How to Use Indices in Kotlin: Syntax and Examples

In Kotlin, you can use indices to get the valid index range of a collection like a list or array. Use collection.indices to loop through or access elements safely by their index.

Syntax

The indices property returns an IntRange representing all valid indices of a collection. You can use it in loops or to check index bounds.

• collection.indices: Gets the range of valid indices.
• for (i in collection.indices): Loop through all indices.
• collection[i]: Access element at index i.

kotlin

val list = listOf("a", "b", "c")
for (i in list.indices) {
    println("Index: $i, Value: ${list[i]}")
}

Output

Index: 0, Value: a
Index: 1, Value: b
Index: 2, Value: c

Example

This example shows how to use indices to loop through a list and print each index with its value.

kotlin

fun main() {
    val fruits = listOf("Apple", "Banana", "Cherry")
    for (index in fruits.indices) {
        println("Fruit at index $index is ${fruits[index]}")
    }
}

Output

Fruit at index 0 is Apple
Fruit at index 1 is Banana
Fruit at index 2 is Cherry

Common Pitfalls

One common mistake is to assume indices start at 1 or to access an index outside the valid range, which causes an IndexOutOfBoundsException. Always use indices to safely loop through valid indexes.

Wrong way (may cause error):

for (i in 1..list.size) {
    println(list[i]) // Error: index out of bounds at last iteration
}

Right way:

for (i in list.indices) {
    println(list[i])
}

Quick Reference

• collection.indices: Gets valid index range (0 until size).
• Use in for loops to access elements by index.
• Prevents out-of-bound errors by limiting index range.

Key Takeaways

Use collection.indices to get the valid index range of a collection.

Loop with for (i in collection.indices) to safely access elements by index.

Avoid accessing indices outside indices to prevent errors.

Indices always start at 0 and go up to size - 1.

Using indices makes your code safer and clearer.