Why scope functions reduce boilerplate in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an instance of a StringBuilder and append text using a scope function.

Kotlin

val result = StringBuilder().[1] {
    append("Hello")
    append(" World")
}.toString()

Drag options to blanks, or click blank then click option'

Aapply

Balso

Crun

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'let' returns the lambda result, not the object itself.

The apply function allows you to configure an object within its scope and returns the object itself, reducing boilerplate when initializing.

2fill in blank

medium

Complete the code to safely access the length of a nullable string using a scope function.

Kotlin

val length = nullableString?.[1] {
    length
} ?: 0

Drag options to blanks, or click blank then click option'

Aapply

Blet

Crun

Dalso

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'apply' returns the object, not the lambda result.

The let function is useful for executing code only if the object is not null and returns the lambda result.

3fill in blank

hard

Fix the error in the code by choosing the correct scope function to execute a block and return its result.

Kotlin

val result = "Hello".[1] {
    this.length
}

Drag options to blanks, or click blank then click option'

Aapply

Balso

Cwith

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'apply' returns the object, not the lambda result.

The run function executes the block with the object as receiver and returns the lambda result, suitable here to get the length.

4fill in blank

hard

Fill both blanks to create a mutable list and add elements using scope functions.

Kotlin

val list = mutableListOf<Int>().[1] {
    add(1)
    add(2)
}.[2] {
    println("List size: $size")
}

Drag options to blanks, or click blank then click option'

Aapply

Blet

Calso

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'let' for the first blank returns the lambda result, not the list.

apply is used to configure the list by adding elements, returning the list itself. also is used to perform an additional action (printing) without changing the list.

5fill in blank

hard

Fill all three blanks to filter a list and create a map of word lengths using scope functions.

Kotlin

val words = listOf("apple", "banana", "cherry")
val lengths = words.filter { it.length [1] 5 }.[2] {
    associateWith { it.length [3] 0 }
}

Drag options to blanks, or click blank then click option'

A>

Bapply

C?:

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'apply' instead of 'let' for the second blank returns the object, not the transformed result.

The filter uses > to select words longer than 5. The let function is used to transform the filtered list. The Elvis operator ?: provides a default value if length is null (though length is non-null here, it's used illustratively).

Practice

1. What is the main reason Kotlin scope functions reduce boilerplate code?

easy

A. They let you access an object inside a block without repeating its name

B. They automatically generate variable names for you

C. They replace all loops with simpler syntax

D. They convert functions into classes

Solution

1. Step 1: Understand boilerplate in Kotlin

   Boilerplate means writing the same object name multiple times when calling its properties or functions.

2. Step 2: Role of scope functions

   Scope functions let you work inside a block where the object is implicit, so you don't repeat its name.

3. Final Answer:

   They let you access an object inside a block without repeating its name -> Option A

4. Quick Check:

   Scope functions reduce repeated object names [OK]

Hint: Scope functions hide object name repetition inside blocks [OK]

Common Mistakes:

• Thinking scope functions generate variable names
• Confusing scope functions with loops
• Believing scope functions convert functions to classes

2. Which of the following is the correct syntax to use the let scope function in Kotlin?

easy

A. let { object -> println(object) }

B. let.object { println(this) }

C. object.let() println(it)

D. object.let { it -> println(it) }

Solution

1. Step 1: Recall let syntax

   The let function is called on an object with a lambda receiving the object as it or a named parameter.

2. Step 2: Check each option

   object.let { it -> println(it) } correctly calls object.let { it -> ... }. Others have wrong syntax or order.

3. Final Answer:

   object.let { it -> println(it) } -> Option D

4. Quick Check:

   let called on object with lambda [OK]

Hint: Remember: object.let { it -> ... } is correct [OK]

Common Mistakes:

• Placing let before the object
• Omitting parentheses or braces
• Using wrong lambda parameter names

3. What will be the output of this Kotlin code?

val name = "Alice"
val result = name.run {
    println(this)
    length
}
println(result)

medium

A. Alice\n5

B. Alice\nAlice

C. 5\n5

D. Compilation error

Solution

1. Step 1: Understand run behavior

   run executes the block with this as the object and returns the last expression's value.

2. Step 2: Trace the code

   Inside run, println(this) prints "Alice". The block returns length which is 5. Then println(result) prints 5.

3. Final Answer:

   Alice 5 -> Option A

4. Quick Check:

   run prints object then returns last expression [OK]

Hint: Remember run returns last expression, prints inside block [OK]

Common Mistakes:

• Confusing this with it
• Expecting run to return the object
• Ignoring the printed output inside the block

4. Identify the error in this Kotlin code using apply:

val builder = StringBuilder()
builder.apply {
    append("Hello")
}
println(builder.toString())

medium

A. append should be called outside apply

B. Missing return statement inside apply block

C. No error; code works and prints "Hello"

D. apply cannot be used with StringBuilder

Solution

1. Step 1: Understand apply usage

   apply runs the block with this as the object and returns the object itself.

2. Step 2: Check code correctness

   The code appends "Hello" inside apply and then prints the builder content. This is valid and prints "Hello".

3. Final Answer:

   No error; code works and prints "Hello" -> Option C

4. Quick Check:

   apply returns object, no return needed [OK]

Hint: apply returns object; no explicit return needed [OK]

Common Mistakes:

• Thinking return is required inside apply
• Believing apply can't be used with StringBuilder
• Trying to call append outside apply unnecessarily

5. You want to initialize a mutable list and add elements to it without repeating the list name. Which scope function helps reduce boilerplate and returns the list itself after adding elements?

hard

A. let

B. apply

C. run

D. also

Solution

1. Step 1: Identify scope function behavior

   apply runs a block with this as the object and returns the object itself, perfect for initialization.

2. Step 2: Compare with other functions

   run returns last expression, let returns lambda result, also returns object but uses it. apply is best for adding elements and returning the list.

3. Final Answer:

   apply -> Option B

4. Quick Check:

   Initialize and return object = apply [OK]

Hint: apply returns object after block; ideal for initialization [OK]

Common Mistakes:

• Choosing let which returns lambda result
• Confusing also with apply
• Using run which returns last expression, not object