Let function with safe calls in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to safely print the length of the string using let.

Kotlin

val name: String? = "Kotlin"
name[1] { println(it.length) }

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

A.let

B?.let

C?:let

D!!let

Attempts:

3 left

💡 Hint

Common Mistakes

Using .let without safe call causes error if variable is null.

Using !!let is invalid syntax.

Using ?:let is incorrect because ?: is the Elvis operator.

The safe call operator ?. is used before let to ensure the block runs only if 'name' is not null.

2fill in blank

medium

Complete the code to safely convert the string to uppercase using let.

Kotlin

val input: String? = "hello"
val result = input[1] { it.uppercase() } ?: "default"

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

A.let

B?:let

C!!let

D?.let

Attempts:

3 left

💡 Hint

Common Mistakes

Using .let without safe call causes NullPointerException if input is null.

Using !!let is invalid syntax.

Using ?:let is incorrect; ?: is the Elvis operator.

Using ?.let ensures the uppercase conversion happens only if 'input' is not null; otherwise, "default" is used.

3fill in blank

hard

Fix the error in the code to safely print the first character of the string using let.

Kotlin

val text: String? = null
text[1] { println(it.first()) }

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

A.let

B!!let

C?.let

D?:let

Attempts:

3 left

💡 Hint

Common Mistakes

Using .let without safe call causes NullPointerException.

Using !!let is invalid syntax.

Using ?:let is incorrect.

The safe call operator ?. before let prevents calling let on a null value, avoiding runtime errors.

4fill in blank

hard

Fill both blanks to safely print the length of the string if it is not null and longer than 3 characters.

Kotlin

val word: String? = "code"
word[1] { if (it.length [2] 3) println(it.length) }

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

A?.let

B>

C<

D.let

Attempts:

3 left

💡 Hint

Common Mistakes

Using .let without safe call causes error if word is null.

Using < instead of > changes the logic.

Using ?:let is invalid.

Use ?.let to safely call let only if word is not null, and > to check if length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a list of lengths only for words longer than 4 characters.

Kotlin

val words: List<String?> = listOf("apple", "bat", "carrot")
val lengths = words.mapNotNull { it[1] { if (it.length [2] 4) it[3] { it.length } else null } }
println(lengths)

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

A?.let

B>

C<

D.let

Attempts:

3 left

💡 Hint

Common Mistakes

Using .let without safe call causes errors if any word is null.

Using < instead of > changes the filter condition.

Misplacing .let inside the lambda.

Use ?.let to safely call let on each word, > to check length, and .let inside the lambda to return length or null.

Practice

1. What does the Kotlin expression nullableVar?.let { println(it) } do?

easy

A. Prints the value of nullableVar only if it is not null.

B. Always prints null regardless of nullableVar.

C. Throws a NullPointerException if nullableVar is null.

D. Assigns nullableVar to it without printing.

Solution

1. Step 1: Understand safe call operator usage

   The ?. operator checks if nullableVar is not null before proceeding.

2. Step 2: Analyze the let function behavior

   If nullableVar is not null, let executes the lambda, printing the value.

3. Final Answer:

   Prints the value of nullableVar only if it is not null. -> Option A

4. Quick Check:

   ?.let runs code only if not null [OK]

Hint: Remember: ?.let runs code only when value is not null [OK]

Common Mistakes:

• Thinking it prints null when variable is null
• Assuming it throws an exception on null
• Confusing let with always running code

2. Which of the following is the correct syntax to safely print a nullable string name using let?

easy

A. name?.let { println(it) }

B. name.let { println(it) }

C. name?.let println(it)

D. name?.let { println(name) }

Solution

1. Step 1: Identify safe call usage

   To avoid null errors, ?.let must be used, not just .let.

2. Step 2: Check lambda syntax correctness

   The lambda must be enclosed in braces { } and use it to refer to the value.

3. Final Answer:

   name?.let { println(it) } -> Option A

4. Quick Check:

   Safe call + let with braces = correct syntax [OK]

Hint: Use ?.let { } with braces and 'it' inside [OK]

Common Mistakes:

• Omitting the safe call operator (?.)
• Missing braces around lambda
• Using variable name inside lambda instead of 'it'

3. What is the output of this Kotlin code?

val number: Int? = 5
number?.let { println(it * 2) }

medium

A. No output

B. 10

C. null

D. 5

Solution

1. Step 1: Check if number is null

   The variable number holds 5, which is not null, so ?.let runs.

2. Step 2: Calculate inside let block

   The lambda multiplies it (5) by 2 and prints 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Non-null value runs let, prints doubled value [OK]

Hint: If variable not null, let runs and prints result [OK]

Common Mistakes:

• Expecting no output if value is not null
• Confusing printed value with original variable
• Thinking it prints null when variable is non-null

4. Identify the error in this Kotlin code snippet:

val text: String? = null
text.let { println(it.length) }

medium

A. It prints 0 because null is treated as empty string.

B. It compiles but does not print anything.

C. It throws NullPointerException because safe call is missing.

D. It prints 'null' as a string.

Solution

1. Step 1: Check safe call usage

   The code uses text.let without ?., so it runs even if text is null.

2. Step 2: Understand null dereference

   Inside the lambda, it.length is called on null, causing a NullPointerException.

3. Final Answer:

   It throws NullPointerException because safe call is missing. -> Option C

4. Quick Check:

   Missing ?. causes null error [OK]

Hint: Always use ?.let to avoid null errors [OK]

Common Mistakes:

• Assuming let skips null without safe call
• Thinking null acts like empty string
• Ignoring possibility of NullPointerException

5. Given a nullable list of strings val names: List<String>? = listOf("Anna", "Bob", null, "Cara"), which Kotlin code safely prints only non-null names using let with safe calls?

hard

A. names?.let { it.forEach { println(it) } }

B. names.forEach { it.let { println(it) } }

C. names?.forEach { println(it) }

D. names?.forEach { it?.let { println(it) } }

Solution

1. Step 1: Handle nullable list safely

   Use names?.forEach to avoid errors if names is null.

2. Step 2: Filter out null elements inside the list

   Inside the loop, use it?.let to print only non-null names.

3. Final Answer:

   names?.forEach { it?.let { println(it) } } -> Option D

4. Quick Check:

   Safe calls on list and elements print only non-null [OK]

Hint: Use ?.forEach and it?.let to skip nulls safely [OK]

Common Mistakes:

• Not using safe call on list causing errors
• Printing null elements without filtering
• Using let without safe call on nullable elements