Let function behavior and use cases in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using let?

Consider the following Kotlin code snippet. What will it print when run?

Kotlin

val name: String? = "Anna"
val result = name?.let { it.uppercase() } ?: "No name"
println(result)

A null

B anna

C No name

D ANNA

💡 Hint

Remember that let runs the block only if the variable is not null.

The variable 'name' is not null, so let runs and converts it to uppercase, printing 'ANNA'.

❓ Predict Output

intermediate

What does this Kotlin code print when using let with a nullable variable?

Analyze this Kotlin code and determine its output.

Kotlin

val number: Int? = null
val output = number?.let { it * 2 } ?: -1
println(output)

A 0

B -1

C null

D 2

💡 Hint

If the variable is null, let block does not run and the Elvis operator provides the default.

Since 'number' is null, let is skipped and the Elvis operator returns -1.

🔧 Debug

advanced

Why does this Kotlin code cause a compilation error?

Examine the code below and identify the reason for the compilation error.

Kotlin

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

A Because 'text' is nullable, 'it.length' causes a compilation error.

B Because 'let' requires a non-null receiver, 'text.let' is invalid.

C Because 'it' inside let is nullable, accessing 'length' without safe call causes error.

D Because 'println' cannot print nullable types directly.

💡 Hint

Consider the type of 'it' inside the let block when the receiver is nullable.

Inside let, 'it' is the nullable type String?, so accessing 'length' without safe call or null check causes error.

🧠 Conceptual

advanced

What is a common use case for Kotlin's let function?

Choose the best description of a typical use case for the let function in Kotlin.

A To execute a block only when a variable is not null and transform its value.

B To declare a new variable with a different type inside a function.

C To replace the need for loops by iterating over collections.

D To create an extension function that modifies the original object.

💡 Hint

Think about how let helps with null safety and chaining.

Let is often used to run code only if a variable is not null and to transform or use its value safely.

❓ Predict Output

expert

What is the output of this Kotlin code using nested let functions?

Analyze the following Kotlin code and determine what it prints.

Kotlin

val a: String? = "foo"
val b: String? = null
val result = a?.let { x -> b?.let { y -> "$x and $y" } } ?: "No result"
println(result)

A No result

B foo and

C foo and null

D foo and foo

💡 Hint

Remember that the inner let runs only if 'b' is not null.

Since 'b' is null, the inner let does not run, so the whole expression is null and Elvis operator returns 'No result'.

Practice

1. What does the Kotlin let function primarily do when called on an object?

easy

A. Creates a new object identical to the original

B. Executes a block of code with the object as its argument and returns the block's result

C. Deletes the object after running the block

D. Converts the object to a string representation

Solution

1. Step 1: Understand the purpose of let

   The let function takes the object it is called on and passes it as an argument to a block of code.

2. Step 2: Identify the return value of let

   The result of the block inside let is returned as the final value of the let call.

3. Final Answer:

   Executes a block of code with the object as its argument and returns the block's result -> Option B

4. Quick Check:

   let runs block and returns result [OK]

Hint: Remember: let runs code and returns block result [OK]

Common Mistakes:

• Thinking let creates a new object
• Assuming let deletes the object
• Confusing let with toString conversion

2. Which of the following is the correct syntax to use let on a nullable variable name to print it only if it's not null?

easy

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

B. let(name) { println(it) }

C. name.let { println(it) }

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

Solution

1. Step 1: Recognize nullable safe call usage

   To safely call let only when name is not null, use the safe call operator ?..

2. Step 2: Check syntax correctness

   name?.let { println(it) } runs the block only if name is not null, printing the value.

3. Final Answer:

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

4. Quick Check:

   Safe call with let = name?.let { ... } [OK]

Hint: Use ?.let to run code only if variable is not null [OK]

Common Mistakes:

• Using let without safe call on nullable variable
• Incorrect syntax like let(name) or let? after variable
• Forgetting the safe call operator

3. What is the output of the following Kotlin code?

val number = 5
val result = number.let { it * 2 }
println(result)

medium

A. 10

B. 5

C. it * 2

D. Compilation error

Solution

1. Step 1: Understand the let block execution

   The variable number is 5. The let block multiplies it (which is 5) by 2.

2. Step 2: Calculate the result and print

   The expression it * 2 evaluates to 10, which is assigned to result and printed.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   5 * 2 = 10 [OK]

Hint: let returns block result, so 5 * 2 = 10 [OK]

Common Mistakes:

• Thinking let returns original object
• Confusing it with string output
• Expecting syntax error due to block

4. Identify the error in this Kotlin code snippet using let:

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

medium

A. Prints 'null' string

B. No error, prints 0

C. Compilation error due to missing safe call

D. NullPointerException at runtime

Solution

1. Step 1: Analyze nullable variable usage

   The variable text is nullable and assigned null. Calling let directly runs the block even if text is null.

2. Step 2: Understand runtime behavior

   Inside the block, it.length is accessed. Since it is null, this causes a NullPointerException at runtime.

3. Final Answer:

   NullPointerException at runtime -> Option D

4. Quick Check:

   Accessing length on null causes runtime error [OK]

Hint: Use ?.let to avoid null pointer errors [OK]

Common Mistakes:

• Assuming let skips block if null without safe call
• Expecting compile error instead of runtime error
• Thinking it prints 'null' string

5. Given a nullable list of integers val numbers: List<Int>? = listOf(1, 2, 3), how can you use let to safely calculate and print the sum only if the list is not null and not empty?

hard

A. numbers.let { println(it.sum()) }

B. numbers.let { if (it.isNotEmpty()) println(it.sum()) }

C. numbers?.let { if (it.isNotEmpty()) println(it.sum()) }

D. numbers?.let { println(it.sum()) }

Solution

1. Step 1: Use safe call to handle null list

   Since numbers is nullable, use ?.let to run code only if it's not null.

2. Step 2: Check for empty list inside let block

   Inside the block, check it.isNotEmpty() to ensure the list has elements before summing.

3. Step 3: Print sum only if conditions met

   If the list is not empty, print the sum using println(it.sum()).

4. Final Answer:

   numbers?.let { if (it.isNotEmpty()) println(it.sum()) } -> Option C

5. Quick Check:

   Safe call + empty check + sum print = numbers?.let { if (it.isNotEmpty()) println(it.sum()) } [OK]

Hint: Combine ?.let with isNotEmpty() check inside block [OK]

Common Mistakes:

• Not using safe call on nullable list
• Skipping empty check causing wrong output
• Using let without safe call causing runtime error