Map transformation in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Map transformation in Kotlin?

A Map transformation in Kotlin means creating a new Map by changing the keys, values, or both from an existing Map without modifying the original one.

beginner

How do you transform only the values of a Map in Kotlin?

You use the mapValues function, which applies a transformation to each value while keeping the keys the same.

beginner

What does the mapKeys function do in Kotlin?

The mapKeys function creates a new Map by transforming the keys of the original Map while keeping the values unchanged.

beginner

Show a simple example of transforming a Map's values by doubling them.

Given val map = mapOf("a" to 1, "b" to 2),
val doubled = map.mapValues { it.value * 2 }
Result: {"a"=2, "b"=4}

intermediate

Can you transform both keys and values of a Map at the same time in Kotlin?

Yes, by using the map function on the Map's entries and then converting the result back to a Map with toMap().

Which Kotlin function transforms only the values of a Map?

A mapKeys

B mapValues

C map

D filter

The mapValues function changes only the values, keeping keys the same.

What does mapKeys do?

A Filters keys

B Transforms values only

C Transforms both keys and values

D Transforms keys only

mapKeys changes keys but keeps values unchanged.

How do you transform both keys and values of a Map?

A Use mapValues and mapKeys together

B Use filter

C Use map on entries and then toMap()

D Use reduce

Using map on entries lets you change both keys and values, then toMap() converts back to a Map.

What is the result of mapOf("x" to 3).mapValues { it.value + 1 }?

A {"x"=4}

B {"x"=3}

C {"x"=1}

D {}

The value 3 is increased by 1, resulting in 4.

Which function keeps the original Map unchanged and returns a new transformed Map?

A Both A and C

B mapValues

C mapKeys

D None

Both mapValues and mapKeys return new Maps without changing the original.

Explain how to transform only the values of a Map in Kotlin and give a simple example.

Think about a function that changes values but keeps keys the same.

Describe how to transform both keys and values of a Map in Kotlin.

Use map on entries and then convert back to Map.

Practice

1. What does the map function do when applied to a list in Kotlin?

easy

A. It removes elements from the list based on a condition.

B. It creates a new list by applying a transformation to each element.

C. It sorts the list in ascending order.

D. It changes the original list by modifying its elements.

Solution

1. Step 1: Understand the purpose of map

   The map function applies a given transformation to each element of the list.

2. Step 2: Check the effect on the list

   It returns a new list with transformed elements without changing the original list.

3. Final Answer:

   It creates a new list by applying a transformation to each element. -> Option B

4. Quick Check:

   map transforms each item and returns new list [OK]

Hint: Remember: map returns a new list, original stays same [OK]

Common Mistakes:

• Thinking map modifies the original list
• Confusing map with filter or sort
• Assuming map removes elements

2. Which of the following is the correct syntax to transform a list of integers by doubling each value using map in Kotlin?

easy

A. val result = list.map { it * 2 }

B. val result = list.map(it * 2)

C. val result = list.map -> it * 2

D. val result = list.map { it + 2 }

Solution

1. Step 1: Identify correct lambda syntax

   The map function requires a lambda inside curly braces: { it * 2 }.

2. Step 2: Check the transformation logic

   Doubling means multiplying by 2, so it * 2 is correct.

3. Final Answer:

   val result = list.map { it * 2 } -> Option A

4. Quick Check:

   Correct lambda syntax with doubling [OK]

Hint: Use curly braces and 'it' for lambda in map [OK]

Common Mistakes:

• Missing curly braces around lambda
• Using parentheses instead of braces
• Wrong operation inside lambda

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

val numbers = listOf(1, 2, 3)
val result = numbers.map { it + 5 }
println(result)

medium

A. [6, 7, 8]

B. [1, 2, 3, 5]

C. [5, 6, 7]

D. [1, 2, 3]

Solution

1. Step 1: Apply the map transformation

   Each element in numbers is increased by 5: 1+5=6, 2+5=7, 3+5=8.

2. Step 2: Form the new list

   The new list is [6, 7, 8], which is printed.

3. Final Answer:

   [6, 7, 8] -> Option A

4. Quick Check:

   Each number +5 = [6, 7, 8] [OK]

Hint: Add 5 to each element inside map to get output [OK]

Common Mistakes:

• Confusing original list with transformed list
• Adding 5 only once instead of to each element
• Misreading the output format

4. Identify the error in this Kotlin code snippet:

val list = listOf(1, 2, 3)
val newList = list.map(it * 3)
println(newList)

medium

A. The multiplication operator is incorrect.

B. Using map on a list is not allowed.

C. The list should be mutable to use map.

D. Missing curly braces around the lambda expression.

Solution

1. Step 1: Check lambda syntax in map

   The map function requires a lambda inside curly braces, but here parentheses are used without braces.

2. Step 2: Confirm correct usage

   Correct syntax is list.map { it * 3 } to apply multiplication.

3. Final Answer:

   Missing curly braces around the lambda expression. -> Option D

4. Quick Check:

   Lambda needs braces in map [OK]

Hint: Always use curly braces for lambda in map [OK]

Common Mistakes:

• Using parentheses instead of braces for lambda
• Thinking map needs mutable list
• Misunderstanding operator usage

5. Given a list of strings val words = listOf("apple", "banana", "cherry"), how can you create a map where each word is a key and its length is the value using Kotlin's map transformation?

hard

A. val wordLengths = words.associateWith { it.length }

B. val wordLengths = words.map { it.length to it }

C. val wordLengths = words.map { it to it.length } .toMap()

D. val wordLengths = words.map { it.length }

Solution

1. Step 1: Understand the goal

   We want a map where keys are words and values are their lengths.

2. Step 2: Use map to create pairs and convert to map

   words.map { it to it.length } creates pairs (word, length). Then toMap() converts the list of pairs to a map.

3. Final Answer:

   val wordLengths = words.map { it to it.length } .toMap() -> Option C

4. Quick Check:

   Map pairs then convert to map [OK]

Hint: Use map to pairs and then toMap() for map creation [OK]

Common Mistakes:

• Using associateWith is correct but not the asked map transformation
• Mapping only lengths returns list, not map
• Swapping key and value in pairs