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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 x

The 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))



Conclusion

In this page (written and validated by ) you learned about Scala For Expressions . What's Next? If you are interested in completing Scala tutorial, your next topic will be learning about: Scala Functions.



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