Map transformation in Kotlin - Step-by-Step Execution

Concept Flow - Map transformation

Start with original Map

↓

Apply transformation function

↓

Create new Map with transformed entries

↓

Return new transformed Map

We start with an original map, apply a function to each entry, and build a new map with the transformed keys and/or values.

Execution Sample

Kotlin

val original = mapOf("a" to 1, "b" to 2)
val transformed = original.mapValues { it.value * 10 }
println(transformed)

This code multiplies each value in the map by 10 and prints the new map.

Execution Table

Step	Entry (key to value)	Transformation Applied	Resulting Entry	New Map State
1	"a" to 1	1 * 10	"a" to 10	{"a"=10}
2	"b" to 2	2 * 10	"b" to 20	{"a"=10, "b"=20}
3	All entries processed	-	-	Final map: {"a"=10, "b"=20}

💡 All entries processed, transformation complete.

Variable Tracker

Variable	Start	After 1	After 2	Final
original	{"a"=1, "b"=2}	{"a"=1, "b"=2}	{"a"=1, "b"=2}	{"a"=1, "b"=2}
transformed	{}	{"a"=10}	{"a"=10, "b"=20}	{"a"=10, "b"=20}

Key Moments - 2 Insights

Why does the original map not change after transformation?

Because mapValues creates a new map with transformed values without modifying the original map, as shown in execution_table rows 1 and 2 where 'original' stays the same.

What does 'it.value * 10' mean in the transformation?

'it' refers to each map entry, and 'it.value' accesses the value part. Multiplying by 10 changes the value while keeping the key same, as seen in the transformation applied column.

Visual Quiz - 3 Questions

Look at the execution table, what is the new map state after processing the first entry?

A {"a"=1, "b"=2}

B {"b"=20}

C {"a"=10}

D {}

💡 Hint

Check the 'New Map State' column at Step 1 in the execution_table.

At which step does the transformed map contain both entries?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'New Map State' column to see when both keys appear.

If the transformation was changed to 'it.key.uppercase()', what would happen?

A Keys become uppercase, values unchanged

B No change

C Keys and values both become uppercase

D Values become uppercase, keys unchanged

💡 Hint

mapValues only changes values, keys stay the same as shown in the example.

Concept Snapshot

Map transformation in Kotlin:
- Use mapValues or mapKeys to create a new map
- mapValues changes values, keys stay same
- mapKeys changes keys, values stay same
- Original map is not modified
- Transformation function applies to each entry
- Result is a new map with transformed entries

Full Transcript

This visual execution shows how Kotlin transforms a map using mapValues. We start with an original map with keys "a" and "b" and values 1 and 2. Each value is multiplied by 10 to create a new map. The original map remains unchanged. Step by step, the new map builds up with transformed entries. This helps beginners see how map transformations work without changing the original data.

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