For loop with index (withIndex) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the withIndex() function do in a Kotlin for loop?

It allows you to loop over a collection while accessing both the index and the value of each element at the same time.

beginner

How do you access the index and value inside a for loop using withIndex()?

You can use destructuring syntax: for ((index, value) in collection.withIndex()) where index is the position and value is the element.

intermediate

Why is using withIndex() better than manually managing an index variable in a loop?

Because it is cleaner, less error-prone, and Kotlin handles the index automatically, so you don’t have to update it yourself.

beginner

Write a simple Kotlin for loop using withIndex() to print each index and value from a list of fruits.

Example:
val fruits = listOf("Apple", "Banana", "Cherry")
for ((index, fruit) in fruits.withIndex()) {
println("Fruit at index $index is $fruit")
}

beginner

Can withIndex() be used with arrays and other collections in Kotlin?

Yes, withIndex() works with arrays, lists, and any iterable collection to provide index-value pairs.

What does withIndex() return when called on a list?

A Only the values of the list

B Only the indexes of the list

C A sequence of index-value pairs

D A reversed list

withIndex() returns a sequence of pairs where each pair contains the index and the corresponding value from the list.

How do you write a for loop to get both index and value from a list items?

A for (item in items.withIndex())

B for ((index, item) in items.withIndex())

C for (index, item in items)

D for (index in items)

Option B uses destructuring to get both index and value from withIndex().

Which of these is a benefit of using withIndex() in Kotlin loops?

A It filters out null values

B It automatically sorts the list

C It converts the list to a map

D It provides both index and value without manual index tracking

withIndex() helps you get index and value easily without managing an index variable yourself.

What happens if you use for (index in list.withIndex()) without destructuring?

A You get a pair object containing index and value

B You get the index only

C You get the value only

D You get a compile error

Without destructuring, the loop variable is a pair containing both index and value.

Can withIndex() be used with arrays in Kotlin?

A Yes, arrays and other iterable collections

B Only with maps

C No, only lists

D Only with strings

withIndex() works with arrays, lists, and other iterable collections.

Explain how to use withIndex() in a Kotlin for loop to access both index and value.

Think about how you get both position and element in the loop.

Why is using withIndex() preferred over manually incrementing an index variable in loops?

Consider what problems manual index management can cause.

Practice

1. What does the withIndex() function do in a Kotlin for loop?

easy

A. It filters elements based on their index.

B. It sorts the list before looping.

C. It provides both the index and the value of each element during iteration.

D. It reverses the list before looping.

Solution

1. Step 1: Understand the purpose of withIndex()

   The withIndex() function returns an iterable of pairs containing the index and the value of each element in the collection.

2. Step 2: Identify what the loop receives

   Using withIndex() in a for loop allows access to both the position (index) and the element (value) simultaneously.

3. Final Answer:

   It provides both the index and the value of each element during iteration. -> Option C

4. Quick Check:

   withIndex() = index + value [OK]

Hint: Remember: withIndex() gives index and value together [OK]

Common Mistakes:

• Thinking it sorts or filters the list
• Assuming it reverses the collection
• Confusing index with value only

2. Which of the following is the correct syntax to loop over a list items with index in Kotlin?

easy

A. for ((index, value) in items.withIndex()) { /* code */ }

B. for (index, value in items) { /* code */ }

C. for (index; value in items.withIndex()) { /* code */ }

D. for index, value in items.withIndex() { /* code */ }

Solution

1. Step 1: Recall Kotlin destructuring syntax in loops

   Kotlin allows destructuring pairs using parentheses in the for loop header, like for ((index, value) in ...).

2. Step 2: Match syntax with withIndex()

   The correct syntax uses items.withIndex() and destructures the pair into index and value.

3. Final Answer:

   for ((index, value) in items.withIndex()) { /* code */ } -> Option A

4. Quick Check:

   Destructure pairs with (index, value) [OK]

Hint: Use parentheses to destructure pairs in for loop [OK]

Common Mistakes:

• Omitting parentheses around (index, value)
• Using semicolon instead of comma
• Missing 'in' keyword

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

val data = listOf("apple", "banana", "cherry")
for ((i, v) in data.withIndex()) {
    println("$i: $v")
}

medium

A. 0: apple
1: banana
2: cherry

B. apple: 0
banana: 1
cherry: 2

C. 1: apple
2: banana
3: cherry

D. Error: cannot destructure

Solution

1. Step 1: Understand the list and loop

   The list contains three fruits: "apple", "banana", "cherry". The loop uses withIndex() to get index and value.

2. Step 2: Trace the loop output

   Each iteration prints the index and value separated by colon and space. Indices start at 0.

3. Final Answer:

   0: apple 1: banana 2: cherry -> Option A

4. Quick Check:

   Index starts at 0, prints index: value [OK]

Hint: Index starts at 0 in withIndex() loops [OK]

Common Mistakes:

• Assuming index starts at 1
• Swapping index and value in output
• Expecting error from destructuring

4. Identify the error in this Kotlin code snippet:

val numbers = arrayOf(10, 20, 30)
for (i, num in numbers.withIndex()) {
    println("$i -> $num")
}

medium

A. Cannot use withIndex() on arrays.

B. Missing parentheses around (i, num) in the for loop.

C. The println statement syntax is incorrect.

D. Variable names i and num are reserved keywords.

Solution

1. Step 1: Check for correct destructuring syntax

   In Kotlin, destructuring pairs in a for loop requires parentheses: for ((i, num) in ...).

2. Step 2: Verify other parts of the code

   Using withIndex() on arrays is valid. The println syntax and variable names are correct.

3. Final Answer:

   Missing parentheses around (i, num) in the for loop. -> Option B

4. Quick Check:

   Destructure pairs with parentheses [OK]

Hint: Always use parentheses to destructure pairs in for loops [OK]

Common Mistakes:

• Omitting parentheses in destructuring
• Thinking withIndex() doesn't work on arrays
• Misreading println syntax

5. Given a list of database table names tables = listOf("users", "orders", "products"), how can you create a map of index to table name using a Kotlin for loop with withIndex()?

hard

A. val map = mutableMapOf()
for (t in tables) {
map[t] = t.length
}

B. val map = mutableMapOf()
for ((i, t) in tables.withIndex()) {
map[t] = i
}

C. val map = mutableMapOf()
for (i in tables.withIndex()) {
map[i] = tables[i]
}

D. val map = mutableMapOf()
for ((i, t) in tables.withIndex()) {
map[i] = t
}

Solution

1. Step 1: Understand the goal

   We want a map where keys are indices (Int) and values are table names (String).

2. Step 2: Use withIndex() to get index and value

   Loop over tables.withIndex() and assign map[i] = t to build the map correctly.

3. Final Answer:

   val map = mutableMapOf() for ((i, t) in tables.withIndex()) { map[i] = t } -> Option D

4. Quick Check:

   Index to value mapping with withIndex() [OK]

Hint: Use withIndex() and map[i] = value to build index-value map [OK]

Common Mistakes:

• Swapping key and value types in map
• Not destructuring with parentheses
• Using wrong loop variable types