Flow builder and collect in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a flow that emits numbers from 1 to 3.

Kotlin

val numberFlow = flow {
    emit([1])
    emit(2)
    emit(3)
}

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

A0

B5

C4

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 as the first emitted value.

Using a number outside the range 1 to 3.

The emit function sends values to the flow. Here, the first emitted value should be 1.

2fill in blank

medium

Complete the code to collect and print each value from the flow.

Kotlin

runBlocking {
    numberFlow.[1] { value ->
        println(value)
    }
}

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

Aemit

Blaunch

Ccollect

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'emit' instead of 'collect'.

Using 'launch' which starts a coroutine but does not collect flow values.

The collect function is used to receive and process each value emitted by the flow.

3fill in blank

hard

Fix the error in the flow builder by completing the code to emit squares of numbers from 1 to 3.

Kotlin

val squareFlow = flow {
    for (i in 1..3) {
        emit(i [1] i)
    }
}

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication.

Using division or subtraction which do not calculate squares.

The multiplication operator * is used to calculate the square of each number.

4fill in blank

hard

Fill both blanks to create a flow that emits even numbers from 1 to 6.

Kotlin

val evenFlow = flow {
    for (num in 1..6) {
        if (num [1] 2 == 0) {
            emit(num[2]0)
        }
    }
}

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

A%

B*

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication or subtraction in the condition instead of modulo.

Modifying the number when emitting instead of emitting it as is.

The modulo operator % checks if a number is even by seeing if the remainder when divided by 2 is zero. The plus operator + is used here to emit the number as is (no change).

5fill in blank

hard

Fill all three blanks to create a flow that emits uppercase strings from a list and collects them.

Kotlin

val words = listOf("apple", "banana", "cherry")

val upperFlow = flow {
    for (word in words) {
        emit(word[1])
    }
}

runBlocking {
    upperFlow.[2] { value ->
        println(value[3])
    }
}

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

A.uppercase()

Bcollect

C.toUpperCase()

D.lowercase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase conversion instead of uppercase.

Using 'emit' instead of 'collect' to gather values.

.uppercase() converts strings to uppercase in Kotlin 1.5+. The collect function gathers emitted values. .toUpperCase() is a valid method to print the string in uppercase (older style).

Practice

1. What is the primary purpose of the flow {} builder in Kotlin?

easy

A. To create a stream of values that can be emitted over time

B. To immediately execute a block of code and return a single value

C. To collect values from a list synchronously

D. To define a suspend function without any emissions

Solution

1. Step 1: Understand the role of flow {}

   The flow {} builder creates a stream that can emit multiple values asynchronously over time.

2. Step 2: Compare with other options

   Options B, C, and D describe synchronous or non-stream behaviors, which do not match flow {} usage.

3. Final Answer:

   To create a stream of values that can be emitted over time -> Option A

4. Quick Check:

   Flow builder = stream of values [OK]

Hint: Remember: flow builds streams, not single immediate values [OK]

Common Mistakes:

• Confusing flow with suspend functions returning single values
• Thinking flow collects values instead of emitting them
• Assuming flow is synchronous

2. Which of the following is the correct syntax to collect values from a flow named myFlow inside a suspend function?

easy

A. myFlow.collect(value -> println(value))

B. myFlow.collect { println(value) }

C. myFlow.collect { value -> println(value) }

D. myFlow.collect { value => println(value) }

Solution

1. Step 1: Identify correct lambda syntax in Kotlin

   Kotlin uses curly braces with parameter and arrow: { param -> action }.

2. Step 2: Match syntax with options

   myFlow.collect { value -> println(value) } correctly uses { value -> println(value) }. Options A lacks braces, B misses parameter declaration (value undefined), D uses wrong arrow symbol (=>).

3. Final Answer:

   myFlow.collect { value -> println(value) } -> Option C

4. Quick Check:

   Correct lambda syntax = myFlow.collect { value -> println(value) } [OK]

Hint: Use { param -> action } syntax for Kotlin lambdas [OK]

Common Mistakes:

• Omitting lambda parameter name
• Using Java-style arrows (->) incorrectly
• Using parentheses instead of curly braces

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

import kotlinx.coroutines.flow.*
suspend fun main() {
    val numbers = flow {
        emit(1)
        emit(2)
        emit(3)
    }
    numbers.collect { println(it * 2) }
}

medium

A. 1
2
3

B. 2
4
6

C. 3
6
9

D. Compilation error

Solution

1. Step 1: Analyze emitted values

   The flow emits 1, 2, and 3 sequentially.

2. Step 2: Understand the collect block

   Each emitted value is multiplied by 2 and printed, so outputs are 2, 4, and 6.

3. Final Answer:

   2 4 6 -> Option B

4. Quick Check:

   Emitted values * 2 = 2,4,6 [OK]

Hint: Multiply emitted values inside collect to get output [OK]

Common Mistakes:

• Printing original values without multiplication
• Expecting a list output instead of separate prints
• Assuming compilation error due to missing coroutine context

4. Identify the error in this Kotlin flow code snippet:

val flow = flow {
    emit(1)
    emit(2)
}
flow.collect { println(it) }

medium

A. collect should be called with parentheses

B. Incorrect flow builder syntax

C. emit cannot be called inside flow builder

D. Missing suspend modifier on the function calling collect

Solution

1. Step 1: Check flow and collect usage

   The flow builder and emit calls are correct.

2. Step 2: Understand collect context

   Collect is a suspend function and must be called from a suspend function or coroutine scope. Calling it directly without suspend context causes error.

3. Final Answer:

   Missing suspend modifier on the function calling collect -> Option D

4. Quick Check:

   collect requires suspend context [OK]

Hint: Remember: collect is suspend, needs suspend function or coroutine [OK]

Common Mistakes:

• Trying to call collect outside suspend or coroutine
• Confusing emit usage inside flow
• Thinking collect syntax is wrong

5. You want to create a flow that emits only even numbers from 1 to 5 and then collect and print them. Which code snippet correctly achieves this?

hard

A. val flow = flow {
for (i in 1..5) {
if (i % 2 == 0) emit(i)
}
}
suspend fun main() {
flow.collect { println(it) }
}

B. val flow = flowOf(1, 2, 3, 4, 5).filter { it % 2 == 0 }
fun main() {
flow.collect { println(it) }
}

C. val flow = flow {
emit(2)
emit(4)
}
flow.collect { println(it) }

D. val flow = flow {
for (i in 1..5) emit(i)
}
flow.collect { if (it % 2 == 0) println(it) }

Solution

1. Step 1: Check flow emission of even numbers

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } emits only even numbers inside the flow builder using a for loop and condition.

2. Step 2: Verify collection and printing

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } collects the flow inside a suspend function and prints each emitted value correctly.

3. Final Answer:

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } -> Option A

4. Quick Check:

   Emit filtered values inside flow, collect in suspend [OK]

Hint: Filter inside flow builder to emit only needed values [OK]

Common Mistakes:

• Calling collect outside suspend function
• Filtering after emitting all values instead of inside flow
• Not using suspend function for collect