Kotlin for Loops

• for loops are similar to Java, but the syntax is different
• Unlike in Scala, Kotlin for loops are not expressions

Basic syntax

Kotlin for loops have this basic syntax:

for (e in elements) { // do something with e ...}

For example, given a list:

val nums = listOf(1,2,3)

here’s a single-line algorithm:

for (n in nums) println(n)

The REPL shows how it works:

>>> for (n in nums) println(n)
1
2
3

You can also use multiple lines of code in your algorithm:

for (n in 1..3) {
    println(n)
}

Here are a couple of other examples:

// directly on listOf
for (n in listOf(1,2,3)) println(n)

// 1..3 creates a range
for (n in 1..3) println(n)

Using for with Maps

You can also use a for loop with a Kotlin Map (which is similar to a Java HashMap). This is how you create a Map of movie names and ratings:

val ratings = mapOf(
    "Lady in the Water"  to 3.0, 
    "Snakes on a Plane"  to 4.0, 
    "You, Me and Dupree" to 3.5
)

Given that map, you can print the movie names and ratings with this for loop:

for ((name,rating) in ratings) println("Movie: $name, Rating: $rating")

Here’s what that looks like in the REPL:

>>> for ((name,rating) in ratings) println("Movie: $name, Rating: $rating")
Movie: Lady in the Water, Rating: 3.0
Movie: Snakes on a Plane, Rating: 4.0
Movie: You, Me and Dupree, Rating: 3.5

In this example, name corresponds to each key in the map, and rating is the name I assign for each value in the map.

for is not an expression

If you’re coming to Kotlin from Scala, it’s important to note that for is NOT an expression. That means that code like this does not work:

// error
val x = for (n in 1..3) {
    return n * 2
}