Why collection operations replace loops in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter a list of numbers to keep only even numbers.

Kotlin

val evens = numbers.[1] { it % 2 == 0 }

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

Afilter

Bmap

Creduce

DforEach

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of filter returns transformed elements, not filtered ones.

The filter function selects elements matching the condition.

2fill in blank

medium

Complete the code to transform a list of strings to their lengths.

Kotlin

val lengths = words.[1] { it.length }

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

Afilter

BforEach

Cfold

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using filter returns a subset, not transformed elements.

The map function transforms each element to another value.

3fill in blank

hard

Fix the error in the code to sum all numbers in the list.

Kotlin

val total = numbers.[1] { acc, num -> acc + num }

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

AforEach

Bmap

Creduce

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using map returns a list, not a single combined value.

The reduce function combines elements using the lambda to produce a single result.

4fill in blank

hard

Fill both blanks to create a map of words to their uppercase forms, filtering only words longer than 3 letters.

Kotlin

val result = words.[1] { it.length > 3 }.[2] { it to it.uppercase() }

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

Afilter

Bmap

Creduce

DforEach

Attempts:

3 left

💡 Hint

Common Mistakes

Using map before filter changes all elements before filtering.

First, filter keeps words longer than 3 letters, then map transforms each word to a pair of the word and its uppercase.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths, filtering words starting with 'a'.

Kotlin

val result = words.[1] { it.startsWith("a") }.[2] { it.uppercase() }.[3] { it to it.length }

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

Afilter

Bmap

Cassociate

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Using reduce instead of associate does not create a map.

First, filter selects words starting with 'a', then map converts them to uppercase, and finally associate creates a map from uppercase words to their lengths.

Practice

1. Why do Kotlin collection operations like map and filter often replace traditional loops?

easy

A. They make the code simpler and easier to read.

B. They run faster than any loop.

C. They require less memory than loops.

D. They only work with numbers.

Solution

1. Step 1: Understand the purpose of collection operations

   Collection operations like map and filter apply actions to each item in a collection in a clear way.

2. Step 2: Compare with traditional loops

   Traditional loops can be longer and harder to read, while collection operations simplify the code.

3. Final Answer:

   They make the code simpler and easier to read. -> Option A

4. Quick Check:

   Collection operations simplify code = A [OK]

Hint: Collection operations simplify and clarify loops [OK]

Common Mistakes:

• Thinking collection operations always run faster
• Believing they use less memory by default
• Assuming they only work with numbers

2. Which of the following is the correct Kotlin syntax to replace a loop that prints each item in a list items using a collection operation?

easy

A. items.printEach()

B. items.forEach { println(it) }

C. forEach(items) { println(it) }

D. items.each { println(it) }

Solution

1. Step 1: Recall the correct collection operation syntax

   The forEach function is called on the collection like items.forEach { ... }.

2. Step 2: Check each option

   items.forEach { println(it) } uses correct syntax. Options A, B, and D are invalid Kotlin syntax.

3. Final Answer:

   items.forEach { println(it) } -> Option B

4. Quick Check:

   Correct forEach syntax = C [OK]

Hint: Remember: collection.forEach { action } [OK]

Common Mistakes:

• Using forEach as a standalone function
• Calling non-existent methods like printEach or each
• Incorrect placement of braces

3. What is the output of this Kotlin code?

val numbers = listOf(1, 2, 3, 4)
val result = numbers.filter { it % 2 == 0 }.map { it * 10 }
println(result)

medium

A. [20, 40]

B. [10, 20]

C. [2, 4]

D. [1, 2, 3, 4]

Solution

1. Step 1: Apply the filter operation

   The filter keeps only even numbers: 2 and 4.

2. Step 2: Apply the map operation

   Each filtered number is multiplied by 10: 2*10=20, 4*10=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 transform [OK]

Common Mistakes:

• Confusing filter and map order
• Expecting original list output
• Multiplying before filtering

4. Identify the error in this Kotlin code that tries to print all names in a list:

val names = listOf("Anna", "Bob", "Cara")
names.map { println(it) }

medium

A. The code is correct and prints all names.

B. The list is empty, so nothing prints.

C. println cannot be used inside map.

D. map is used incorrectly; forEach should be used for printing.

Solution

1. Step 1: Understand the purpose of map

   map transforms each item and returns a new list; it is not meant for side effects like printing.

2. Step 2: Identify correct operation for printing

   forEach is designed for performing actions like printing on each item without creating a new list.

3. Final Answer:

   map is used incorrectly; forEach should be used for printing. -> Option D

4. Quick Check:

   Use forEach for actions, map for transformations [OK]

Hint: Use forEach for actions, map for creating new lists [OK]

Common Mistakes:

• Using map for side effects like printing
• Assuming map prints automatically
• Confusing map and forEach purposes

5. Given a list of user ages ages = listOf(12, 17, 20, 25, 30), which Kotlin collection operation chain correctly returns a list of ages over 18 multiplied by 2?

hard

A. ages.map { it * 2 }.filter { it > 18 }

B. ages.filter { it * 2 > 18 }

C. ages.filter { it > 18 }.map { it * 2 }

D. ages.map { if (it > 18) it * 2 else it }

Solution

1. Step 1: Filter ages over 18

   We first keep only ages greater than 18: 20, 25, 30.

2. Step 2: Multiply filtered ages by 2

   Then multiply each by 2: 40, 50, 60.

3. Final Answer:

   ages.filter { it > 18 }.map { it * 2 } -> Option C

4. Quick Check:

   Filter then map for correct result [OK]

Hint: Filter first, then map to transform [OK]

Common Mistakes:

• Mapping before filtering changes values incorrectly
• Filtering after mapping filters wrong values
• Using map with conditional without filtering