For loop over collections in Kotlin - Mini Project: Build & Apply

For loop over collections

📖 Scenario: You work at a small bookstore. You have a list of book titles that you want to display one by one to your customers.

🎯 Goal: Learn how to use a for loop 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 a for loop with variable book to iterate over the list

Print each book title inside the loop

Print the total number of books after the loop

💡 Why This Matters

🌍 Real World

Bookstores and libraries often need to show lists of books to customers or users. Looping through collections helps display or process each item easily.

💼 Career

Knowing how to loop over collections is a basic skill for any programmer. It helps in tasks like processing data, generating reports, or building user interfaces.

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 here
// Your code here

Hint

Use val books = listOf(...) to create the list with the exact book titles.

2

Create a counter variable

Create a variable called count and set it to 0 to count the number of books.

Kotlin

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

Hint

Use var count = 0 to create a variable that can change.

3

Use a for loop to print each book

Use a for loop with variable book to go through each item in books. Inside the loop, print the book and increase count by 1.

Kotlin

val books = listOf("The Hobbit", "1984", "Pride and Prejudice", "To Kill a Mockingbird")
var count = 0
// Write a for loop to print each book and increase count
// Your code here

Hint

Use for (book in books) { ... } to loop. Use println(book) to print each title. Use count++ to add one to count.

4

Print the total number of books

After the loop, print the text Total books: followed by the value of count using a Kotlin println statement with string interpolation.

Kotlin

val books = listOf("The Hobbit", "1984", "Pride and Prejudice", "To Kill a Mockingbird")
var count = 0
for (book in books) {
    println(book)
    count++
}
// Print the total number of books here
// Your code here

Hint

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

Practice

1. What does a for loop do when used with a collection in Kotlin?

easy

A. It sorts the collection automatically.

B. It creates a new collection from the existing one.

C. It removes items from the collection one by one.

D. It repeats a block of code for each item in the collection.

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop runs code repeatedly for each element in a collection like a list or array.

2. Step 2: Identify what happens in the loop

   Each item is accessed one by one, and the code inside the loop runs for each item.

3. Final Answer:

   It repeats a block of code for each item in the collection. -> Option D

4. Quick Check:

   For loop = repeat for each item [OK]

Hint: For loops run code once per item in a collection [OK]

Common Mistakes:

• Thinking for loop creates or removes items
• Confusing for loop with sorting or filtering
• Believing for loop runs only once

2. Which of the following is the correct syntax for a for loop over a list named items in Kotlin?

easy

A. for item in items { println(item) }

B. for (item in items) { println(item) }

C. for item : items { println(item) }

D. for (item : items) println(item)

Solution

1. Step 1: Recall Kotlin for loop syntax

   Kotlin requires parentheses around the loop variable and collection: for (item in items) { ... }

2. Step 2: Check each option

   for (item in items) { println(item) } matches the correct syntax with parentheses and braces.

3. Final Answer:

   for (item in items) { println(item) } -> Option B

4. Quick Check:

   Parentheses + in keyword = correct syntax [OK]

Hint: Use parentheses and 'in' keyword in Kotlin for loops [OK]

Common Mistakes:

• Omitting parentheses around loop header
• Using colon ':' instead of 'in'
• Missing braces for loop body

3. What is the output of this Kotlin code?

val numbers = listOf(1, 2, 3)
for (num in numbers) {
    print(num * 2)
}

medium

A. 246

B. 123

C. 6

D. Error

Solution

1. Step 1: Understand the loop and list

   The list contains 1, 2, 3. The loop goes through each number.

2. Step 2: Calculate output for each iteration

   Each number is multiplied by 2 and printed without spaces: 1*2=2, 2*2=4, 3*2=6, so output is "246".

3. Final Answer:

   246 -> Option A

4. Quick Check:

   Print doubles of 1,2,3 = 246 [OK]

Hint: Multiply and print each item in order [OK]

Common Mistakes:

• Assuming print adds spaces or newlines
• Confusing multiplication with addition
• Expecting a list output instead of concatenated string

4. Identify the error in this Kotlin code snippet:

val fruits = listOf("apple", "banana", "cherry")
for fruit in fruits {
    println(fruit)
}

medium

A. Missing parentheses around loop header

B. Using wrong collection type

C. Incorrect print statement

D. No error, code is correct

Solution

1. Step 1: Check for correct for loop syntax

   Kotlin requires parentheses around the loop header: for (fruit in fruits) { ... }

2. Step 2: Identify the missing parentheses

   The code misses parentheses around 'fruit in fruits', causing a syntax error.

3. Final Answer:

   Missing parentheses around loop header -> Option A

4. Quick Check:

   Parentheses required in Kotlin for loops [OK]

Hint: Always put parentheses around for loop header [OK]

Common Mistakes:

• Omitting parentheses in for loop
• Using wrong loop variable syntax
• Assuming print statement causes error

5. Given a list of names, how would you use a Kotlin for loop to print only names longer than 4 characters? Choose the correct code snippet.

hard

A. for (name in names) { println(name.length) > 4 }

B. for (name in names) println(name.length > 4)

C. for (name in names) { if (name.length > 4) println(name) }

D. for (name in names) { if (name > 4) println(name) }

Solution

1. Step 1: Understand the condition to filter names

   We want to print names only if their length is greater than 4 characters.

2. Step 2: Check each option for correct syntax and logic

   for (name in names) { if (name.length > 4) println(name) } correctly uses an if statement inside the loop to check length and print the name.

3. Final Answer:

   for (name in names) { if (name.length > 4) println(name) } -> Option C

4. Quick Check:

   Use if inside for to filter by length [OK]

Hint: Use if condition inside for loop to filter items [OK]

Common Mistakes:

• Comparing string directly to number
• Printing boolean instead of name
• Incorrect syntax in if or print statements