For loop with index (withIndex) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of for loop with withIndex on a list

What is the output of this Kotlin code snippet?

val fruits = listOf("apple", "banana", "cherry")
for ((index, fruit) in fruits.withIndex()) {
    println("$index: $fruit")
}

Kotlin

val fruits = listOf("apple", "banana", "cherry")
for ((index, fruit) in fruits.withIndex()) {
    println("$index: $fruit")
}

A

apple: 0
banana: 1
cherry: 2

B

0: apple
1: banana
2: cherry

C

apple
banana
cherry

D

0: apple
1: banana
2: cherry
3: null

💡 Hint

Remember that withIndex() provides both the index and the element in the loop.

The withIndex() function returns an iterable of IndexedValue objects, each containing an index and the corresponding element. The loop destructures these into index and fruit, printing them in order.

📝 Syntax

intermediate

Identify the syntax error in for loop with withIndex

Which option contains a syntax error in using withIndex() in a for loop?

Kotlin

val numbers = listOf(10, 20, 30)
for ((i, num) in numbers.withIndex()) {
    println("Index $i has value $num")
}

A for ((i, num) in numbers.withIndex()) { println(i + num) }

B } )mun + i(nltnirp { ))(xednIhtiw.srebmun ni )mun ,i(( rof

C for ((i, num) in numbers.withIndex()) println(i + num)

D for (i, num in numbers.withIndex()) { println(i + num) }

💡 Hint

Check how destructuring declarations are used in Kotlin for loops.

Option D misses parentheses around the destructuring variables, which is required syntax in Kotlin for destructuring in for loops.

❓ optimization

advanced

Optimizing iteration with withIndex on large lists

Given a large list, which approach is more efficient to print index and value pairs?
Options:
A) Using withIndex() in a for loop
B) Using indices property and accessing elements by index
C) Using forEachIndexed function
D) Using a while loop with manual index increment

A for ((index, value) in list.withIndex()) { println("$index: $value") }

B for (i in list.indices) { println("$i: ${list[i]}") }

C list.forEachIndexed { index, value -> println("$index: $value") }

D

var i = 0
while (i < list.size) { println("$i: ${list[i]}"); i++ }

💡 Hint

Consider readability and internal optimizations of Kotlin standard library functions.

forEachIndexed is optimized and concise, often preferred for readability and performance over manual loops or withIndex in large collections.

🔧 Debug

advanced

Debug the unexpected output in for loop with withIndex

What causes this Kotlin code to print incorrect indices?

val items = listOf("a", "b", "c")
for ((index, item) in items.withIndex()) {
    if (item == "b") continue
    println("$index: $item")
}

A The continue statement skips printing but does not skip index increment, so indices remain correct.

B The loop should use break instead of continue to avoid index issues.

C The index variable is not updated correctly because of continue, causing wrong indices.

D Using continue inside for loop with withIndex causes index to reset to 0.

💡 Hint

Think about how continue affects loop iteration and index values.

The continue statement skips the current iteration's print but the loop still increments index correctly. Indices remain consistent.

🧠 Conceptual

expert

Understanding withIndex behavior on different collections

Which statement about withIndex() in Kotlin is TRUE?

A withIndex() can be used on any Iterable and returns an IndexedValue sequence with index starting at 0.

B withIndex() modifies the original collection by adding index properties to elements.

C withIndex() returns indices and values only for List collections, not for Sets.

D withIndex() returns a Map of index to element for any collection.

💡 Hint

Recall what types of collections implement Iterable and how withIndex works.

withIndex() works on any Iterable, producing a sequence of IndexedValue objects with zero-based indices without modifying the original collection.