For loop over collections in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a for loop over a list

What is the output of this Kotlin code?

Kotlin

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

A a b c

B applebananacherry

C abc

D fruitfruitfruit

💡 Hint

Look at what the print statement outputs for each fruit.

The loop prints the first character of each fruit name without spaces, so the output is 'abc'.

❓ Predict Output

intermediate

For loop with index over a list

What will be printed by this Kotlin code?

Kotlin

val numbers = listOf(10, 20, 30)
for ((index, value) in numbers.withIndex()) {
    println("$index:$value")
}

A

0:10
1:20
2:30

B

index:value
index:value
index:value

C

10:0
20:1
30:2

D

0-10
1-20
2-30

💡 Hint

withIndex() gives pairs of index and value.

The loop prints each index and its corresponding value separated by a colon, each on a new line.

🔧 Debug

advanced

Identify the error in this for loop over a map

What error does this Kotlin code produce?

Kotlin

val map = mapOf("a" to 1, "b" to 2)
for (key, value in map) {
    println("$key -> $value")
}

A SyntaxError: Unexpected destructuring declaration

B TypeError: Cannot iterate over map

C No error, prints 'a -> 1' and 'b -> 2'

D RuntimeException: Invalid loop variable

💡 Hint

Check how to destructure map entries in Kotlin for loops.

The for loop syntax for destructuring map entries requires parentheses around the pair: for ((key, value) in map). Missing parentheses cause a syntax error.

❓ Predict Output

advanced

Output of nested for loops over collections

What is the output of this Kotlin code?

Kotlin

val list1 = listOf(1, 2)
val list2 = listOf("a", "b")
for (num in list1) {
    for (char in list2) {
        print("$num$char ")
    }
}

A 1 2 a b

B 1a 1b 2a 2b

C a1 b1 a2 b2

D 12 ab

💡 Hint

The outer loop runs over numbers, inner over characters.

The code prints each number combined with each character, separated by spaces, in order.

🧠 Conceptual

expert

Number of iterations in a for loop over a set

Given this Kotlin code, how many times will the loop run?

Kotlin

val set = setOf("x", "y", "z")
var count = 0
for (item in set) {
    count++
}
println(count)

A Cannot determine because sets are unordered

B 0

C 1

D 3

💡 Hint

Count how many items are in the set.

The set has 3 unique items, so the loop runs 3 times, incrementing count each time.

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