Array creation and usage in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code?

Consider the following Kotlin code snippet. What will it print when run?

Kotlin

val arr = arrayOf(1, 2, 3, 4)
println(arr[2])

A 3

B 2

C 4

D IndexOutOfBoundsException

💡 Hint

Remember that Kotlin arrays are zero-indexed.

Arrays in Kotlin start at index 0, so arr[2] accesses the third element, which is 3.

❓ Predict Output

intermediate

What does this Kotlin code print?

Look at this Kotlin code. What is the output?

Kotlin

val arr = IntArray(3) { it * 2 }
println(arr.joinToString(","))

A 0,2,4

B 1,2,3

C 0,1,2

D 2,4,6

💡 Hint

The lambda uses the index to calculate each element.

IntArray(3) creates an array of size 3. Each element is set to index * 2, so elements are 0, 2, 4.

❓ Predict Output

advanced

What is the output of this Kotlin code with array mutation?

What will this Kotlin code print?

Kotlin

val arr = arrayOf("a", "b", "c")
arr[1] = "z"
println(arr.joinToString())

A a, b, c

B a, z, c

C a, z, b

D Compilation error

💡 Hint

Arrays in Kotlin are mutable by default.

The second element is changed from 'b' to 'z', so the array prints 'a, z, c'.

❓ Predict Output

advanced

What error does this Kotlin code produce?

What happens when this Kotlin code runs?

Kotlin

val arr = arrayOf(1, 2, 3)
println(arr[3])

A Prints 3

B Prints null

C ArrayIndexOutOfBoundsException

D Compilation error

💡 Hint

Check the valid indices for the array.

The array has indices 0,1,2. Accessing arr[3] is out of bounds and throws an exception.

🧠 Conceptual

expert

How many elements does this Kotlin array contain?

Given this Kotlin code, how many elements are in the array 'arr'?

Kotlin

val arr = Array(5) { i -> i * i }

A 0

B 4

C 6

D 5

💡 Hint

The first argument to Array() is the size.

Array(5) creates an array with 5 elements indexed 0 to 4.

Practice

1. What is the correct way to create an array of integers with values 1, 2, and 3 in Kotlin?

easy

A. val arr = Array(3) { 1, 2, 3 }

B. val arr = arrayOf(1, 2, 3)

C. val arr = IntArray(1, 2, 3)

D. val arr = [1, 2, 3]

Solution

1. Step 1: Understand array creation syntax

   Kotlin uses arrayOf() to create an array with given elements.

2. Step 2: Check each option

   val arr = arrayOf(1, 2, 3) uses arrayOf(1, 2, 3) which is correct. val arr = [1, 2, 3] uses square brackets which is invalid in Kotlin. val arr = IntArray(1, 2, 3) which uses invalid constructor arguments for IntArray. val arr = Array(3) { 1, 2, 3 } tries to pass multiple values in a lambda which is incorrect.

3. Final Answer:

   val arr = arrayOf(1, 2, 3) -> Option B

4. Quick Check:

   Use arrayOf() for arrays with elements [OK]

Hint: Use arrayOf() to create arrays with elements [OK]

Common Mistakes:

• Using square brackets [] like in other languages
• Confusing IntArray with Array<Int>
• Trying to pass multiple values in Array constructor lambda

2. Which of the following is the correct syntax to create an array of size 5 with all elements initialized to zero in Kotlin?

easy

A. val arr = Array(5, 0)

B. val arr = Array(0, 0, 0, 0, 0)

C. val arr = Array(5) { 0 }

D. val arr = IntArray(5, 0)

Solution

1. Step 1: Understand Array constructor usage

   Kotlin's Array(size) { init } creates an array of given size, initializing each element with the lambda result.

2. Step 2: Evaluate options

   val arr = Array(5) { 0 } correctly uses Array(5) { 0 } to create an array of 5 zeros. val arr = Array(0, 0, 0, 0, 0) is invalid syntax. val arr = Array(5, 0) is invalid syntax. val arr = IntArray(5, 0) is invalid because IntArray constructor takes only size or size with init lambda, not two parameters.

3. Final Answer:

   val arr = Array(5) { 0 } -> Option C

4. Quick Check:

   Use Array(size) { value } to initialize [OK]

Hint: Use Array(size) with lambda to initialize all elements [OK]

Common Mistakes:

• Passing two parameters to Array constructor incorrectly
• Confusing IntArray constructor parameters
• Passing multiple values directly to Array constructor

3. What will be the output of the following Kotlin code?

val arr = arrayOf(10, 20, 30)
println(arr[1])

medium

A. 20

B. 10

C. 30

D. IndexOutOfBoundsException

Solution

1. Step 1: Understand array indexing

   Arrays in Kotlin are zero-indexed, so arr[0] is 10, arr[1] is 20, and arr[2] is 30.

2. Step 2: Check the printed value

   The code prints arr[1], which is 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Array index 1 holds 20 [OK]

Hint: Remember arrays start at index 0 in Kotlin [OK]

Common Mistakes:

• Confusing index 1 with first element
• Expecting 30 instead of 20
• Thinking arrays start at 1

4. Identify the error in the following Kotlin code snippet:

val arr = arrayOf(1, 2, 3)
arr[3] = 4
println(arr.joinToString())

medium

A. arr is immutable, cannot assign new value

B. No error, output is '1, 2, 3, 4'

C. Syntax error in array declaration

D. IndexOutOfBoundsException at arr[3] assignment

Solution

1. Step 1: Check array size and indexing

   The array has 3 elements with indices 0, 1, 2. Accessing index 3 is out of bounds.

2. Step 2: Understand runtime behavior

   Assigning arr[3] = 4 causes an IndexOutOfBoundsException at runtime.

3. Final Answer:

   IndexOutOfBoundsException at arr[3] assignment -> Option D

4. Quick Check:

   Array indices must be within 0 to size-1 [OK]

Hint: Array indices go from 0 to size-1 only [OK]

Common Mistakes:

• Trying to assign value outside array bounds
• Thinking array size auto-expands
• Confusing immutability with index errors

5. You want to create an array of size 4 where each element is its index squared (0, 1, 4, 9). Which Kotlin code correctly does this?

hard

A. val arr = Array(4) { index -> index * index }

B. val arr = arrayOf(4) { index -> index * index }

C. val arr = intArrayOf(4) { index -> index * index }

D. val arr = Array(4) { -> it * it }

Solution

1. Step 1: Understand array initialization with lambda

   Kotlin's Array(size) { index -> value } creates an array where each element is initialized by the lambda using the index.

2. Step 2: Check each option

   val arr = Array(4) { index -> index * index } uses explicit lambda parameter index and correctly computes index squared. val arr = arrayOf(4) { index -> index * index } is invalid because arrayOf does not take a size and lambda. val arr = intArrayOf(4) { index -> index * index } is invalid syntax for the intArrayOf factory function. val arr = Array(4) { -> it * it } has invalid lambda syntax where it is undefined.

3. Final Answer:

   val arr = Array(4) { index -> index * index } -> Option A

4. Quick Check:

   Use Array(size) with index lambda for computed values [OK]

Hint: Use Array(size) with index lambda for computed elements [OK]

Common Mistakes:

• Confusing it with index in lambda
• Using arrayOf() for dynamic initialization
• Not using lambda for element computation