Derived state in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a derived state that holds the length of the text.

Android Kotlin

val text = remember { mutableStateOf("") }
val textLength = remember [1] {
    text.value.length
}

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

AmutableStateOf

BderivedStateOf

CrememberUpdatedState

DstateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf instead of derivedStateOf causes the length not to update automatically.

Forgetting to use remember causes recomposition issues.

The derivedStateOf function creates a state that automatically updates when its dependencies change. Here, it tracks text.value.length.

2fill in blank

medium

Complete the code to create a derived state that returns true if the text length is greater than 5.

Android Kotlin

val text = remember { mutableStateOf("") }
val isLongText = remember [1] {
    text.value.length > 5
}

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

AderivedStateOf

BmutableStateOf

CrememberUpdatedState

DstateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf will not update automatically when text.value changes.

Using rememberUpdatedState is not suitable for derived state.

derivedStateOf is used to create a state that depends on other states and updates automatically. Here, it checks if the text length is greater than 5.

3fill in blank

hard

Fix the error in the code by completing the blank to create a derived state that returns the uppercase version of the text.

Android Kotlin

val text = remember { mutableStateOf("") }
val upperText = remember [1] {
    text.value.uppercase()
}

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

AderivedStateOf

BmutableStateOf

CrememberUpdatedState

DstateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf causes the derived state not to update automatically.

Using rememberUpdatedState is not appropriate here.

derivedStateOf is the correct function to create a derived state that updates automatically when text.value changes.

4fill in blank

hard

Fill both blanks to create a derived state that returns true if the text contains the letter 'a' and its length is more than 3.

Android Kotlin

val text = remember { mutableStateOf("") }
val condition = remember [1] {
    text.value.contains('a') && text.value.length [2] 3
}

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

AderivedStateOf

B>

C<

DmutableStateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf instead of derivedStateOf.

Using the less than operator instead of greater than.

derivedStateOf creates the derived state. The condition checks if the text contains 'a' and if its length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a derived state that returns a map of words to their lengths, but only for words longer than 4 characters.

Android Kotlin

val words = listOf("apple", "bat", "carrot", "dog")
val wordLengths = remember [1] {
    words.filter { it.length [2] 4 }
        .associateWith { it.length [3] }
}

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

AderivedStateOf

B>

Ctrue

DmutableStateOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableStateOf instead of derivedStateOf.

Using less than operator instead of greater than.

Leaving the third blank empty or incorrect.

derivedStateOf creates the derived state. The filter uses the greater than operator to select words longer than 4. The associateWith maps each word to its length, so the lambda returns it.length which is a value, so true is used as a placeholder to complete syntax.

Practice

1. What is the main purpose of derivedStateOf in Android Kotlin Compose?

easy

A. To create a value that automatically updates when its dependencies change

B. To store data permanently on the device

C. To launch a new screen in the app

D. To handle user input events

Solution

1. Step 1: Understand what derived state means

   Derived state is a value computed from other state values that updates automatically when those values change.

2. Step 2: Identify the role of derivedStateOf

   derivedStateOf creates such a derived value in Compose, so the UI stays in sync without extra storage.

3. Final Answer:

   To create a value that automatically updates when its dependencies change -> Option A

4. Quick Check:

   Derived state = auto-updating computed value [OK]

Hint: Derived state updates automatically from other states [OK]

Common Mistakes:

• Confusing derived state with permanent storage
• Thinking it triggers navigation
• Assuming it handles user input directly

2. Which of the following is the correct syntax to create a derived state in Jetpack Compose?

easy

A. val derivedValue = mutableStateOf { someState.value * 2 }

B. val derivedValue = remember { mutableStateOf(0) }

C. val derivedValue = state { someState.value * 2 }

D. val derivedValue = derivedStateOf { someState.value * 2 }

Solution

1. Step 1: Recall the correct function for derived state

   The function derivedStateOf is used to create derived state in Compose.

2. Step 2: Check the syntax correctness

   val derivedValue = derivedStateOf { someState.value * 2 } uses derivedStateOf with a lambda returning a computed value, which is correct syntax.

3. Final Answer:

   val derivedValue = derivedStateOf { someState.value * 2 } -> Option D

4. Quick Check:

   Use derivedStateOf with a lambda for derived state [OK]

Hint: Look for derivedStateOf with a lambda block [OK]

Common Mistakes:

• Using mutableStateOf instead of derivedStateOf
• Forgetting the lambda braces {}
• Using state instead of derivedStateOf

3. Given the code below, what will be the output of derivedValue.value after count.value = 5?

val count = mutableStateOf(0)
val derivedValue = derivedStateOf { count.value * 3 }
count.value = 5
println(derivedValue.value)

medium

A. 15

B. 5

C. 0

D. Error at runtime

Solution

1. Step 1: Understand the initial states

   Initially, count.value is 0, so derivedValue.value is 0 * 3 = 0.

2. Step 2: Update count and check derived value

   After setting count.value = 5, derivedValue.value recalculates as 5 * 3 = 15 automatically.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   Derived state recalculates on dependency change [OK]

Hint: Derived state updates instantly when dependencies change [OK]

Common Mistakes:

• Assuming derivedValue does not update after count changes
• Confusing derivedValue with count.value
• Expecting runtime error due to state change

4. Identify the error in the following code that tries to create a derived state:

val count = mutableStateOf(10)
val derivedValue = derivedStateOf(count.value * 2)

medium

A. mutableStateOf cannot be used with integers

B. derivedValue should be mutableStateOf, not derivedStateOf

C. derivedStateOf requires a lambda, not a direct value

D. No error, code is correct

Solution

1. Step 1: Check the usage of derivedStateOf

   derivedStateOf expects a lambda expression to compute the value lazily and track dependencies.

2. Step 2: Identify the mistake in the code

   The code passes count.value * 2 directly, which evaluates immediately, not inside a lambda.

3. Final Answer:

   derivedStateOf requires a lambda, not a direct value -> Option C

4. Quick Check:

   derivedStateOf needs a lambda block {} [OK]

Hint: Always use curly braces {} with derivedStateOf [OK]

Common Mistakes:

• Passing direct value instead of lambda
• Confusing mutableStateOf and derivedStateOf
• Assuming no error in syntax

5. You want to display a message in your Compose UI that shows "Even" or "Odd" depending on a number state. Which is the best way to implement this using derived state?

val number = mutableStateOf(0)
// Your code here
Text(text = evenOdd.value)

hard

A. val evenOdd = derivedStateOf(number.value % 2)

B. val evenOdd = derivedStateOf { if (number.value % 2 == 0) "Even" else "Odd" }

C. val evenOdd = remember { mutableStateOf("Even") }

D. val evenOdd = mutableStateOf(if (number.value % 2 == 0) "Even" else "Odd")

Solution

1. Step 1: Understand the requirement for dynamic text

   The text must update automatically when number.value changes, showing "Even" or "Odd".

2. Step 2: Choose the correct derived state implementation

   val evenOdd = derivedStateOf { if (number.value % 2 == 0) "Even" else "Odd" } uses derivedStateOf with a lambda that returns "Even" or "Odd" based on number.value, which updates automatically.

3. Final Answer:

   val evenOdd = derivedStateOf { if (number.value % 2 == 0) "Even" else "Odd" } -> Option B

4. Quick Check:

   Use derivedStateOf with conditional logic for dynamic UI text [OK]

Hint: Use derivedStateOf with if-else for dynamic text [OK]

Common Mistakes:

• Using mutableStateOf without updating on number change
• Forgetting to use a lambda in derivedStateOf
• Using derivedStateOf incorrectly without returning string