Why expressions over statements matters in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign the maximum of two numbers to max using an expression.

Kotlin

val max = if (a > b) [1] else b

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

Aa

B0

Ca + b

Db

Attempts:

3 left

💡 Hint

Common Mistakes

Using if as a statement without returning a value.

Assigning a fixed value like 0 instead of a.

The if expression returns a if a is greater than b, otherwise b. This assigns the maximum value to max.

2fill in blank

medium

Complete the code to assign the result of the expression that returns the length of the string or 0 if the string is null.

Kotlin

val length = str?.length ?: [1]

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

Anull

Bstr.length

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using null as default which causes type mismatch.

Using str.length which can cause a null pointer exception.

The Elvis operator ?: returns 0 if str?.length is null, ensuring length is always an integer.

3fill in blank

hard

Fix the error in the expression that tries to assign the result of a when expression to a variable.

Kotlin

val result = when (x) {
    1 -> "One"
    2 -> "Two"
    else -> [1]
}

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

Anull

B"Other"

Cprint("Other")

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using print("Other") which returns Unit, not String.

Returning null which may cause type mismatch.

The when expression must return a value for all cases. Using "Other" returns a string, matching the other branches.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths, including only words longer than 3 characters.

Kotlin

val lengths = words.filter { word -> word.length > 3 }.map { word -> [1] to [2] }.toMap()

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

Aword

Bword.length

Cword.uppercase()

Dword.count()

Attempts:

3 left

💡 Hint

Common Mistakes

Using word.uppercase() as key which changes the word.

Using word.count() which is not a Kotlin String method.

The map keys are the words themselves (word) and the values are their lengths (word.length), filtered by length > 3.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, including only words longer than 4 characters.

Kotlin

val result = words.filter { word -> word.length [3] 4 }.map { word -> [1] to [2] }.toMap()

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

Aword.uppercase()

Bword.length

C>

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using word instead of uppercase for keys.

Using < instead of > for filtering.

The map keys are uppercase words (word.uppercase()), values are their lengths (word.length), filtered by words longer than 4 characters (> 4).

Practice

1. Why are expressions preferred over statements in Kotlin programming?

easy

A. Because expressions return values and can be used anywhere a value is needed

B. Because statements are faster to execute

C. Because statements always cause errors

D. Because expressions do not require variables

Solution

1. Step 1: Understand the difference between expressions and statements

   Expressions produce values and can be used wherever a value is expected, while statements perform actions but do not return values.

2. Step 2: Identify why expressions are preferred

   Using expressions allows writing clearer and more concise code by directly returning values without extra variables or statements.

3. Final Answer:

   Because expressions return values and can be used anywhere a value is needed -> Option A

4. Quick Check:

   Expressions return values = C [OK]

Hint: Remember: expressions produce values, statements do not [OK]

Common Mistakes:

• Confusing speed with value return
• Thinking statements always cause errors
• Believing expressions don't need variables

2. Which of the following is the correct Kotlin syntax for using an expression in an if statement?

easy

A. if (x > 0) { return "Positive" } else { return "Non-positive" }

B. if x > 0 then "Positive" else "Non-positive"

C. val result = if (x > 0) "Positive" else "Non-positive"

D. val result = if x > 0 { "Positive" } else { "Non-positive" }

Solution

1. Step 1: Review Kotlin's expression syntax for if

   Kotlin allows if to be used as an expression that returns a value, using parentheses and else branch.

2. Step 2: Identify the correct syntax

   val result = if (x > 0) "Positive" else "Non-positive" correctly assigns the result of the if expression to a variable. Other options have syntax errors or incorrect usage.

3. Final Answer:

   val result = if (x > 0) "Positive" else "Non-positive" -> Option C

4. Quick Check:

   Correct if expression syntax = B [OK]

Hint: Use parentheses and else for if expressions [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using incorrect keywords like 'then'
• Trying to assign without else branch

3. What will be the output of this Kotlin code?

val max = if (10 > 5) 10 else 5
println(max)

medium

A. 5

B. 10

C. true

D. Compilation error

Solution

1. Step 1: Evaluate the if expression condition

   The condition 10 > 5 is true, so the expression returns 10.

2. Step 2: Assign and print the value

   The variable max is assigned 10, and println(max) outputs 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Condition true returns 10 = A [OK]

Hint: Check condition result to find expression value [OK]

Common Mistakes:

• Confusing output with false branch
• Expecting boolean output
• Thinking code causes error

4. Identify the error in this Kotlin code snippet:

val result
if (x > 0) {
  result = "Positive"
} else {
  result = "Non-positive"
}
println(result)

medium

A. No error, code runs fine

B. Missing parentheses in if condition

C. Cannot assign string to 'result'

D. Variable 'result' must be initialized before use

Solution

1. Step 1: Check variable declaration and initialization

   The variable result is declared without initialization, which is not allowed for val in Kotlin.

2. Step 2: Understand Kotlin's val initialization rules

   val must be initialized at declaration or in constructor; here it is uninitialized and assigned later, causing error.

3. Final Answer:

   Variable 'result' must be initialized before use -> Option D

4. Quick Check:

   Uninitialized val causes error = D [OK]

Hint: val needs immediate initialization or constructor assignment [OK]

Common Mistakes:

• Thinking val can be assigned later
• Ignoring initialization errors
• Assuming if syntax is wrong

5. How can you rewrite this Kotlin code using expressions to make it shorter and clearer?

var status: String
if (score >= 60) {
  status = "Pass"
} else {
  status = "Fail"
}
println(status)

hard

A. val status = if (score >= 60) "Pass" else "Fail"
println(status)

B. if (score >= 60) println("Pass") else println("Fail")

C. var status = "Pass" if (score >= 60) else "Fail"
println(status)

D. val status
status = if score >= 60 then "Pass" else "Fail"
println(status)

Solution

1. Step 1: Identify the current code structure

   The code uses a statement-based if to assign status variable based on score.

2. Step 2: Use if as an expression for assignment

   Kotlin allows if to return a value, so we can assign status directly with the expression, making code shorter and clearer.

3. Final Answer:

   val status = if (score >= 60) "Pass" else "Fail" println(status) -> Option A

4. Quick Check:

   Use if expression for assignment = A [OK]

Hint: Assign if result directly to variable [OK]

Common Mistakes:

• Using invalid syntax for if expression
• Trying to assign without else branch
• Using statements instead of expressions