Iterating collections with forEach in Kotlin - Mini Project: Build & Apply

Iterating collections with forEach

📖 Scenario: You are helping a small bookstore manage its inventory. You have a list of book titles and want to print each title to check the stock.

🎯 Goal: Learn how to use the forEach function in Kotlin to go through each item in a list and print it.

📋 What You'll Learn

Create a list of book titles with exact values

Create a variable to count the number of books

Use forEach to print each book title

Print the total number of books after listing them

💡 Why This Matters

🌍 Real World

Bookstores and libraries often need to list and count their inventory. Using <code>forEach</code> helps to process each item easily.

💼 Career

Knowing how to iterate collections is essential for many programming jobs, especially when working with data lists or user inputs.

1

Create a list of book titles

Create a list called books with these exact titles: "The Hobbit", "1984", "Pride and Prejudice", "To Kill a Mockingbird"

Kotlin

# Create the list of books below
# Your code here

Hint

Use listOf to create a list with the given book titles.

2

Create a variable to count books

Create a variable called bookCount and set it to the size of the books list

Kotlin

val books = listOf("The Hobbit", "1984", "Pride and Prejudice", "To Kill a Mockingbird")
# Create the variable bookCount below
# Your code here

Hint

Use the size property of the list to get the number of books.

3

Use forEach to print each book title

Use books.forEach with a lambda that takes a parameter called book and prints it

Kotlin

val books = listOf("The Hobbit", "1984", "Pride and Prejudice", "To Kill a Mockingbird")
val bookCount = books.size
# Use forEach to print each book title below
# Your code here

Hint

Use forEach with a lambda that prints each book.

4

Print the total number of books

Write a println statement that prints exactly: Total books: 4 using the bookCount variable inside an f-string

Kotlin

val books = listOf("The Hobbit", "1984", "Pride and Prejudice", "To Kill a Mockingbird")
val bookCount = books.size
books.forEach { book ->
    println(book)
}
# Print the total number of books below
# Your code here

Hint

Use println("Total books: $bookCount") to show the total count.

Practice

1. What does the forEach function do when used on a list in Kotlin?

easy

A. It runs a block of code for each item in the list.

B. It creates a new list with modified items.

C. It removes items from the list.

D. It sorts the list in ascending order.

Solution

1. Step 1: Understand the purpose of forEach

   The forEach function is designed to execute a given block of code on every element in a collection.

2. Step 2: Compare options with forEach behavior

   Only It runs a block of code for each item in the list. correctly describes this behavior. Options B, C, and D describe other functions like map, remove, and sort.

3. Final Answer:

   It runs a block of code for each item in the list. -> Option A

4. Quick Check:

   forEach = run code on each item [OK]

Hint: forEach runs code on every element, not changing the list [OK]

Common Mistakes:

• Confusing forEach with map that creates new lists
• Thinking forEach modifies the list itself
• Assuming forEach sorts or filters the list

2. Which of the following is the correct syntax to print each element of a list numbers using forEach in Kotlin?

easy

A. numbers.forEach -> println(it)

B. forEach(numbers) { println(it) }

C. numbers.forEach { println(it) }

D. numbers.forEach(println(it))

Solution

1. Step 1: Recall the correct forEach syntax

   In Kotlin, forEach is called on the collection with a lambda block: collection.forEach { item -> ... }. The shorthand uses it for the current item.

2. Step 2: Check each option's syntax

   numbers.forEach { println(it) } matches the correct syntax. forEach(numbers) { println(it) } wrongly uses forEach as a function with the list as argument. numbers.forEach(println(it)) incorrectly calls println inside forEach parentheses. numbers.forEach -> println(it) uses an invalid arrow syntax.

3. Final Answer:

   numbers.forEach { println(it) } -> Option C

4. Quick Check:

   Correct forEach syntax = numbers.forEach { println(it) } [OK]

Hint: Use collection.forEach { println(it) } to print items [OK]

Common Mistakes:

• Calling forEach as a standalone function
• Passing println(it) directly inside forEach parentheses
• Using arrow (->) outside lambda block

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

val fruits = listOf("apple", "banana", "cherry")
fruits.forEach { println(it.length) }

medium

A. 5 6 6

B. 5
6
6

C. apple banana cherry

D. 3 6 6

Solution

1. Step 1: Understand the code behavior

   The list fruits contains three strings: "apple", "banana", and "cherry". The forEach prints the length of each string.

2. Step 2: Calculate lengths and output format

   "apple" has length 5, "banana" has length 6, "cherry" has length 6. Each println prints on a new line, so output is: 5 6 6

3. Final Answer:

   5 6 6 -> Option B

4. Quick Check:

   Print lengths with println = lines of 5,6,6 [OK]

Hint: println prints each item on a new line [OK]

Common Mistakes:

• Assuming output is on one line separated by spaces
• Confusing string length with number of vowels
• Printing the strings instead of their lengths

4. Identify the error in this Kotlin code snippet:

val nums = listOf(1, 2, 3)
nums.forEach(i -> println(i))

medium

A. Calling println inside forEach is not allowed

B. Missing parentheses after forEach

C. Using val instead of var for list

D. Using arrow (->) instead of curly braces for lambda

Solution

1. Step 1: Check lambda syntax in Kotlin

   Kotlin uses curly braces {} for lambdas, not arrow notation like -> outside braces. The arrow is used inside the lambda to separate parameters from body.

2. Step 2: Identify the incorrect usage

   The code uses i -> println(i) directly after forEach without braces, which is invalid syntax. Correct form is nums.forEach { i -> println(i) }.

3. Final Answer:

   Using arrow (->) instead of curly braces for lambda -> Option D

4. Quick Check:

   Lambda needs braces {} around arrow -> [OK]

Hint: Use { param -> code } for lambdas, not (param -> code) [OK]

Common Mistakes:

• Using arrow notation without braces
• Confusing Kotlin lambda syntax with other languages
• Thinking val vs var affects forEach usage

5. Given a list of integers nums, how can you use forEach to print only the even numbers?

hard

A. nums.forEach { if (it % 2 == 0) println(it) }

B. nums.forEach { println(it % 2 == 0) }

C. nums.forEach { if (it / 2 == 0) println(it) }

D. nums.forEach { println(it) if (it % 2 == 0) }

Solution

1. Step 1: Understand the condition for even numbers

   An even number has zero remainder when divided by 2, so it % 2 == 0 checks if a number is even.

2. Step 2: Use if inside forEach to print only even numbers

   nums.forEach { if (it % 2 == 0) println(it) } correctly uses an if statement inside the lambda to print only when the condition is true. Other options either print booleans or have incorrect syntax.

3. Final Answer:

   nums.forEach { if (it % 2 == 0) println(it) } -> Option A

4. Quick Check:

   Use if inside forEach to filter printing [OK]

Hint: Use if (it % 2 == 0) inside forEach to print evens [OK]

Common Mistakes:

• Printing boolean results instead of numbers
• Using division instead of modulo for even check
• Placing if after println causing syntax error