For loop over collections in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a for loop when used with collections in Kotlin?

A for loop lets you go through each item in a collection one by one, so you can work with each element easily.

beginner

How do you write a basic for loop to print all elements in a list named fruits?

You write:

for (fruit in fruits) {
    println(fruit)
}

This goes through each fruit and prints it.

beginner

Can you use a for loop with different types of collections in Kotlin?

Yes! You can use for loops with lists, sets, arrays, and other collections because they all support iteration.

intermediate

What happens if you modify a collection inside a for loop in Kotlin?

Modifying a collection while looping over it can cause errors or unexpected behavior. It's safer to avoid changing the collection during iteration.

intermediate

How can you access both the index and the value of elements in a collection using a for loop?

Use for ((index, value) in collection.withIndex()) to get both the position and the item at the same time.

Which keyword is used to loop over each element in a Kotlin collection?

A loop

B while

C for

D repeat

The for keyword is used to iterate over collections in Kotlin.

What does this code print?

val nums = listOf(1, 2, 3)
for (n in nums) {
    print(n)
}

A 1 2 3

B 123

C 3 2 1

D Error

The loop prints each number without spaces, so the output is '123'.

How do you get both index and value in a Kotlin for loop?

A for (index in collection)

B for (index, value in collection)

C for (value in collection.indices)

D for ((index, value) in collection.withIndex())

Using withIndex() lets you access both index and value.

Which collection types can you use a for loop with in Kotlin?

A Lists, Sets, Arrays

B Only Lists

C Only Arrays

D Only Sets

Kotlin supports iteration over many collection types including Lists, Sets, and Arrays.

What is a risk when modifying a collection inside a for loop?

A It can cause errors or unexpected behavior

B It speeds up the loop

C It makes the loop skip elements

D It automatically copies the collection

Changing a collection while looping can cause errors or unexpected results.

Explain how to use a for loop to iterate over a list in Kotlin and print each element.

Think about the keyword 'for' and how you name each item in the loop.

Describe how to access both the index and value of elements in a Kotlin collection during a for loop.

Use parentheses and 'in' keyword together.

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