Also function behavior and use cases in Kotlin - Step-by-Step Execution

Concept Flow - Also function behavior and use cases

Start with an object

↓

Call also { block }

↓

Inside block: 'it' refers to object

↓

Perform side actions (e.g., print, modify)

↓

Return original object

↓

Continue with original object

The also function lets you run extra code on an object and then returns the original object unchanged.

Execution Sample

Kotlin

val number = 5
val result = number.also {
    println("Number is $it")
}.times(2)
println(result)

Prints the number using also, then multiplies it by 2 and prints the result.

Execution Table

Step	Code Line	Action	Value of 'it'	Result/Output
1	val number = 5	Assign 5 to number	-	number = 5
2	number.also { println("Number is $it") }	Call also on number; inside block print	5	Prints: Number is 5
3	.times(2)	Multiply original number by 2	-	Result = 10
4	println(result)	Print final result	-	Prints: 10
5	-	End of execution	-	-

💡 Execution stops after printing the final result 10.

Variable Tracker

Variable	Start	After also block	After times(2)	Final
number	5	5	5	5
result	-	-	10	10

Key Moments - 2 Insights

Why does the also function return the original object and not the block's last expression?

Because also is designed to perform side effects without changing the object; see execution_table step 2 where the block prints but the original number is returned.

What does 'it' refer to inside the also block?

'it' refers to the original object the also function is called on, as shown in execution_table step 2 where 'it' is 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is printed inside the also block?

A Number is 10

B Number is 5

C Result is 10

D No output

💡 Hint

Check the 'Result/Output' column at step 2 in execution_table.

At which step does the variable 'result' get the value 10?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Value of result' in variable_tracker after times(2).

If the also block modified 'it' to 7, what would be the value of 'result' after times(2)?

A 14

B 7

C 10

D 5

💡 Hint

also returns the original object unchanged, see key_moments about return behavior.

Concept Snapshot

also function syntax:
object.also { it -> /* side actions */ }

- Runs block with 'it' as the object
- Returns original object unchanged
- Useful for side effects like logging or debugging
- Allows chaining without breaking flow

Full Transcript

The also function in Kotlin lets you run extra code on an object without changing it. When you call also on an object, it passes the object as 'it' inside the block. You can do things like print or modify properties inside the block. After the block runs, also returns the original object unchanged. This is useful when you want to add side effects like logging but keep using the original object in a chain of calls. For example, number.also { println("Number is $it") }.times(2) prints the number and then multiplies it by 2. The execution steps show the value of variables and outputs at each stage, helping you see how also works step-by-step.

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