Parameters with default values in Kotlin - Mini Project: Build & Apply

Parameters with Default Values in Kotlin

📖 Scenario: You are creating a simple greeting function for a friendly chatbot. Sometimes the chatbot knows the user's name, and sometimes it doesn't. You want the chatbot to greet the user by name if given, or use a general greeting if no name is provided.

🎯 Goal: Build a Kotlin function called greet that uses a parameter with a default value. This function will print a greeting message using the name if provided, or a default greeting if no name is given.

📋 What You'll Learn

Create a function named greet with a parameter name of type String that has a default value.

Set the default value of name to "Guest".

Inside the function, print a greeting message using the name parameter.

Call the greet function twice: once without any argument, and once with the argument "Alice".

💡 Why This Matters

🌍 Real World

Functions with default parameters are common in apps and programs to provide flexible behavior without forcing users to always give all details.

💼 Career

Understanding default parameters is important for writing clean, reusable code in Kotlin, which is widely used for Android app development and backend services.

1

Create the greet function with a name parameter

Write a Kotlin function called greet that has one parameter named name of type String. Set the default value of name to "Guest". Inside the function, write a print statement that prints "Hello, $name!" using string interpolation with println.

Kotlin

fun greet(name: String = "Guest") {
    // Your code here
}

Hint

Use fun greet(name: String = "Guest") to create the function with a default parameter. Use println("Hello, $name!") to print the greeting.

2

Call greet without an argument

Call the greet function without passing any argument to use the default name.

Kotlin

fun greet(name: String = "Guest") {
    println("Hello, $name!")
}

// Call greet without argument
// Your code here

Hint

Just write greet() to call the function without any argument.

3

Call greet with the argument "Alice"

Add a call to the greet function passing the argument "Alice" to greet Alice by name.

Kotlin

fun greet(name: String = "Guest") {
    println("Hello, $name!")
}

greet()

// Call greet with "Alice"
// Your code here

Hint

Call the function with greet("Alice") to greet Alice.

4

Print the greetings

Run the program so it prints the greetings for the default name and for "Alice".

Kotlin

fun greet(name: String = "Guest") {
    println("Hello, $name!")
}

greet()
greet("Alice")

// Print greetings
// Your code here

Hint

Just run the program. It should print two greetings: one for the default name and one for Alice.

Practice

1. What happens when you call a Kotlin function with a parameter that has a default value but you don't provide an argument for it?
fun greet(name: String = "Guest") { println("Hello, $name!") }

easy

A. The function uses the default value for the missing parameter.

B. The function throws an error because the argument is missing.

C. The function ignores the parameter and prints nothing.

D. The function requires you to always provide the argument.

Solution

1. Step 1: Understand default parameter behavior

   In Kotlin, if a parameter has a default value, the function can be called without providing that argument.

2. Step 2: Apply to the example function

   Calling greet() will use the default value "Guest" for name, printing "Hello, Guest!".

3. Final Answer:

   The function uses the default value for the missing parameter. -> Option A

4. Quick Check:

   Default parameter means optional argument [OK]

Hint: If argument missing, default value is used automatically [OK]

Common Mistakes:

• Thinking missing argument causes error
• Assuming function ignores missing parameters
• Believing all parameters must be provided

2. Which of the following is the correct way to declare a Kotlin function with a default parameter value?

easy

A. fun add(a: Int, b: Int default 5) { return a + b }

B. fun add(a: Int = 5, b: Int) { return a + b }

C. fun add(a: Int, b: Int) = a + b = 5

D. fun add(a: Int, b: Int = 5) { return a + b }

Solution

1. Step 1: Check Kotlin syntax for default parameters

   In Kotlin, default values are assigned using = after the parameter type, like b: Int = 5.

2. Step 2: Validate each option

   fun add(a: Int, b: Int = 5) { return a + b } correctly uses b: Int = 5. fun add(a: Int = 5, b: Int) { return a + b } places default before non-default parameter, which is allowed but can cause ambiguity if called without named arguments. Options A and C have invalid syntax.

3. Final Answer:

   fun add(a: Int, b: Int = 5) { return a + b } -> Option D

4. Quick Check:

   Default value syntax = parameter: Type = value [OK]

Hint: Use '=' after type to set default value [OK]

Common Mistakes:

• Placing default value before type
• Using 'default' keyword instead of '='
• Assigning default outside parameter list

3. What is the output of the following Kotlin code?

fun multiply(x: Int, y: Int = 2) = x * y

println(multiply(4))

medium

A. 8

B. 4

C. Error: Missing argument for y

D. 2

Solution

1. Step 1: Identify default parameter usage

   The function multiply has a default value of 2 for y. Calling multiply(4) means x=4 and y=2.

2. Step 2: Calculate the result

   Multiply 4 by 2 to get 8.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   Default y=2 used, 4*2=8 [OK]

Hint: If argument missing, default value is used [OK]

Common Mistakes:

• Expecting error for missing argument
• Using only first argument as output
• Confusing default value with zero

4. Identify the error in this Kotlin function declaration:

fun greet(name: String = "User", age: Int) {
    println("Hello, $name! You are $age years old.")
}

medium

A. Cannot use default values with String parameters.

B. Default parameter must be last or followed by named arguments.

C. Function must return a value if default parameters are used.

D. No error, function is correct.

Solution

1. Step 1: Check parameter order rules

   In Kotlin, parameters with default values should be placed after parameters without default values, or you must use named arguments when calling.

2. Step 2: Analyze given function

   Here, name has a default but comes before age which has no default. This causes a problem when calling without named arguments.

3. Final Answer:

   Default parameter must be last or followed by named arguments. -> Option B

4. Quick Check:

   Default parameters after non-default or use named args [OK]

Hint: Place default parameters after non-default ones [OK]

Common Mistakes:

• Putting default parameters before required ones
• Assuming all parameters can have defaults anywhere
• Ignoring named argument calls

5. You want to write a Kotlin function formatMessage that takes a message and an optional prefix with default value "Info:". It should return the combined string. Which function correctly implements this and allows calling with or without the prefix?

hard

A. fun formatMessage(prefix: String = "Info:", message: String) = "$prefix $message"

B. fun formatMessage(message: String, prefix: String) = "$prefix $message"

C. fun formatMessage(message: String, prefix: String = "Info:") = "$prefix $message"

D. fun formatMessage(message: String = "Info:", prefix: String) = "$prefix $message"

Solution

1. Step 1: Define function with default parameter last

   The optional parameter prefix with default value should come after the required message parameter.

2. Step 2: Check each option

   fun formatMessage(message: String, prefix: String = "Info:") = "$prefix $message" correctly places prefix last with default. fun formatMessage(prefix: String = "Info:", message: String) = "$prefix $message" places default before required, which can cause call issues. Options C and D lack default for prefix or have wrong order.

3. Final Answer:

   fun formatMessage(message: String, prefix: String = "Info:") = "$prefix $message" -> Option C

4. Quick Check:

   Default last parameter enables optional argument [OK]

Hint: Put default parameters after required ones for easy calls [OK]

Common Mistakes:

• Placing default parameter before required parameter
• Not providing default value for optional parameter
• Confusing parameter order causing call errors