Parameters with default values in Kotlin - Step-by-Step Execution

Concept Flow - Parameters with default values

Function call

↓

Check if argument provided?

No→Use default value

Yes↓

Assign argument to parameter

↓

Execute function body

↓

Return result or finish

When a function is called, Kotlin checks if each parameter has an argument. If not, it uses the default value. Then it runs the function with those values.

Execution Sample

Kotlin

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

greet()
greet("Alice")

This code defines a function with a default parameter and calls it once without and once with an argument.

Execution Table

Step	Function Call	Parameter 'name'	Action	Output
1	greet()	No argument	Use default "Friend"	Hello, Friend!
2	greet("Alice")	Argument "Alice"	Use argument	Hello, Alice!
3	End	-	-	No more calls

💡 All function calls completed

Variable Tracker

Variable	Start	After call 1	After call 2	Final
name	-	"Friend"	"Alice"	-

Key Moments - 2 Insights

Why does greet() print "Hello, Friend!" even though no argument is given?

Because in step 1 of the execution_table, no argument is passed, so Kotlin uses the default value "Friend" for the parameter 'name'.

What happens if we provide an argument like "Alice"?

As shown in step 2, the argument "Alice" replaces the default, so the function prints "Hello, Alice!".

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'name' during the first function call?

A null

B "Alice"

C "Friend"

D No value

💡 Hint

Check the 'Parameter name' column in row 1 of the execution_table.

At which step does the function use the argument instead of the default?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Action' column in the execution_table for step 2.

If we call greet("Bob"), what will be printed?

A Hello, Bob!

B Hello, Alice!

C Hello, Friend!

D Error

💡 Hint

Based on the pattern in the execution_table, providing an argument replaces the default.

Concept Snapshot

fun functionName(param: Type = defaultValue) {
  // function body
}

- If argument missing, defaultValue is used.
- If argument given, it overrides default.
- Helps avoid overloads and simplifies calls.

Full Transcript

This visual trace shows how Kotlin functions handle parameters with default values. When the function greet is called without an argument, Kotlin uses the default value "Friend" for the parameter 'name' and prints "Hello, Friend!". When called with an argument like "Alice", it uses that argument instead and prints "Hello, Alice!". This feature lets you write simpler functions that work with or without some arguments.

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