How a for loop is translated

1. How a for loop is translated under various conditions.

1. A simple for loop that iterates over a collection is translated to a foreach method call on the collection.
2. A for loop with a guard (see Recipe 3.3) is translated to a sequence of a withFilter method call on the collection followed by a foreach call.
3. A for loop with a yield expression is translated to a map method call on the collection.
4. A for loop with a yield expression and a guard is translated to a withFilter method call on the collection, followed by a map method call.

For example, the below two statements are equal.

scala> val out = for (e <- fruits) yield e.toUpperCase

scala> val out = fruits.map(_.toUpperCase)

The way to check these:

class Main {

  for (i <- 1 to 10) println(i)

}

$ scalac -Xprint:parse Main.scala



2. A yield statement with a for loop and your algorithm to create a new collection from an existing collection. 

• When it begins running, the for/yield loop immediately creates a new, empty collection that is of the same type as the input collection. For example, if the input type is a Vector, the output type will also be a Vector. You can think of this new collection as being like a bucket.
• On each iteration of the for loop, a new output element is created from the current element of the input collection. When the output element is created, it’s placed in the bucket.
• When the loop finishes running, the entire contents of the bucket are returned.