Kotlin for Loop

A for loop repeats a block of code for each item in a sequence. Instead of writing the same instruction ten times, you write it once inside a loop and tell Kotlin how many times to repeat it.

for Loop with a Range

for (i in 1..5) {
    println(i)
}

Output:

1
2
3
4
5

How the Range Works

1..5  means: start at 1, go up to and including 5
      ┌──┬──┬──┬──┬──┐
      │1 │2 │3 │4 │5 │
      └──┴──┴──┴──┴──┘
  i takes each value, one at a time, left to right

Exclusive Upper Bound with until

for (i in 1 until 5) {
    println(i)
}
// Output: 1 2 3 4  (5 is NOT included)

Counting Down with downTo

for (i in 5 downTo 1) {
    println(i)
}
// Output: 5 4 3 2 1

Step: Skip Values

for (i in 0..20 step 5) {
    println(i)
}
// Output: 0 5 10 15 20

for (i in 10 downTo 0 step 3) {
    print("$i ")
}
// Output: 10 7 4 1

Looping Over a List

val fruits = listOf("Apple", "Mango", "Banana", "Grape")

for (fruit in fruits) {
    println(fruit)
}

Output:

Apple
Mango
Banana
Grape

Looping with Index

Use withIndex() when you need both the position and the value:

val cities = listOf("Delhi", "Mumbai", "Chennai")

for ((index, city) in cities.withIndex()) {
    println("$index: $city")
}

Output:

0: Delhi
1: Mumbai
2: Chennai

Looping Over a String

val word = "Kotlin"

for (ch in word) {
    print("$ch-")
}
// Output: K-o-t-l-i-n-

Looping Over a Map

val scores = mapOf("Alice" to 92, "Bob" to 88, "Carol" to 95)

for ((name, score) in scores) {
    println("$name scored $score")
}

Output:

Alice scored 92
Bob scored 88
Carol scored 95

Nested for Loops

for (row in 1..3) {
    for (col in 1..3) {
        print("($row,$col) ")
    }
    println()
}

Output:

(1,1) (1,2) (1,3)
(2,1) (2,2) (2,3)
(3,1) (3,2) (3,3)

Practical Example: Multiplication Table

fun main() {
    val number = 7

    println("Multiplication Table of $number")
    println("──────────────────────────")
    for (i in 1..10) {
        println("$number × $i = ${number * i}")
    }
}

Output:

Multiplication Table of 7
──────────────────────────
7 × 1 = 7
7 × 2 = 14
7 × 3 = 21
7 × 4 = 28
7 × 5 = 35
7 × 6 = 42
7 × 7 = 49
7 × 8 = 56
7 × 9 = 63
7 × 10 = 70