Scala For Expressions
Examples
Basic For Loop
for (x <- 1 to 10)
println("Iteration number " + x)This demonstrates iterating a variable, x, from 1 to 10 and doing something with that value. The return type of this for comprehension is Unit.
Basic For Comprehension
This demonstrates a filter on a for-loop, and the use of yield to create a 'sequence comprehension':
for ( x <- 1 to 10 if x % 2 == 0)
yield xThe output for this is:
scala.collection.immutable.IndexedSeq[Int] = Vector(2, 4, 6, 8, 10)A for comprehension is useful when you need to create a new collection based on the iteration and it's filters.
Nested For Loop
This shows how you can iterate over multiple variables:
for {
x <- 1 to 2
y <- 'a' to 'd'
} println("(" + x + "," + y + ")")(Note that to here is an infix operator method that returns an inclusive range. See the definition here.)
This creates the output:
(1,a)
(1,b)
(1,c)
(1,d)
(2,a)
(2,b)
(2,c)
(2,d)Note that this is an equivalent expression, using parentheses instead of brackets:
for (
x <- 1 to 2
y <- 'a' to 'd'
) println("(" + x + "," + y + ")")In order to get all of the combinations into a single vector, we can yield the result and set it to a val:
val a = for {
x <- 1 to 2
y <- 'a' to 'd'
} yield "(%s,%s)".format(x, y)
// a: scala.collection.immutable.IndexedSeq[String] = Vector((1,a), (1,b), (1,c), (1,d),
(2,a), (2,b), (2,c), (2,d))