Flow operators (map, filter, transform) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to transform each number by doubling it using the map operator.

Kotlin

val numbers = flowOf(1, 2, 3)
val doubled = numbers.[1] { it * 2 }

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

Afilter

Bcollect

Ctransform

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using filter instead of map will only select elements, not transform them.

Using collect is for consuming the flow, not transforming it.

The map operator applies a transformation to each element in the flow.

2fill in blank

medium

Complete the code to keep only even numbers using the filter operator.

Kotlin

val numbers = flowOf(1, 2, 3, 4)
val evens = numbers.[1] { it % 2 == 0 }

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

Amap

Bfilter

Ctransform

Dcollect

Attempts:

3 left

💡 Hint

Common Mistakes

Using map will transform elements but not filter them.

Using collect is for consuming the flow, not filtering.

The filter operator keeps only elements that satisfy the condition.

3fill in blank

hard

Fix the error in the code to emit both original and doubled values using transform.

Kotlin

val numbers = flowOf(1, 2)
val result = numbers.[1] { emit(it); emit(it * 2) }

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

Atransform

Bfilter

Ccollect

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using map only emits one value per input.

Using filter does not emit new values, only filters existing ones.

The transform operator allows emitting multiple values per input.

4fill in blank

hard

Fill both blanks to create a flow of squares of odd numbers.

Kotlin

val numbers = flowOf(1, 2, 3, 4, 5)
val squaresOfOdds = numbers.[1] { it % 2 != 0 }.[2] { it * it }

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

Afilter

Bmap

Ctransform

Dcollect

Attempts:

3 left

💡 Hint

Common Mistakes

Using map before filter will square all numbers before filtering.

Using transform instead of map for simple transformation is more complex.

First, filter keeps odd numbers, then map squares them.

5fill in blank

hard

Fill all three blanks to emit strings for numbers greater than 2, doubled and converted to string.

Kotlin

val numbers = flowOf(1, 2, 3, 4)
val result = numbers.[1] { it [2] 2 }.[3] { (it * 2).toString() }

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

Afilter

B>

Cmap

Dtransform

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will select wrong numbers.

Using transform instead of map for simple transformation is unnecessary.

Filter numbers greater than 2, then map to double and convert to string.

Practice

1. Which Kotlin Flow operator is used to change each item in the flow to a new value?

easy

A. filter

B. map

C. transform

D. collect

Solution

1. Step 1: Understand the purpose of map

   The map operator changes each item in the flow to a new value by applying a function.

2. Step 2: Compare with other operators

   filter keeps items based on a condition, transform can emit multiple items, and collect gathers items but does not change them.

3. Final Answer:

   map -> Option B

4. Quick Check:

   Change each item = map [OK]

Hint: Map means change each item to a new one [OK]

Common Mistakes:

• Confusing map with filter
• Thinking transform only changes one item
• Using collect to change items

2. Which of the following is the correct syntax to filter even numbers from a Kotlin Flow named numbers?

easy

A. numbers.collect { it % 2 == 0 }

B. numbers.map { it % 2 == 0 }

C. numbers.filter { it % 2 == 0 }

D. numbers.transform { emit(it % 2 == 0) }

Solution

1. Step 1: Identify the filter syntax

   The filter operator takes a lambda returning a Boolean to keep items matching the condition.

2. Step 2: Check each option

   numbers.filter { it % 2 == 0 } correctly uses filter { it % 2 == 0 } to keep even numbers. numbers.map { it % 2 == 0 } uses map which changes items to booleans. numbers.transform { emit(it % 2 == 0) } emits booleans instead of filtering. numbers.collect { it % 2 == 0 } misuses collect.

3. Final Answer:

   numbers.filter { it % 2 == 0 } -> Option C

4. Quick Check:

   Filter even numbers = filter { it % 2 == 0 } [OK]

Hint: Filter keeps items; map changes items [OK]

Common Mistakes:

• Using map instead of filter
• Misusing collect as a filter
• Confusing transform syntax

3. What is the output of this Kotlin Flow code?

flowOf(1, 2, 3, 4)
    .filter { it % 2 == 0 }
    .map { it * 10 }
    .toList()

medium

A. [20, 40]

B. [10, 20]

C. [2, 4]

D. [1, 2, 3, 4]

Solution

1. Step 1: Apply filter to keep even numbers

   The flow starts with [1, 2, 3, 4]. After filtering even numbers, we get [2, 4].

2. Step 2: Apply map to multiply each by 10

   Mapping [2, 4] by multiplying each by 10 results in [20, 40].

3. Final Answer:

   [20, 40] -> Option A

4. Quick Check:

   Filter evens then multiply by 10 = [20, 40] [OK]

Hint: Filter first, then map to change values [OK]

Common Mistakes:

• Mapping before filtering
• Confusing output with original list
• Ignoring order of operators

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

flowOf(1, 2, 3)
    .filter { it > 1 }
    .map { it * 2 }
    .transform { emit(it + 1) }

medium

A. Using transform after map is invalid

B. The filter condition syntax is wrong

C. The emit call inside transform is incorrect

D. The code misses a terminal operator like collect

Solution

1. Step 1: Check the flow operators usage

   The operators filter, map, and transform are used correctly in sequence.

2. Step 2: Identify missing terminal operation

   Flows are cold and need a terminal operator like collect or toList to start processing. This code lacks that.

3. Final Answer:

   The code misses a terminal operator like collect -> Option D

4. Quick Check:

   Flows need terminal operator to run [OK]

Hint: Flows need collect or toList to run [OK]

Common Mistakes:

• Forgetting terminal operator
• Thinking transform can't follow map
• Misreading filter condition syntax

5. Given a Flow of integers, how can you use transform to emit both the original number and its square for each item?

hard

A. flow.transform { emit(it); emit(it * it) }

B. flow.map { listOf(it, it * it) }

C. flow.filter { emit(it); emit(it * it) }

D. flow.collect { emit(it); emit(it * it) }

Solution

1. Step 1: Understand transform usage

   transform allows emitting zero or more items per input, so emitting both original and square is possible.

2. Step 2: Check each option's correctness

   flow.transform { emit(it); emit(it * it) } correctly uses transform with two emit calls. flow.map { listOf(it, it * it) } uses map which emits a list, not separate items. flow.filter { emit(it); emit(it * it) } misuses filter. flow.collect { emit(it); emit(it * it) } misuses collect which is terminal and cannot emit.

3. Final Answer:

   flow.transform { emit(it); emit(it * it) } -> Option A

4. Quick Check:

   Emit multiple items per input = transform [OK]

Hint: Use transform to emit multiple items per input [OK]

Common Mistakes:

• Using map to emit multiple items separately
• Misusing filter or collect for emission
• Not calling emit inside transform