price using a String Template.

11. MCQs with Answers #

Q1. Which symbol is used for String Templates in Kotlin to inject a variable into text? a) & b) # c) $ Answer: c) $.

12. Interview Questions #

• Q: Explain the difference between toInt() and toIntOrNull(). Why is the latter preferred for user input?

• Q: What is a String Template in Kotlin, and why is it superior to String Concatenation?

13. Summary #

14. Next Chapter Recommendation #