Parameters with default values in Kotlin - Time & Space Complexity

Time Complexity: Parameters with default values

O(n)

Understanding Time Complexity

We want to see how the time it takes to run a function changes when it has parameters with default values.

Does using default values affect how long the function takes as input grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun greet(name: String = "Guest", times: Int = 1) {
    for (i in 1..times) {
        println("Hello, $name!")
    }
}

fun main() {
    greet()
    greet("Alice", 3)
}

This function prints a greeting message a number of times, using default values if no arguments are given.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that prints the greeting message.
• How many times: It runs exactly times times, which depends on the input parameter.

How Execution Grows With Input

The number of greetings printed grows directly with the times parameter.

Input Size (times)	Approx. Operations
1	1 print operation
3	3 print operations
100	100 print operations

Pattern observation: The work grows in a straight line as times increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the function grows directly with the number of times it prints the message.

Common Mistake

[X] Wrong: "Using default values makes the function run faster or slower depending on the defaults."

[OK] Correct: Default values only fill in missing arguments; they do not change how many times the loop runs. The time depends on the actual times value used.

Interview Connect

Understanding how default parameters affect performance helps you explain your code clearly and shows you think about how input size impacts work done.

Self-Check

What if we changed the loop to run from 1 to times * 2? How would the time complexity change?

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