Iterating collections with forEach in Kotlin - Time & Space Complexity

Time Complexity: Iterating collections with forEach

O(n)

Understanding Time Complexity

When we use forEach to go through a collection, we want to know how the time it takes changes as the collection grows.

We ask: How does the work increase when the list gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

val numbers = listOf(1, 2, 3, 4, 5)
numbers.forEach { number ->
    println(number)
}
    

This code goes through each number in the list and prints it out.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The forEach loop visits each item once.
• How many times: Exactly once per item in the list.

How Execution Grows With Input

As the list gets bigger, the number of print actions grows the same way.

Input Size (n)	Approx. Operations
10	10 print actions
100	100 print actions
1000	1000 print actions

Pattern observation: The work grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line with the list size.

Common Mistake

[X] Wrong: "Using forEach is slower than a regular for loop because it has extra overhead."

[OK] Correct: Both do the same number of steps over the list. The difference is tiny and does not change how time grows with list size.

Interview Connect

Understanding how forEach scales helps you explain your code choices clearly and shows you know how loops affect performance.

Self-Check

"What if we nested a forEach inside another forEach? How would the time complexity change?"

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