For loop with index (withIndex) in Kotlin - Step-by-Step Execution

Concept Flow - For loop with index (withIndex)

Start with collection

↓

Get first element and index

↓

Execute loop body with element and index

↓

Get next element and index

↓

More elements?

No→Exit loop

↩Back to Execute loop body

The loop goes through each item in a collection, giving you both the item and its position (index) to use inside the loop.

Execution Sample

Kotlin

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

This code loops over a list of fruits, printing each fruit with its index number.

Execution Table

Step	index	fruit	Action	Output
1	0	Apple	Start loop, get first element and index	Print '0: Apple'
2	1	Banana	Next element and index	Print '1: Banana'
3	2	Cherry	Next element and index	Print '2: Cherry'
4	-	-	No more elements, exit loop	Loop ends

💡 All elements processed, loop ends after index 2

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
index	-	0	1	2	-
fruit	-	"Apple"	"Banana"	"Cherry"	-

Key Moments - 3 Insights

Why does the index start at 0 instead of 1?

Indexes in Kotlin collections start at 0 by design, as shown in execution_table rows 1-3 where index values are 0, 1, 2.

What happens when the loop reaches the last element?

After processing the last element (index 2), the loop checks for more elements, finds none, and exits as shown in execution_table row 4.

Can I use the index and element separately inside the loop?

Yes, the loop destructures each pair into index and element variables, so you can use them independently as shown in the code sample and execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'fruit' at step 2?

A "Apple"

B "Banana"

C "Cherry"

D null

💡 Hint

Check the 'fruit' column in execution_table row with Step 2

At which step does the loop end according to the execution_table?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look at the 'Action' column where it says 'No more elements, exit loop'

If the list had 4 fruits instead of 3, how many steps would the execution_table have before exit?

A 5 steps

B 3 steps

C 4 steps

D 6 steps

💡 Hint

Each element adds one step plus one exit step, see current 3 elements with 4 steps total

Concept Snapshot

for ((index, element) in collection.withIndex()) {
    // use index and element
}
- Loops over collection with index starting at 0
- index and element are available inside loop
- Loop ends after last element is processed

Full Transcript

This visual execution shows how a Kotlin for loop with withIndex() works. The loop goes through each item in a list, giving both the item's position (index) and the item itself. The index starts at 0 and increases by 1 each time. The loop runs until all items are processed, then it stops. Variables index and fruit change each step, showing the current position and item. Key moments clarify why index starts at 0, what happens at the last element, and how to use index and element separately. The quiz tests understanding of variable values at steps, loop ending step, and how the number of steps changes with list size.