Also function behavior and use cases in Kotlin - Mini Project: Build & Apply

Understanding the Kotlin also Function

📖 Scenario: Imagine you are organizing a list of books and want to log some information while building the list. Kotlin's also function helps you do extra actions on an object without changing it.

🎯 Goal: You will create a list of book titles, use the also function to print each book as it is added, and finally print the full list.

📋 What You'll Learn

Create a mutable list of strings called books with initial titles

Create a variable newBook with a book title

Use the also function to print the newBook before adding it to books

Print the final books list

💡 Why This Matters

🌍 Real World

The <code>also</code> function is useful when you want to do extra work with an object, like logging or debugging, without changing the object itself.

💼 Career

Understanding <code>also</code> helps you write cleaner Kotlin code, which is valuable for Android development and backend Kotlin projects.

1

Create the initial list of books

Create a mutable list of strings called books with these exact titles: "Kotlin Basics", "Advanced Kotlin", and "Kotlin Coroutines".

Kotlin

# Create the mutable list called books with the given titles
# Your code here

Hint

Use mutableListOf to create a list you can add to later.

2

Create a new book title variable

Create a variable called newBook and set it to the string "Kotlin Flow".

Kotlin

val books = mutableListOf("Kotlin Basics", "Advanced Kotlin", "Kotlin Coroutines")
# Create the variable newBook with the title "Kotlin Flow"
# Your code here

Hint

Use val to create a read-only variable with the exact string.

3

Use also to print the new book before adding

Use the also function on newBook to print "Adding book: " followed by the book title, then add the book to the books list.

Kotlin

val books = mutableListOf("Kotlin Basics", "Advanced Kotlin", "Kotlin Coroutines")
val newBook = "Kotlin Flow"
# Use also on newBook to print and then add to books
# Your code here

Hint

The also function lets you do something with the object (like print) and then returns it so you can add it to the list.

4

Print the final list of books

Write a println statement to print the books list.

Kotlin

val books = mutableListOf("Kotlin Basics", "Advanced Kotlin", "Kotlin Coroutines")
val newBook = "Kotlin Flow"
newBook.also { println("Adding book: $it") }.let { books.add(it) }
# Print the books list
# Your code here

Hint

Use println(books) to show the full list after adding the new book.

Practice

1. What does the also function do in Kotlin?

easy

A. Runs extra code on an object and returns the original object

B. Transforms the object and returns a new one

C. Stops the execution of the program

D. Creates a copy of the object

Solution

1. Step 1: Understand the purpose of also

   The also function is used to run some extra code on an object without changing it.

2. Step 2: Check the return value of also

   It returns the original object, not a transformed copy.

3. Final Answer:

   Runs extra code on an object and returns the original object -> Option A

4. Quick Check:

   also returns original object [OK]

Hint: Remember: also returns the original object unchanged [OK]

Common Mistakes:

• Thinking it returns a transformed object
• Confusing it with map or let
• Assuming it stops program execution

2. Which of the following is the correct syntax to use also with a variable num in Kotlin?

easy

A. also(num) { println(it) }

B. num.also -> { println(it) }

C. num.also println(it)

D. num.also { println(it) }

Solution

1. Step 1: Recall the syntax of also

   The also function is called on an object with a lambda block: object.also { ... }.

2. Step 2: Check each option for correct syntax

   num.also { println(it) } uses the correct dot call and lambda syntax. Options A, C, and D have syntax errors or wrong usage.

3. Final Answer:

   num.also { println(it) } -> Option D

4. Quick Check:

   Correct also syntax uses dot and lambda [OK]

Hint: Use dot and curly braces for lambda with also [OK]

Common Mistakes:

• Calling also as a standalone function
• Missing curly braces for lambda
• Using arrow without lambda properly

3. What is the output of this Kotlin code?

val list = mutableListOf(1, 2, 3)
val result = list.also { it.add(4) }
println(result)

medium

A. Compilation error

B. [1, 2, 3]

C. [1, 2, 3, 4]

D. Runtime exception

Solution

1. Step 1: Understand what also does here

   The also block adds 4 to the mutable list. Since the list is mutable, it changes the original list.

2. Step 2: Check the returned value

   also returns the original list, now modified with 4 added.

3. Final Answer:

   [1, 2, 3, 4] -> Option C

4. Quick Check:

   also returns original object after modification [OK]

Hint: also returns original object, so changes show in output [OK]

Common Mistakes:

• Thinking also returns a new list
• Ignoring that list is mutable
• Expecting original list without 4

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

val text = "Hello"
text.also { it = it + " World" }
println(text)

medium

A. println syntax is incorrect

B. Cannot reassign 'it' inside also lambda

C. also cannot be used with strings

D. Missing return statement in also

Solution

1. Step 1: Analyze the lambda parameter 'it'

   In also, 'it' is a val (read-only), so you cannot reassign it inside the lambda.

2. Step 2: Check the code for reassignment

   The code tries to do it = it + " World", which is illegal and causes a compilation error.

3. Final Answer:

   Cannot reassign 'it' inside also lambda -> Option B

4. Quick Check:

   Lambda parameter 'it' is read-only [OK]

Hint: Remember: 'it' in also lambda is read-only [OK]

Common Mistakes:

• Trying to reassign 'it' inside lambda
• Confusing also with let for transformations
• Ignoring immutability of lambda parameters

5. Given a list of user names, how can you use also to log each name while returning the original list unchanged?

val users = listOf("Alice", "Bob", "Carol")
// Insert code here
println(users)

hard

A. users.also { it.forEach { name -> println(name) } }

B. users.map { println(it) }

C. users.let { it.forEach { println(it) } }

D. users.forEach { println(it) }

Solution

1. Step 1: Understand the goal

   We want to log each name but keep the original list unchanged and returned.

2. Step 2: Check each option's behavior

   users.also { it.forEach { name -> println(name) } } uses also to run logging side effects and returns the original list. Options B and C return transformed or Unit types, and D returns Unit.

3. Final Answer:

   users.also { it.forEach { name -> println(name) } } -> Option A

4. Quick Check:

   also runs side effects and returns original object [OK]

Hint: Use also for side effects without changing the object [OK]

Common Mistakes:

• Using map which changes the list
• Using let which returns last lambda result
• Using forEach alone which returns Unit