The Kotlin for loop explained with example code

The for loop in Kotlin works a bit differently than the for loop in traditional programming languages like Java.

For example in Java, a for loop is a combination of 3 statements as follows:

for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

/* 
Output:
0
1
2
3
4
*/

But the for loop in Kotlin doesn’t have a 3 statements part like in Java.

Instead, the Kotlin for loop allows you to iterate through many things using a simple for ... in syntax.

Here’s the same Java loop above refactored to Kotlin code:

for (i in 0..4) {
    println(i)
}

/* 
Output:
0
1
2
3
4
*/

The code above is a valid Kotlin for loop. The variable i is used to iterate through a range of numbers between 0 to 4 and print the number to the console.

The in operator is the magic in Kotlin. It allows you to loop through anything that allows iteration.

In another example, you can loop through a String value as shown below:

var myString = "Hello!"

for (i in myString) {
  println(i)
}

/* 
Output:
H
e
l
l
o
!
*/

You can also loop through any array type using the same syntax. Here’s an example of looping over an Int array:

val numberArr: Array<Int> = arrayOf(10, 20, 30)

for (i in numberArr){
    println(i)
}

/* 
Output:
10
20
30
*/

And you can also loop over the index stored inside the indices property of the array. Here’s another example of looping over a String array:

val nameArr: Array<String> = arrayOf("Jack", "John", "Dan" )

for (i in nameArr.indices){
    println("The value at index $i is ${nameArr[i]}")
}

/* 
Output:
The value at index 0 is Jack
The value at index 1 is John
The value at index 2 is Dan
*/

Conclusion

To summarize, Kotlin only has one valid syntax of the for loop, and that is the for ... in loop.

The for loop in Kotlin can be used to loop through anything that provides an iterator:

• A range of numbers
• A String value
• An Array object
• A HashMap object

But keep in mind that some objects like an Array also have the forEach() method, so you may want to use that over a for loop.

When you’re in doubt whether the Kotlin for loop can iterate through an object, just try writing the statement.

If Kotlin doesn’t complain, it means you can iterate through it 😉

Nice work on learning about Kotlin for loop! 👍