Scala list append a class in list and retrieve item - list

Question

new to scala so missing basic of list.
I'm trying to make a list of following class
case class Person (val name: String, val age:Int, val email: String)
How do I make a list immutable list of Person class?
I tried:-
val list: List[Person] = List(Person("",0,"")) // compilation ok
list.::(person) // compile ok
But when I check size it is 1. It should be 2. Also I don't see the added person in the list.
Please let me know how do I make list of Person class, inset new person and then retrieve person if either age or name or email or by index as well.
Please don't refer me to api doc http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.List
It is hard to understand :(

Answer 1

Try this.
val list2 = list.::(person)
Remember list is immutable.
list2 should have size 2.

Answer 2

Keeping in mind immutable lists as aforementioned, for
case class Person (val name: String = "",
val age:Int = 0,
val email: String = "")
(note you can set construction defaults to each field) and
val list: List[Person] = List(Person("",0,"")) // compilation ok
we can construct a second List[Person] also with the List concatenation operator ++ as follows,
val list2 = list ++ List(Person(age=1))
list2: List(Person(,0,), Person(,1,))
To retrive a Person by a given criterium, for instance in this example, by age, consider collect as follows,
val list3 = list2.collect { case p#Person(_, age, _) if age == 1 => p }
List[Person] = List(Person(,1,))
Here we extract the field of interest, age, and check for a condition (whether it equates to 1), thus we select those entries in list2 that hold the condition; to deliver such entries, we use a bind p on each Person instance; perhaps more canonical (and unnecessarily repetitive) approach would be
val list3 = list2.collect {
case Person(name, age, email) if age == 1 => Person(name, age, email) }
A similar approach relies in filter as follows,
list2.filter(p => p.age == 1)
And with a for comprehension, like this
for ( p#Person(n, a, e) <- list2 if a == 1 ) yield p
A more elaborated approach to defining a selection over a list of entries includes the definition of a method that takes a predicate as argument and evaluates it against each entry; in addition this can be encapsulated in an implicit class for ease of use; thus consider this example,
implicit class PersonsOps( val persons: List[Person] ) extends AnyVal {
def select(pred: Person => Boolean) = persons.filter(pred)
}
and so we can define an anonymous function in the call to byAge, namely (x: Person) => x.age == 1; hence
list2.select( (x: Person) => x.age == 1 )
or equivalently
list2.select( _.age == 1 )
Note in this illustrative example select and filter are equivalent or interchangeable, yet select may be enriched/extended with additional requirements.

Answer 3

As Soumya Simanta said, you have an immutable list, so you have to assign the List returned by the :: operator to a new val. By the way you can use following notation
val list2 = person :: list
which is equivalent to
val list2 = list.::(person)
To get the information of your Person instances you can use the following line:
list2 foreach { p: Person => println(p.email) }
foreach is a method of List. It applies a function (e.g. { p: Person => println(p.email) }) to all elements of the list.

Answer 4

list.::(person)
retrieves a new list, containing both elements: the old and the new. However, the operation does not change your list called list. Try this way:
var list: List[Person] = List(Person("",0,"")) // compilation ok
list = list.::(person) // compile ok