What is the alternative in writing the following code using while construct?
val list = List(1,2,3)
for (v <- list) println(v)
3 Answers
val list = List(1,2,3)
val iter = list.iterator
while (iter.hasNext) println(iter.next())
Comments
A possible imperative traversal :
var current = list
while(!current.isEmpty) {
println(current.head)
current = current.tail
}
3 Comments
Howard
I believe this approach is bad as it need to modify the
current?
Rex Kerr
@Howard - Well, that's what you get when you use a while loop. While loops are inherently mutating constructs (or they're pointless except for warming your CPU).
Didier Dupont
Using an iterator does about the same think. It just encapsulates the var. The loop above works for list only, and is meant to show how iteration works on list. iterators abstract that away, which is a of course a good thing.
val list = List(1,2,3)
var i = 0
while (i < list.length) {
println(list(i))
i += 1
}
1 Comment
Luigi Plinge
This is not a good way because calling
length requires a full traversal of the list and accessing element n by index requires the traversal of n elements.
forloop, this is aforcomprehension. It's just syntax sugar forlist.foreach { v => println(v) }which would be more idiomatically written aslist foreach println. This is as clear and obvious as it gets.for (x <- expr1) body"for loops"; by nature they're side-effecting and have type Unit, as would a while loop. With ayieldclause the book calls it a "for expression". The term "for comprehension" is listed in the glossary as another name for a for expression, but is not used elsewhere.