Kotlin for Beginners — #1: “for” loop

Kotlin ‘for’ loop in 2 min (Updated — Jun, 2020)

All you need to know about ‘for’ loop in Kotlin

Photo by Tine Ivanič on Unsplash

for loop iterates over anything that is iterable (anything that has an iterator() function that provides an Iterator object), or anything that is itself an Iterator.

— Kotlin Doucmentation

1. in — Simplest for loop which iterates over every element of the list

val fruits = listOf("Apple", "Banana", "Cherries", "Guava")
for (fruit in fruits) {
    println(fruit)
}
// Prints: Apple, Banana, Cherries, Guava

2. ..— Iterate over a range between 0 (included) to 5 (included)

for (number in 0..5) {
    println(number)
}
// Prints: 0 through 5

3. until— For range between 0 (included) to 5 (excluded)

for (number in 0 until 5) {
    println(number)
}
// Prints: 0 through 4

4. downTo — Iterate decreasingly from 5 (included) to 0(included)

for (number in 5 downTo 0) {
    println(number)
}
// Prints: 5 to 0

5. step — Jump while iterating over numbers

for (number in 0..10 step 2) {
    println(number)
} 
// Prints: 0, 2, 4, 6, 8, 10

for (number in 10 downTo 0 step 2) {
    println(number)
}
// Prints: 10, 8, 6, 4, 2, 0
val fruits = listOf("Apple", "Banana", "Cherries", "Guava")
for (index in fruits.indices step 2) {
    println(fruits[index])
}
// Prints: Apple, Cherries

6. withIndex() — Iterate over the list with an index of the current item.

val fruits = listOf("Apple", "Banana", "Cherries", "Guava")
for ((index, fruit) in fruits.withIndex()) {
    println("$index - $fruit")
}
// Prints: 0 - Apple, 1 - Banana, 2 - Cherries, 3 - Guava

7. indices — Iterate a list through indexes

val fruits = listOf("Apple", "Banana", "Cherries", "Guava")
for (index in fruits.indices) {
    println(fruits[index])
}
// Prints: Apple, Banana, Cherries, Guava

8. Iterate over Map using entries as objects

val fruitMap = mapOf("A" to "Apple", 
                     "B" to "Banana", 
                     "C" to "Cherries", 
                     "G" to "Guava")
    
for (entry in fruitMap) {
    println(entry.key + "-" + entry.value)
}
// Prints: A-Apple, B-Banana, C-Cherries, G-Guava

for ((key, value) in fruitMap) {
    println(key + "-" + value)
}
// Prints: A-Apple, B-Banana, C-Cherries, G-Guava

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About the Author

Anand K Parmar is a Software Engineer, who loves designing and building Mobile Apps. He is a writer publishing articles on Computer Science, Programming, and Personal Finance. Find him here LinkedIn or Twitter. Check out his latest articles underneath.

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