Scala ‘for loop’ examples and syntax

Besides having a bad memory, I haven’t been able to work with Scala much recently, so I’ve been putting together this list of for loop examples.

This page is a work in progress, and as of tonight I haven’t tested some of the examples, but ... if you’re looking for some Scala for loop examples — technically called a for-comprehension or for-expression — I hope these examples are helpful.

Example data structures

Before getting into the examples, it helps to have some simple, common data structures at our disposal, so I’ll use these data structures in the examples that follow:

val names = Seq("chris", "ed", "maurice")
val nums = Seq(1, 2, 3)

I’ll add a few more data structures below as needed, such as in the Map examples.

Basic for-loop examples

First, a few basic Scala for loops:

for (n <- names) println(n)
for (n <- names) println(n.capitalize)    
for (n <- names) {
    // imagine this requires several lines
    println(n.capitalize)
}

Using generators in for-loops

Next, here’s a for loop that uses a single generator:

for (i <- 1 to 3) println(i)

Multiple generators:

val nums = Seq(1,2,3)
val letters = Seq('a', 'b', 'c')
val res = for {
    n <- nums
    c <- letters
} yield (n, c)

If you paste that block of code in the REPL, it will yield this result:

res: Seq[(Int, Char)] = List((1,a), (1,b), (1,c), 
                             (2,a), (2,b), (2,c),
                             (3,a), (3,b), (3,c))

Scala for-loop generators with guards

Here’s one generator with one guard (an if condition), on one line:

for (i <- 1 to 10 if i < 4) println(i)

Here’s the same thing on multiple lines:

for {
    i <- 1 to 10
    if i < 4
} println(i)

This example shows several guards:

for {
    i <- 1 to 10
    if i > 3
    if i < 6
    if i % 2 == 0
} println(i)

Scala for/yield examples (for-expressions)

A common use case is to use a for loop with yield to create a new data structure from an existing data structure. Here’s a simple example:

val names2 = for (e <- names) yield e.capitalize

Here’s a multiline version:

val lengths = for (e <- names) yield {
    // imagine that this required multiple lines of code
    e.length
}

Note that for/yield is the same as a basic map example:

val out = for (e <- names) yield e.capitalize
val out = names.map(_.capitalize)

Scala for-loop counters (and zip, zipWithIndex)

You can use a counter in a for loop like this:

for (i <- 0 until names.length) {
    println(s"$i is ${names(i)}")
}

For a zero-based counter you can also use zipWithIndex:

for ((name, count) <- names.zipWithIndex) {
    println(s"$count is $name")
}

Beware: zipWithIndex creates a new sequence from the existing sequence, you may want to call view before invoking zipWithIndex:

val zwi2 = names.view.zipWithIndex

The zipWithIndex counter starts at 0; zip lets you control where the counter starts from:

for ((name,count) <- names.view.zip(Stream from 1)) {
    println(s"$count is $name")
}

Can also use foreach with zipWithIndex:

names.zipWithIndex.foreach { d =>
    println(s"${d._2} is ${d._1}")
}

Note that these examples work as shown because zip and zipWithIndex both return a sequence of Tuple2 elements.

Using a for loop with a Scala Map

Here are some examples of how to use a for loop with a Scala Map (mutable or immutable):

val nameMap = Map("firstName" -> "Ed", "lastName" -> "Chigliak")
for ((k,v) <- nameMap) {
    println(s"key: $k, value: $v")
}

val result = for ((k,v) <- nameMap) yield {
    s"key: $k, value: $v"
}
println(result)

That’s not the most useful example because result ends up as a List:

List(key: firstName, value: Ed, key: lastName, value: Chigliak)

but I can’t think of a good example right now, and I wanted to show a for/yield example with a Map.

Here’s another example with for and a Map:

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

for ((name,rating) <- ratings) println(s"Movie: $name, Rating: $rating")

That’s the same as the first Map example, but I wanted to show a Map with foreach:

ratings.foreach {
    case(movie, rating) => println(s"key: $movie, value: $rating")
}

ratings.foreach(x => println(s"key: ${x._1}, value: ${x._2}"))
ratings.keys.foreach((movie) => println(movie))
ratings.keys.foreach(println)
ratings.values.foreach((rating) => println(rating))

Multiple futures in a for loop

You can use a Scala Future with a for comprehension, but you have to make sure you create the future(s) before the comprehension, like this:

val result1 = Future { do1() }
val result2 = Future { do2() }
val result3 = Future { do3() }

Do that first, then merge the futures’ results inside the for loop:

val result = for {
    r1 <- result1
    r2 <- result2
    r3 <- result3
} yield (r1 + r2 + r3)

For more details on this, see my How to use multiple Scala Futures in a for loop example.

foreach examples

You can also use the foreach method on the Scala collections classes:

names.foreach(println)
names.foreach(e => println(e.toUpperCase))
names.foreach {
    // imagine this requires multiple lines
    e => println(e.toUpperCase)
}

See the Map examples above for examples of using foreach with a Map.

Summary

As a quick summary, if you wanted to see some Scala for loop examples in a concise format, I hope this is helpful. For more details on the for comprehension, see these links:

• How to loop over a Scala collection with a for loop
• Scala: How to create a "for comprehension" (for/yield loop)
• How to loop over Scala collections with for and foreach
• How to loop over a Scala collection with a for loop
• Scala: How to use zipWithIndex or zip to create loop counters
• Scala: How to use zipWithIndex or zip to create loop counters
• How to traverse a Map in Scala (for loop, foreach)