For loop over collections in Kotlin

Introduction

A for loop helps you repeat actions for each item in a list or collection. It makes working with groups of things easy and organized.

When you want to print each name in a list of friends.

When you need to add up all numbers in a collection.

When you want to check each item in a shopping cart.

When you want to change or use every element in a list one by one.

Syntax

Kotlin

for (item in collection) {
    // do something with item
}

item is a temporary name for each element as you loop.

collection can be a list, set, or any group of items.

Examples

This prints each fruit name from the list.

Kotlin

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

This prints each number doubled.

Kotlin

val numbers = listOf(1, 2, 3, 4)
for (num in numbers) {
    println(num * 2)
}

This prints each character in uppercase.

Kotlin

val chars = setOf('a', 'b', 'c')
for (ch in chars) {
    println(ch.uppercaseChar())
}

Sample Program

This program goes through each animal in the list and prints a loving message.

Kotlin

fun main() {
    val animals = listOf("dog", "cat", "bird")
    for (animal in animals) {
        println("I love my $animal")
    }
}

Important Notes

You can use for loops with any collection like lists, sets, or arrays.

Inside the loop, you can use the current item to do calculations, print, or change values.

Remember, the loop runs once for each item in the collection.

Summary

A for loop repeats actions for every item in a collection.

Use it to easily work with lists, sets, or arrays.

It helps avoid writing repetitive code for each item.

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