Chaining scope functions in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of chained scope functions with let and also

What is the output of this Kotlin code snippet?

Kotlin

val result = "hello".let {
    println(it.uppercase())
    it.length
}.also {
    println(it * 2)
}
println(result)

A

hello
10
5

B

HELLO
5
10

C

HELLO
10
5

D

HELLO
5
5

💡 Hint

Remember that let returns the last expression inside its block, and also returns the receiver.

The let block prints the uppercase string "HELLO" and returns the length 5.
The also block receives 5, prints 5 * 2 = 10, and returns the original receiver which is 5.
Finally, println(result) prints 5.

❓ Predict Output

intermediate

Result of chaining apply and run

What will be printed by this Kotlin code?

Kotlin

val builder = StringBuilder().apply {
    append("Hi")
}.run {
    append(" there")
    toString()
}
println(builder)

A there

B Hi thereHi

C Hi

D Hi there

💡 Hint

apply returns the receiver, run returns the last expression.

apply appends "Hi" and returns the StringBuilder.
run appends " there" and returns the string representation.
So builder is "Hi there".

🔧 Debug

advanced

Identify the error in chained let and with

What error does this Kotlin code produce?

Kotlin

val text = "abc".let {
    it.uppercase()
}.with {
    println(this)
    length
}
println(text)

A Unresolved reference: length

B Type mismatch: inferred type is Unit but Int was expected

C No error, prints ABC and 3

D NullPointerException

💡 Hint

The let block returns the result of the last expression: it.uppercase(), which is a String.

This code produces an error. The let block returns a String "ABC".
However, with is a top-level function that takes the receiver as its first argument, but here it is called as an extension function, which is invalid.
Therefore, the compiler reports an error: Unresolved reference: with.

❓ Predict Output

advanced

Output of nested run and also with mutable list

What is the output of this Kotlin code?

Kotlin

val list = mutableListOf(1, 2, 3).run {
    add(4)
    also {
        it.removeAt(0)
    }
    size
}
println(list)

A 3

B 4

C 5

D Compilation error

💡 Hint

Remember what run returns and what also returns.

run returns the last expression, which is size (an Int).
also returns the receiver list but is not the last expression.
After adding 4 and removing the first element, the list has 3 elements.
So list is 3.

🧠 Conceptual

expert

Understanding return values in chained scope functions

Given the Kotlin code below, what is the final value of result?

Kotlin

val result = "Kotlin".also {
    println(it.lowercase())
}.let {
    it.length
}.apply {
    println(this)
}

A Unit

B 6

C Compilation error

D Kotlin

💡 Hint

Check what apply returns and what let returns.

also prints "kotlin" and returns the original string "Kotlin".
let returns the length 6.
apply is called on the integer 6 (scope functions are extensions on Any, so work on Int), prints 6, and returns the receiver 6.
Thus, result is 6.

Practice

1. What does chaining scope functions in Kotlin help you achieve?

easy

A. Perform multiple operations on one object without repeating its name

B. Create multiple objects at once

C. Write loops faster

D. Automatically convert types

Solution

1. Step 1: Understand the purpose of scope functions

   Scope functions let you run code blocks on an object, often returning the object or a result.

2. Step 2: Recognize chaining benefit

   Chaining lets you call many scope functions one after another on the same object without repeating its name.

3. Final Answer:

   Perform multiple operations on one object without repeating its name -> Option A

4. Quick Check:

   Chaining scope functions = clean, repeated operations [OK]

Hint: Chaining avoids repeating object names in multiple calls [OK]

Common Mistakes:

• Thinking chaining creates new objects
• Confusing chaining with loops
• Believing chaining changes data types automatically

2. Which of the following is the correct syntax to chain apply and also on an object obj?

easy

A. obj.apply { }.also { }

B. obj.apply() also()

C. apply(obj) { } also { }

D. obj.apply { } also { }

Solution

1. Step 1: Recall correct chaining syntax

   In Kotlin, scope functions are called with dot notation and lambda blocks inside curly braces.

2. Step 2: Check each option

   obj.apply { }.also { } uses correct dot chaining with braces. obj.apply() also() misses dots and braces. apply(obj) { } also { } uses wrong function call style. obj.apply { } also { } misses dot before also.

3. Final Answer:

   obj.apply { }.also { } -> Option A

4. Quick Check:

   Dot + braces = correct chaining syntax [OK]

Hint: Use dot and braces for each scope function call [OK]

Common Mistakes:

• Omitting dots between functions
• Using parentheses instead of braces for lambdas
• Calling scope functions like regular functions without object

3. What is the output of this Kotlin code?

val result = "hello".also { println(it.uppercase()) }.length
println(result)

medium

A. hello\n5

B. 5\nHELLO

C. HELLO\nhello

D. HELLO\n5

Solution

1. Step 1: Understand also behavior

   also runs the lambda with the object as it, prints uppercase "HELLO", then returns the original string "hello".

2. Step 2: Calculate result

   After also, length is called on "hello", which is 5. Then println(result) prints 5.

3. Final Answer:

   HELLO 5 -> Option D

4. Quick Check:

   also prints uppercase, length returns 5 [OK]

Hint: also returns original object, so length is on original string [OK]

Common Mistakes:

• Thinking also returns uppercase string
• Confusing print order
• Expecting length to be printed before uppercase

4. Find the error in this Kotlin code chaining scope functions:

val text = "kotlin"
text.let { it.uppercase() }.apply { println(this) }

medium

A. apply cannot be called after let because let returns a different type

B. Missing lambda braces in apply

C. let does not return the original object, so apply prints wrong value

D. No error, code works fine

Solution

1. Step 1: Analyze let return value

   let returns the lambda result, here it.uppercase(), a new String "KOTLIN".

2. Step 2: Check apply receiver

   apply is called on the uppercase string, so this inside apply is "KOTLIN" and println(this) prints "KOTLIN".

3. Step 3: Identify the problem

   The original text is "kotlin", but apply prints the uppercase string, not the original. This may be unexpected if you want to print original.

4. Final Answer:

   let does not return the original object, so apply prints wrong value -> Option C

5. Quick Check:

   let returns lambda result, not original object [OK]

Hint: Remember let returns lambda result, not original object [OK]

Common Mistakes:

• Assuming let returns original object
• Thinking apply needs braces
• Believing no error exists

5. Given a mutable list val nums = mutableListOf(1, 2, 3), which chained scope function call correctly adds 4, prints the list, and returns the updated list?

hard

A. nums.let { it.add(4) }.also { println(it) }

B. nums.apply { add(4) }.also { println(it) }

C. nums.also { add(4) }.apply { println(this) }

D. nums.run { add(4); println(this) }

Solution

1. Step 1: Understand return values of scope functions

   apply returns the original object after running the block, also returns the original object too, let returns lambda result, run returns lambda result.

2. Step 2: Check each option

   nums.apply { add(4) }.also { println(it) } adds 4 inside apply, then prints list in also, returning the updated list. nums.let { it.add(4) }.also { println(it) }'s let returns Unit from add, so also prints Unit. nums.also { add(4) }.apply { println(this) } has syntax error ('add' unresolved in also block, needs it.add(4)). nums.run { add(4); println(this) } returns Unit from run.

3. Final Answer:

   nums.apply { add(4) }.also { println(it) } -> Option B

4. Quick Check:

   apply returns object, also prints it [OK]

Hint: Use apply to modify and also to print, both return original object [OK]

Common Mistakes:

• Using let which returns Unit from add
• Expecting run to return the list
• Confusing order of operations in chaining